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Turbo FAQ 101, aftermarket exhausts, and answers to common
turbo questions
Introduction
This article contains both basic and advanced consumer-level information
about turbos so everyone will be able to get something from it. Some of
the information is applicable to any turbo car, some is VW TDI specific but will
add to your knowledge base. It will first describe
basic turbo theory, then more advanced turbo theory including turbo components, exhaust
flow, and turbo properties. More advanced topics such as calculating and interpreting
turbo flow graphs or wheel
trim and component selection is not included because it is not applicable to most consumers. Here is a
table of contents for the most frequently asked questions about turbos.
Do you have information or corrections that should be added to this article? Post your comments in the myturbodiesel.com forums
Table of contents
The absolute basics
Turbocharger basics
Turbos and direct injection
Turbo basic parts
Turbo components and system
Component selection and turbo
design
A/R ratio
Turbo selection as part of a whole system
and volumetric efficiency
Internal vs. External wastegate
Turbo exhaust flow
Why change from your OEM exhaust?
Test pipes vs. catalytic converters and the biodiesel
clogging effect
Downpipe - single and split
Variable geometry turbo vs.
fixed geometry turbo in your TDI
The CHRA
The turbo runaway in a diesel engine
Oil supply and turbo timers
The intercooler as a heat sink
More useful turbocharging information
Common turbo myths
More boost does not always equal more power
Turbocharging your own car
Port matching
Sequential twin turbos vs. symmetrical twin turbos vs. single turbo
Centrifugal superchargers
Let's start with the basics: A turbocharger is an exhaust gas-driven turbine that compresses the intake air, increasing the horsepower and torque of an engine by increasing volumetric efficiency. This means that by compressing the air and increasing the density, you use a given volume of engine displacement more efficiently (volumetric efficiency). Denser air means more air atoms and more fuel atoms can be added into the engine. This makes more power.
As a side note, you may have heard of nitrous or the brand name "NOS". Using nitrous oxide as an additive to gasoline engines dramatically increases the amount of oxygen in the engine's combustion cylinders, cools the air charge, and allows more fuel to be burned. More fuel + more air = more power. Top fuel dragsters use a fuel of 85% nitromethane and about 15% methane in engines only about the size of a ford mustang engine but with power measured in the thousands of horsepower (hp). Since the fuel is over 50% oxygen, it's concentration of energy possible from a given space, its volumetric efficiency, is much greater than an engine burning only regular air which is only about 21% oxygen.
Most modern turbocharged engines seem to be 4 cylinders and often have as much or more power than a 6 cylinder non turbo engine. You may be asking yourself why don't car manufacturers turbocharge all cars? Because it costs more money to design and build, larger engines usually have better low end power, and they can charge a premium for larger engines and that V8 sticker. And in many cases (like the Corvette) a big engine just works! Although you can add turbocharging to a non turbo car, it's not recommended for most people, more details in 1000 answered questions: turbocharging your own car.
So the ultimate goal of turbocharging is to increase air density to make more oxygen available to burn. The energy from this burning is what pushes the piston down, creating energy. This increase in air density, or boost, is normally expressed in pressure. In the US, the most commonly used unit of pressure is pounds/sq. in, or psi. Other common units of pressure are bar, or kPa. To help understand when the engine is under boost and under vacuum, consider these examples. If the engine is off, a vacuum/boost gauge would read 0. This means that the gauge is measuring a difference of 0 psi between the intake and ambient pressure. If the engine is running at idle, the gauge may show a negative reading, for example -7. This means that the intake is under vacuum and has a lower pressure than ambient air. If you press on the throttle pedal 100% while the engine is under load, the gauge will indicate a change from vacuum to boost, or positive pressure. This means that the turbo has pressurized the intake air more than ambient by whatever amount the gauge shows. Part of this is that most boost gauges get their reading from the intake manifold or piping. Also note that most diesel cars will not show any significant vacuum or negative reading in the intake manifold because there is no throttle plate to draw a vacuum against. Outside of North America, some cars may also show ambient atmospheric pressure instead of relative pressure. In other words, when at rest, the gauge will show about 14 psi or 1 bar instead of 0.
Turbocharging in diesel engines and direct injection
Modern passenger car diesels are almost all turbocharged for many reasons. Diesel engines are naturally very robust because of the characteristics of the diesel cycle and high compression ratio. Well designed turbo engines require a stronger design to withstand pressures. And because of a smaller range of rpm in a diesel engine, a turbo setup does not have to be expected to perform over the wider rpm range of a typical gasoline engine.
Another reason is that because all modern diesels use a form of fuel injection called direct injection. There are many variations on how high the pressure is, how long the injection is, what the brand name of injection type is called, and how many injections there are. For the VW TDI, please see 1000q: direct injection pumpe duse and common rail. In any case, direct injection is far superior to port injection used in older diesels and most gasoline cars. Which will burn faster: loose thin shredded paper or thick paper tightly wadded into a ball? The higher pressure of direct injection results in finer atomization and a more thorough burn. Another advantage of direct injection in gasoline cars is heat absorption and finer control over detonation. When fuel is finely misted into the combustion cylinders, the absorption of heat helps avoid any uncontrolled detonation of the fuel. A safe engine tune on a turbo gasoline engine when under boost uses more fuel than a non turbo engine to help control this detonation. The ratio of air to fuel (AF ratio) in a diesel engine is variable, since diesel engines can run well within a large range. As a very rough estimate, your TDI (including chipped and modified cars) runs between 15:1-50:1 AF, being smoky at the rich end. A turbo gasoline engine's AF is about 13:1 to 11:1 under heavy boost, whereas a nonturbo engine should be about 14:1. A turbo car's computer will change fuel delivery when under boost by increasing fueling and making it richer. Please note that this is adjusted to the amount of air it thinks the engine is getting, not the amount of air the engine is actually getting. This can be because any air leaks could let out the amount of air that has already been measured. See 1000q: boost and vacuum leak checking for a simple procedure to test for boost leaks.
Here is a picture of a common rail direct injection system on a gasoline
car. The silver tubing is the shared fuel hard line, the common rail, and the
long black cylinders
tilted to the right are spark plugs.
| For a better idea of where the injectors fit in the engine, here is the same
engine after assembly. The intake port is marked with a blue arrow, the
exhaust port is marked with a yellow arrow. The injector is outlined in
red. A direct injector will always have the nozzle sticking into the
combustion chamber.
For more details on the TDI diesel system, see 1000q: direct injection pump duse and common rail
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The turbocharger's basic parts are the compressor side, turbine side, and the center housing which connects the two sides. The turbine, or exhaust side, has a small pinwheel-like turbine that is spun by exhaust gasses. Built into the housing is an internal wastegate that lets excess exhaust gas and pressure out. If the turbo is an external wastegate, the wastegate is not built into the turbine housing and is somewhere else.
Below are some pictures of an internal vs. external wastegate turbo. The
differences are discussed in more detail in: 1000
answered questions: turbocharging: internal vs. external wastegate.
The other side, the compressor or intake side, also has a pinwheel-like impeller, powered by a straight shaft
from the turbine wheel. Its job is to compress the intake air. The
center housing, or center hub rotating assembly (CHRA), is the part that houses
the shaft and bearings that the two wheels spin on, and normally contains oil
and coolant to lubricate it all. Note that the turbo used in the VW TDI is
oil cooled only.
The wheels and shaft can often reach speeds of 50,000 to 200,000 rpm which is why they require proper cooling and lubrication. Warning: do not reroute the CHRA oil or coolant lines without first considering any possible complications. A bent line could cause the CHRA to be starved of oil or coolant, damaging the turbo. After engine shutdown, the turbo cools off and causes some circulation in the turbo oil and coolant lines. Rerouting these lines improperly can starve the CHRA of this natural circulation, possibly causing long term damage.
| In most cases, the CHRA is fed from the
same oil supply as the engine. In some models of turbos the engine oil may
also be the only method of cooling the turbo, although most modern turbos have separate
coolant lines.
Note: the turbo pictured at right is a ball bearing turbo and had some damage to the wheels. Also note that the VW TDI turbo is cooled by oil only.
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The turbo output and performance is chosen carefully by engineers for a certain range of performance. I'll describe sizing and characteristics of these components in detail in this section. Choosing a smaller turbo and turbo components will give better low end power but cannot move air efficiently at the top end. A larger turbo and turbo components has better top end but less low end power potential than a smaller turbo.
The first issue is fitment. If a larger turbo will not fit into the old turbo's space or has a different style housing than the old, you have to change other components such as the exhaust manifold, downpipe, and motor mounts to get it to fit and bolt on. Some VW turbos have the exhaust manifold and turbo exhaust housing as one piece so they must be changed out as a single unit or replaced as a separate manifold and turbo. If the turbo inlet or outlet is facing the wrong direction, you may have to clock the turbo by loosening the housings and rotating them to orient the turbo in the correct direction.
Another issue is housing size, this has a big effect on turbo efficiency. If a compressor's housing is too big, the exhaust gasses will not be efficiently directed onto the wheel. If the housing is too small, exhaust gasses will back up and create a bottleneck in the airflow. Modern diesel cars use a variable vane geometry turbo to change the way exhaust gasses are directed in the housing to maximize efficiency, but almost no gasoline cars use this type of turbo. The most dangerous condition is when a housing and/or impeller is sized improperly and creates a large area of performance in which the impeller compresses the air so much that becomes too hot and unable to be efficiently cooled. This can cause uncontrolled detonation in an engine, probably causing severe damage to the engine.
The next thing to examine is the size of the pinwheel like exhaust compressor wheel and intake turbine wheel. Like a larger or smaller pinwheel, the larger a turbo wheel is, the more air it can move. For garrett turbos, the wheel size is called trim size. Trim size is a ratio of the inducer/exducer sizes squared times 100. Basically, a larger trim means a larger wheel which can move more air. They can also be constructed out of ceramic, but are limited by top end speed due to the ceramic material.
Turbo selection as part of a whole system and volumetric efficiency
The most important characteristic of each turbo component is that they have to work well as a whole. If the exhaust housing was small, but it's turbine was large, the airflow will get choked. If the exhaust housing is large but the turbine was small, the airflow will not be efficiently directed at the turbine. The exhaust and intake side also should be in harmony. The intake side compresses a certain amount of air into the motor and should expect an appropriately sized exhaust side to flow the exhaust gas back out. The whole system has to work in harmony to achieve efficient operation. Using a water funnel as an analogy, even if you put more and more water into it, there is a range of how much water can come out the bottom. An inappropriately sized and/or matched component will prevent the components from working in the area of good efficiency, performance, and value. Each mod should have a set of supporting mods working towards an overall goal. Before choosing components and modifying your car, have an estimate of about how much power you want, then design the modifications around making that overall goal. Meeting that power goal with the smallest turbo, the least turbo boost, and the most efficient intercooler, will all reduce engine stresses and maximize engine response.
Also keep in mind that changing turbo components are only a part of increasing volumetric efficiency (VE). Adding camshafts, porting or tuning the intake manifold and cylinder heads, all change the volumetric efficiency and will further contribute to the efficiency of the engine. Note that modifying camshafts are more applicable to gasoline engines, but everything else listed can work on diesel engines too.
An important term to know when talking about turbo housings is the AR ratio. How does knowing this effect you in daily driving? Not at all, unless you plan on using anything other than the stock spec. turbo, but it is still a very useful aspect of turbo technology to know. The aspect ratio, or AR is the ratio of the area of the cone to radius from the center hub. Basically, if you were to measure the cross section on any point on the turbo and divide by the distance from the center of that cross section to the center of the turbine wheel, you would get the AR ratio. Ideally, this ratio should remain the same as you move in and out of the turbo housing because the housing gets smaller as you get closer to the center. This spiral shaped cone is called a volute. It begins right about the point in the housing where you can no longer see into it, after the inside diameter changes from the shape of the flange opening to the shape of the volute. Basically, it concentrates airflow at a point on the turbo wheels. Everything else being equal, increasing the AR will reduce spool up but increase top end performance by allowing more air to flow. Decreasing the AR will increase spool up but reduce top end performance. A larger AR will allow more air to flow through its passages.
However, note that I said "everything else being equal". A/R ratio is most useful when comparing flow capacities between like housings with similar exterior dimensions and different size volutes. In other words, turbo A with a .86 AR does not always flow more air than turbo B with a .64 AR. Turbo B could be a turbo 5 feet tall used in a power plant, and turbo A could be a 6 inch tall small motor turbo. When comparing AR ratio, the housings must be otherwise identical.
The shape of the volute can also dramatically effect air flow because of how it directs the air. The closer to the shape of a teardrop the volute takes, the easier energy is transferred around the housing and into the turbine wheel. For example, Mitsubishi makes many turbos for many different applications. The 7cm Mitsubishi exhaust housing has a compromised volute (teardrop cut in half) for water line clearance to the bearing housing and ease of casting, pictured right. |
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The exhaust housing may also house an internal wastegate. An internal wastegate is a hole cast into the exhaust housing and a flap door that opens to let excess gasses out instead of spinning the turbo and creating more boost on the intake side. If you look below, you can see pictures comparing an internal and external wastegate. If the shaft or wheels over speed, damage could result. A turbo wheel can spin from 0-100,000+ rpm. Metal turbine wheels are not as prone to damage as ceramic wheels, the VW TDI all use metal wheels. The wastegate door is is opened and closed by a spring loaded wastegate actuator. The wastegate actuator is basically a vacuum diaphragm which is normally closed from resistance from a spring . Once it receives boost pressure on one side of the vacuum diaphragm, it overcomes the spring pressure and pushes a lever that opens the wastegate. One method that chip tuners use to build power in turbodiesel cars is to reprogram the car's computer to hold the wastegate closed at higher than stock pressures to make more boost.
In the below picture, you can see how the exhaust housing directs the exhaust gases onto the turbo wheel. The internal wastegate has a trap door that opens at a certain pressure, the external wastegate turbo requires a separate component, the external wastegate, placed before the turbo to vent excess gasses. You can also see how the air flow out the external wastegate turbo matches the exhaust pipe, but the internal wastegate has an empty spot where air turbulence can form. Note that some newer turbos have built in dividers and you can also make or buy an exhaust pipe with a divider to improve exhaust flow.
An external wastegate is superior to an internal wastegate in terms of boost control and airflow. The piping exiting the exhaust housing can be made to match the size and shape of the exhaust turbine, creating a smooth transition from the turbo to the exhaust. This translates into more power everywhere in the rpm range. However, it often has a higher price since you have to pay for the wastegate separately from the turbo and takes up more space since it requires extra exhaust piping for the external wastegate. A good compromise between an internal wastegate and external wastegate is a split downpipe, see below for more details on downpipes.
Also note that some diesel turbos do not have a wastegate. Many VW
turbos, specifically the VNT turbos, have a VNT actuator in the same spot as the
wastegate actuator on conventional turbos. One of it's purposes is
analogous to a conventional wastegate because it can redirect the turbo gases on the turbine wheel to
control it's speed instead of dumping gasses out through the wastegate.
1996-1999 3rd generation (mk3) VW TDI turbos use a conventional turbo
with an internal wastegate, all later generations used a VNT turbo. Note that if the solenoid controlling
pressure/vac to the wastegate malfunctions, the default position is to open the
wastegate. This is because if it were to fail in the closed position, the
engine would create too much boost and incur serious damage. A
misdiagnosed "failed turbo" is often just limp mode, see 1000q:
diagnosing VW TDI limp mode for more details.
You want the least backpressure in the exhaust after the turbo for maximum performance, no exceptions. The problem is that you have to balance maximum performance with emissions and difficulty/cost of fabrication, etc.. Note that this does not apply to with non turbo or supercharged cars, where some exhaust backpressure is normal as a result of keeping exhaust velocity and the scavenging effect from individual cylinders high. Non turbo cars that keep their catalytic converters are not as significantly penalized by backpressure as turbo cars are. In most cases, non turbo exhausts want to restrict the piping diameter to some extent to keep exhaust gas velocity high and receive backpressure as a byproduct. With turbo exhausts, there is no scavenging effect downstream of the turbo, so you want the least amount of backpressure after the turbo for the maximum performance and efficiency. A turbo exhaust should have the highest energy differential across the turbo (the exhaust gasses are also hot and have a lot of energy) to get the turbo spooled up, and the least backpressure and high velocity exhaust gases after the turbo. This is because a turbo gets its energy by a pressure ratio. Image a waterwheel: you want the pressure highest before the waterwheel and lowest after the waterwheel to give it the most energy.
Below are some more details on individual components of exhaust systems. Because you want the least backpressure in a turbo car's exhaust, the ideal exhaust system would produce the least backpressure immediately after the turbo. Due to routing, emissions equipment, pipe diameter, exhaust gas temperatures/pressures, the perfect diameter changes from car to car, setup to setup. It's very difficult to know this without extensive testing, so as a rough rule of thumb, a consistent or increasing diameter exhaust as you head downstream towards the tailpipe is best in most cases. Mandrel bent exhausts are also always better. A mandrel bend is when piping is bent with a mandrel, or insert, to keep the inner diameter consistent at the bend. Crush bends reduce the diameter at the bend and reduce smooth exhaust flow. Most factory exhausts are non-mandrel bent crush style bends, so switching to a mandrel bent exhaust will increase power and efficiency of the turbos and engine with no other modifications You also want to avoid very restrictive mufflers, sudden changes in piping diameter, and sharp bends. As a rough rule of thumb, each 90o bend in the piping has about the same resistance to airflow as 25 feet of straight piping!
For an extreme level of modification, you could also switch to an equal
length runner exhaust manifold for the turbo (the part that is between the turbo
and cylinder head). Equal length runners make sure that the exhaust pulses
are timed so that they take the same amount of time to hit the turbine and to
keep cylinder reversion balanced across all cylinders. The stock VW exhaust
manifold and most stock turbo exhaust manifolds are the log style manifold
due to a number of factors. The log style is much cheaper and easier to
make, are usually cast in one piece of iron so that they don't have weak welds
that can crack with repeated heat cycling, expansion, and stress, and also take
up less
room. However, the amount of custom fabrication is so high that you would
be better off spending your money and time improving other areas in the turbo
system first. This is because most turbodiesel passenger cars serve as daily drivers
and are not yet at a level where a tubular manifold would be an economical power
upgrade. Below is a picture of a tubular header.
Note how the exhaust runners are the same length. Since this article was
written, some off the shelf turbo manifolds have become available for the TDI.
Because you can just buy the part as a bolt on part instead of doing custom
fabrication, I would definitely suggest an upgraded manifold if you are upsizing
the turbo, budget permitting. See 1000q:
basic performance upgrades for the TDI for more details.
Why change from your OEM exhaust system?
You may be wondering how much gain in exhaust flow you will gain over your OEM exhaust. There will always be an increase in efficiency in switching to a quality aftermarket exhaust. Unless you make a measurement of backpressure, there is no way to quantitatively know how much. Even two identical cars may be slightly different due to manufacturing tolerances. Much like any other modification to your car, custom parts will cost a lot more than if it were a mass produced part by a parts supplier. All you can know for sure is that it will be an improvement over your OEM exhaust as long as the replacement parts are quality pieces.
The OEM part has to conform to emissions and noise regulations that vary country to country, be easily produced and fabricated thousands of times, and may only be, as an example, 75% efficient. By replacing it with a part that is 95% efficient, you might end up spending $$$. As a result, work with your budget to reach your power goals, realistically. If it's worth the money is ultimately up to you, some people would rather spend the money on something other than a car.
So why didn't your car maker just give you a 95% efficient exhaust? Remember that if all the parts on your car were just one level better, it would result in a car that is for example, $5000 more expensive. If they put all the luxury car parts on an economy car, it wouldn't be an economy car would it? Car makers have to balance the quality of parts on a car to get the most perceived consumer desirability out of it. Since the interior and exterior are what buyers see and touch, some car makers prefer to spend the money there.
Backpressure in the exhaust housing
One way to test how much back pressure you have is to take a reading. You don't need to measure your backpressure constantly, only after you make some modification to the exhaust system or engine. You just have to tap the exhaust system before the turbo with any pressure gauge. An oil pressure gauge or low range air pressure gauge will both measure the backpressure in the exhaust. I suggest putting an air filter or fuel filter inline to dampen the exhaust pulses so you can get a steady measurement. Once you hit boost, note the peak hold value. Once the pressure has peaked, you have reached the engine's max VE. As a rule of thumb, you don’t want more than a 1:1.5 ratio of boost to backpressure. For example, if you are making 10psi of boost you don't want more than 15-18 psi of backpressure. If so, then the turbine side could benefit from more air flow and you’ll make more horsepower for every pound of boost you run. Keep in mind that the turbo wheels are not easily changed except by turbo rebuilding professionals, so for most users, the basic rule of thumb should be:
Between the exhaust ports and the turbine housing, you want as much energy going through that turbo. This means metals that don't soak up the heat, heat reflecting coatings, short piping, and tubular headers. Keep in mind that if the turbine housing can't flow enough air, these improvements will be lessened at the turbo housing. Also keep in mind that while an exhaust manifold made from stainless steel can be welded into to a better flowing manifold, it will get red hot if driven hard and will be more prone to cracking at the welds compared to a cast iron manifold, so keep in mind these other factors when comparing manifolds.
After the turbine
housing, you want the greatest heat and pressure differential. This means a downpipe
that is not coated and free flow exhausts. Test pipes or
straight exhausts would be considered more or less free flow exhausts.
Test pipes vs. catalytic converters and the biodiesel clogging effect
Tests pipes are basically pipes that replace the section of exhaust that contains the catalytic converters. It is for off-road use only and is illegal in every state! In fact, removing the catalytic converters and the O2 sensors will cause error codes to appear in many cars, especially obd2+ cars. OBD2+ gasoline cars often have an O2 sensor before and after the catalytic converter. Note that VW diesels did not use an O2 sensor in the exhaust except 2004-2006 cars that use pumpe duse and later TDI. If removed and not worked around with a chip or resistor, it could set a check engine light and can cause a failure of any required emissions testing or inspections, preventing you from registering your car in some states. There are also fines for removing or tampering with factory emissions equipment on cars.
If there are so many negatives to test pipes, then why do many people use them? Power and economy are both increased with test pipes, especially in turbo engines. In designing a turbo system, the engineers want to have the highest energy differential before and after the turbine wheel. This energy (exhaust gas velocity, heat, pressure) differential transfers energy to the turbo system. By removing the catalytic converters and that restriction in the exhaust system, you create greater a pressure differential for the turbo between the compressor and turbine side and let the turbo work "easier" and better. Keep in mind that this is for turbo cars only! Non-turbo or supercharged cars do not have turbos and the potential performance gains are not as great.
Another factor is that while the catalytic converters act as a restriction in exhaust flow, they also add energy and velocity by burning off unburned hydrocarbons in an exothermic oxidation. This is still not enough to overcome their restriction in flow, but it's not like stuffing a potato in the exhaust pipe if that's what you were imagining. A catalytic converter is actually honeycombed or grid-like, to allow exhaust to flow through.
All in all, especially for diesel applications, I would recommend leaving the catalytic converters in place. Leaving the catalytic converters in place will both clean the exhaust emissions, make the exhaust much cleaner and quieter, and is less expensive then making custom piping. The TDI is an excellent daily driver and I didn't want to tolerate the increased smoke, odor, and emissions for the trade off in increased peak power and throttle response. If you want an all out sports car, the TDI will not satisfy you and if it does, you never wanted a real sports car. In the end, it's up to you to determine if you want to remove it, but remember that in some states with emissions or inspection requirements, you may not pass without a muffler or catalytic converter. Another reason to bypass the exhaust filters is if you are using biodiesel.
Biodiesel, especially homebrew or contaminated biodiesel may cause the newest generation of diesel exhaust filters to become clogged with particulates. Some diesel filters, especially the Bluetec filter system sold in Mercedes, upcoming Audis and VW TDI, use a series of filters to catch diesel particulate emissions. At a set interval, the car's computer dramatically raises the exhaust gas temperatures to burn off the particulates and clean the filters. Home made biodiesel may put excessive byproducts and unreacted chemicals into the filters and cause them to become clogged. This is also a problem if you use the older non ultra low sulphur fuel, no longer available in the USA or Europe but still used in some parts of the world. There is still an ongoing debate since the Bluetec filter system is still so new and the urea injection systems are not widely tested with biodiesel. The filter is what gives petrol diesel such low emissions and the irony is that biodiesel is already a low emissions fuel.
Below is a picture of what a quality test pipe might look like. A resonator is welded on the left side to help quiet any droning resonating "booming" noise that many free flow exhausts will make at certain rpm. A louvered resonator causes turbulence and reduces exhaust flow but is quieter than a perforated hole resonator which has little effect on flow but is not as quiet. Remember that loud = tickets and a catalytic converter is the best way to reduce emissions and keep the exhaust on the quiet side. Pictured below is a venturi, this can also help control exhaust resonation.
A common complaint with free flow straight pipe exhausts is exhaust
resonation noise. In fact, many people have it but don't acknowledge it
because they think it's just loud and actually like it. Resonation differs
from loudness because it has a certain boominess, rattling, buzzing, or hollow
vibration sounds at certain RPM. There are many possible causes, but some
ways to get rid of it are installing a venturi along with a resonator at
strategic positions along the exhaust, controlling flutter of the exhaust by
smoothing out sharp corners in the exhaust or downpipe, or slowing the exhaust
velocity by restoring the catalytic converter (contrary to performance increases
but it will make the car better for daily driving).
The downpipe - split and single pipe
A downpipe is the exhaust pipe immediately after the turbo. It could also be called an up-pipe, but due to the configuration of most engines, the exhaust is normally directed down after exiting the turbo. It normally is a single pipe that collects the exhaust from both the turbine output and wastegate output. From the above picture, you can see that there is also a lot of empty room for exhaust gases to become turbulent upon exiting the turbine in an internal wastegate housing. When the wastegate opens, the tumbling exhaust coming out of the wastegate collides with the spinning air exiting the turbine. This scenario is devastating to the goal of smooth airflow. This area of turbulence saps power by not allowing the air around the turbine to be evacuated as smoothly as possible. Note that this is not a problem for a housing without an internal wastegate, such as external wastegate and many diesel VNT turbos. Also note that some turbos have the initial section of downpipe as part of the exhaust housing.
A split downpipe is a downpipe with two separated pipes, one for the turbine exhaust, and one
for the wastegate exhaust. It may have a machined separator for the empty
space between the turbine outlet and wastegate or a section of pipe. By smoothing out the
airflow, it enhances airflow all throughout the rpm range. The two split
pipes then rejoin down the exhaust path. Here are some pictures of split
downpipes. One has a split that is longer than the other. The point of diminishing returns is
about 12"-18" for uninterrupted flow before rejoining the wastegate
piping to the main exhaust flow. The second picture below also has detail of the machined wastegate separator at one end instead
of using a section of pipe to separate the exhaust streams like in the first
picture below .
This last pictured downpipe is also slightly different in that it has an expansion chamber, a chamber where the diameter of the piping expands as you go downstream. A gradual expansion at the turbine outlet via a straight conical diffuser of 7-12° is ideal, depending on factors such as space within the engine bay, exhaust gas velocity, temperature, and volume. Too great or too abrupt of a transition, and you get flow separation and turbulence, reducing flow. Ideally, the best flow would be achieved by a "trumpet" shaped downpipe that exits into an area below the car, but this is obviously not legal or safe as it would be very loud without mufflers and the exhaust fumes would quickly injure or even kill you since the exhaust would surround and maybe leak into the cabin. You want the highest exhaust velocity after the turbine, and while bigger normally equals better, too large of an exhaust will cool the exhaust, reduce it's velocity, and create excess backpressure. A side effect of a more gradual expansion and wastegate pipe is that it sounds much smoother than a pipe which could cause resonation at certain rpm due to the fluttering of the exhaust.
Also note that
these downpipes all have O2 sensor bungs welded in them because they are for
gasoline cars, although exhaust flow theory is the same for diesel cars.
Keep in mind that many diesel turbos, especially the newer diesel turbos do not have a wastegate, they use the variable nozzles (VNT) within the exhaust housing
to control turbo speeds. Without a wastegate, an excellent downpipe would
look the same as the below picture, except without the smaller pipe for the
wastegate. Also note how the welds are ground down on the inside to smooth
out the flow as the pipe diameter gradually increases.
| Lastly, some newer turbos have a divider
already built into the exhaust
housing to be
used with a matching downpipe. Here is an example from a Mitsubishi
Evolution, a high performance turbo gasoline car. It uses two doors and a
divorced exhaust housing matched to the downpipe.
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Many turbodiesel engines feature a turbo technology that is only now being used in gasoline engines known as variable vane, variable geometry, or variable nozzle technology. Gasoline engines exhaust temperatures tend to be higher than similar diesel exhaust gases resulting in damage and short lifespans on the early generation of gasoline variable turbos introduced on mass production gasoline cars in the 1980s. Gasoline engines also require the turbo to be more efficient over a larger range of rpm as compared to a diesel engine. Today, advances in turbo design and metallurgy have made these turbos more reliable on both gasoline and diesel cars, although only one gasoline car, the newest Porsche 911 turbo, uses it.
Variable vane, variable geometry, or variable nozzle technology change the angle which the exhaust pushes against the exhaust turbine. Different types of variable turbos have different ways of accomplishing this with vanes or nozzles. By optimizing the speed and angle of the exhaust moving through the exhaust housing and hitting the turbine, it maximizes the efficiency of the turbo. By keeping the turbo speed higher over a greater range, it produces more low end power with sustained top end with about equal amounts of airflow compared to a traditional turbo. The air is also cooler due to less compression and friction. North American VW turbodiesel engines mk4 (4th generation) and up, starting in 1998 with the New Beetle all use a variable geometry turbocharger.
Click here for an animated picture of a variable geometry turbocharger vane, from a VW TDI with VNT. I didn't put the picture directly on the page because file size is 500kb, if you are on 56k connection please be patient, the picture is pretty cool.
Turbo lag
The period between pushing on the throttle pedal and feeling the rush of acceleration is commonly referred to as lag. Lag is a symptom of the time it takes for the exhaust turbine wheel to overcome its rotational inertia and for the intake impeller to create positive pressure in the intake.
Lag can be reduced by lowering the rotational inertia of the turbine, for example by using lighter parts such as ceramic turbo wheels to allow the spool-up to happen more quickly. Another way to reduce lag is to change the aspect ratio of the turbine by reducing the diameter and increasing the gas-flow path-length. Increasing the upper-deck air pressure and improving the wastegate response helps but there are cost increases and reliability disadvantages as well.
The center housing rotating assembly, or CHRA is the part that contains the bearings that hold the shaft connecting the intake and exhaust wheels, and the coolant and oil lines. Older turbos use bronze journal bearings, a machined bronze cylinder to hold the main shaft connecting the turbo compressor and turbine wheels. Much like a crankshaft bearing, it is lubricated generously by oil from the engine and held in place by a thrust bearing. While pressurized by oil, the journal bearing is floating and spinning on a layer of oil. Some newer turbos use chromium/carbon steel ball bearings to hold the main shaft. The best turbos use ceramic ball bearings that are much more durable than steel ball bearings. Ceramic ball bearings can handle significantly higher safe operating rpm than comparable steel ball bearings.
The advantages of ball bearings include better damping and control over shaft motion. In addition, the opposed angular contact bearing cartridge eliminates the need for a thrust bearing, a common source of damage and oil leaks. Ball bearings also spool faster and harder compared to an identical journal bearing at the same rpm. There is reduced drag on the turbo shaft which increases performance and can be felt. Ball bearings also require much less oil required to provide adequate lubrication than a journal bearing turbo.
| If they receive too much oil, the ball bearings will actually skid in their races, creating wear in one spot, quickly damaging the ball bearings. This lower oil volume also reduces the chance for seal leakage If you exchange your old journal bearing turbo for a new BB turbo and don't change the oil lines, be prepared for smoke due to excess oil leaking out the exhaust side. To the right is a picture of a ball bearing vs. journal bearing oil feed restrictor. The journal bearing oil line is the larger diameter one - quite a large difference! |
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The most problematic part of a turbo is normally the CHRA. The intake
and exhaust housings are just nonmoving cast metal housings. They generally do not
get damaged unless the exhaust side is overheated and cracks, breaks an inlet or
outlet flange, or damage to the
exhaust transmits force to the exhaust housing, cracking it. The turbine blades
generally do not get break or get damaged unless a foreign object falls into the
turbo or air intake. But a worn or damaged CHRA can allow shaft play and also
damage the turbines. Below is a non VNT conventional turbo
disassembled. Note the ball bearing instead of journal bearings and damage
to the compressor wheel. A ball bearing is not rebuildable, the most
reliable way to reuse your old turbo is to reuse the old cast iron housings with
a brand new CHRA and components.
The turbo runaway in a diesel engine
Another problem with the CHRA is that the oil can leak out from worn
seals and cause a runaway. The turbo runaway is a variation of the diesel
engine runaway. Older turbo seals normally use a 270o thrust bearing
on the compressor side that holds the
journal bearing in place. Some newer types use a 360o thrust bearing
that holds the thrust bearing in place even better because they distribute the
load across a wider area, see below for a picture. Note that some VW TDI turbos use 270o
bearings, some use a 360o. The VNT turbos use a 360o
bearing. I wouldn't worry
about the bearings used in the TDI since the difference in wear is marginal. With proper care and
synthetic oil, the thrust bearing can last the life of the turbo.
However, higher boost
pressure and excessive thrust movement (caused by manufacturing issues and worn
bearings) can wear the thrust bearing even more than normal and can leak oil if the seals get too
worn. Both compressor and turbine sides of the turbo can leak oil out the
intake or exhaust sides, respectively. If oil goes out
the exhaust, it will cause black or "blue" smoke and soot and may also shorten the life of
the catalytic converter due to melting or clogging. If oil goes into the intake,
it can cause a more serious problem for diesels: the engine runaway.
In a gasoline engine, oil in the fuel will reduce the octane of the gasoline
and make the engine more likely to detonate. This is bad. In a diesel engine, it can
result in a runaway and destroyed engine, this is also bad. Since your diesel engine will
run off engine oil, it actually increases the rpm by increasing the amount of
fuel consumed (the engine oil). If the rpm rises past a certain point, the
engine will suddenly increase in rpm and draw the engine oil out of the turbo
seals and into the engine. The engine will run faster and faster until it over-speeds and
breaks or runs out of engine oil and seizes, both conditions resulting in total engine
failure and a possible car crash. Once you reach a certain point, even taking your foot off of the
accelerator pedal won't stop it since diesel engines don't have throttles!
The engine will continue to run faster and faster because it only needs air and
fuel to run. Cutting off the diesel fuel won't 100% stop it because it is feeding off
the engine oil.
All mk4 ALH and later cars have anti-shudder valves or throttles that can shut off the air when you turn the ignition key to off. If so equipped, your best way to stop a runaway engine is to shut off the ignition and pull over as soon as practical. You can also put the car in the highest gear, and step firmly on the brakes to slow you down and stall the engine. If you put the car in neutral or go to a lower gear, there will be less resistance on the engine and it will quickly over-speed and fail. Note that this runaway condition can also be caused by a number of other problems such as excessive crankcase pressurization, but since this a turbocharging article, know that a leaking intake turbo seal is among the most common reasons for a runaway engine. Also remember that this cause of runaway engine occurs in diesel engines only, not gasoline engines, and always follow common safety practices! Do not risk getting rear ended on the freeway and personal injury to yourself and others, only pull over as soon as is safe and practical.
The biggest area of concern in the turbo is the oil supply. Insufficient (especially journal bearing turbos) or excess oil (especially in ball bearing turbos) or dirty oil may wear out the bearings, causing wear and shaft play in the turbo. Because of the temperature that turbos can reach, the oil may also break down faster than a comparable non turbo car. Synthetic oil is recommended in turbo applications because they do not break down as quickly as conventional oils. Because the best engine oils for a diesel engine are synthetics, this should be one more reason to switch to synthetic in turbo and diesel applications if you are not already doing so. In addition, since the turbo can get hot when running, an engine idling period of 5-10 seconds once at a complete stop should be enough to let fresh oil circulate to the turbo bearings before engine shutdown. If driving vigorously, a sufficiently long period of up to 1 minute should be enough to let the turbo cool down and receive fresh oil. If the turbo is too hot and does not receive cooler oil upon shutdown, the oil could become burnt and "coking" may occur. Another issue is letting fresh coolant circulate to the CHRA. If you did not see the warning above, I do not recommend rerouting the oil or coolant lines in your turbo unless you know exactly what the consequences are. After engine shut down, the coolant heats and expands in the cartridge if the CHRA is too hot. This creates a natural circulation to drain away the heat and bring in fresh coolant. If you improperly reroute the coolant or oil lines, this could disrupt the natural circulation after shut down.
Use of VW approved engine oils in the TDI is also recommended. The big shift for North American market cars was in 2004 with the introduction of the pumpe duse engine. These engines see very high pressures in the head and should use VW approved engine oil. See 1000q: pumpe duse engine oil and 1000q: non pumpe duse engine oil to see lists of approved oils and reference links to oil manufacturer's websites. Use of the synthetic VW approved oil will help ensure proper lubrication to the turbo.
Some people install a turbo timer to keep the engine idling for a time set by the timer so they can walk away from their car during a cool down period. I do not recommend these products for a number of reasons. First, if you have a manual transmission, you should always put it in first or reverse gear when parking in addition to applying the parking brake, so the convenience of walking away with the car idling is not possible. Also, a turbo timer requires spending money on the timer, cutting wires and introducing an unnecessary failure point. Lastly, for diesel applications, coking is not as common of a problem due to the lower rpm and cooler exhaust gas temperatures of diesel exhaust, and that you should be using synthetic oil anyways which is more resistant to coking. If you are truly concerned about turbo care, just make sure that you drive at medium rpms and low load when the engine is still warming up and just drive sensibly a few minutes before shutting the engine down.
Another component essential to the turbocharging system is the intercooler. As the turbo compresses the air, it heats up - an intercooler lowers the air temps. The ideal gas law states that when all other variables are held constant, if pressure is increased in a system so will temperature. The turbocharger also radiates some heat into the air because it's hot from all of the exhaust gasses passing through the exhaust side of the turbo. The hot under hood air also heats up all of the intake piping (turbo cars have more piping than non turbo cars). The mechanical agitation of the air by the turbo wheel also heats it up a little.
Hotter than ambient air are are some the negatives of turbocharging, the air
gets hotter than what an average nonturbo engine gets. This increases the likelihood
of uncontrolled detonation and engine damage. An intercooler is basically a
heat sink that takes away the heat of the intake charge and cool it as much as possible. Here is a picture of an intercooler in a
Mk4 jetta TDI. The yellow outline
marks the intercooler, the intercooler intake and outlet. The arrow marks the front of the car, where the
cooling ambient air enters from.
The goal of intercooling is to produce the least pressure drop and the most heat transfer to the metal and air or water, whatever the cooling medium is. Any well designed intercooler may have about .5-2.0 psi pressure drop due to pressure losses involved with the process of cooling the air. A good air-air intercooler can cool the air to within 20 degrees of ambient temperature if it has steady airflow to take away the heat. The advantage of a good air-water intercooler is more consistent intake air temperatures since the water (coolant) is not as quickly affected by rapid changes in ambient air temperatures and car speed. An air-air intercooler is preferred for diesels, more details below.
Keep in mind that an intercooler is acting like a heat sink and less like a radiator when boosting. The intercooler gets the hottest after the turbo heats the turbo output air. After absorbing the heat, the intercooler releases the heat into the ambient air or coolant. In a gasoline engine, the engine is operating at vacuum or low boost most of the time. Low boost does not heat the turbo outlet air as much as hard boosting and as a result, doesn't transfer as much heat to the intercooler. In other words, a larger intercooler is not needed unless you need the extra heat sink capability! Most modified gasoline cars would benefit a little from a larger intercooler due to higher than stock boost levels. However, how much it's needed in only lightly modified cars is debatable due to variations between cars, ambient outside temperatures, intended use (street vs. track), desired safety margin and fuel octane, etc.. For example, a large front mount intercooler will cool better than a small intercooler but it may not fit, may be blocked by the bumper, cause overheating problems due to blocking the radiator, etc.. Another issue is that like any other heat sink, after the intercooler absorbs heat, it releases it into ambient air AND the intake air. However, long as the car is in motion, most of the heat is carried away by ambient air.
Because of this, it's best to maximize the intercooler efficiency by leaving it unpainted and keeping the core unobstructed. The VW TDI naturally puts an oily mist on the inside of the intercooler but trying to keep the inside clean is like keeping the oil dipstick clean. Gasoline cars shouldn't have any oil inside the intercooler. Also check for leaves or dirt blocking the face of the intercooler. If you must paint it to help hide the intercooler, use 1-2 passes of radiator paint or even better, a heat shedding coating like Swaintech's "BBE heat emitting coating". Depending on ambient temperature, intercooler size, intake temps, etc., a heat shedding coating can lower intercooler temps by as much as 25o F. You can also spray coolant onto the outside of the intercooler, lowering the temperature of the intake air even more, below ambient air temps. CO2 (compressed carbon dioxide gas), N2O (nitrous), and just regular water all work very well at increasing intercooler effectiveness but only work until your coolant runs out. If you are preparing to race, placing bags of ice on an air-air intercooler or chilling the coolant in a water-air intercooler works well too. Heat coatings won't lower the temps as much as using a coolant but work all the time and don't run out.
A diesel engine has a greater need for an effective heat sink than a comparable gasoline engine. In a diesel engine, turbos are normally smaller compared to a gasoline engine for a number of reasons, for example, the smaller rpm range. They also tend to use higher boost levels than a comparable gasoline engine. I think that even lightly modified VW TDI cars could benefit from more efficient intercooling for maximum peak power. With an air-water intercooler, the more stable temperature is harder to cool because once it's hot, it tends to stay hot longer than an air-air intercooler. An air-air intercooler is also easier to fabricate with less chance for leaks. If there was a small water leak into the intercooler core, it could hydrolock the engine, so for these reasons air-air intercoolers are preferred for diesels.
Keep in mind that the turbo pressure in most cars is regulated by how much pressure seen at the intake manifold, not at the turbo! Some also measure the air temp at or near the manifold. Regardless of intercooler efficiency, pressure at the intake manifold should remain about the same. For example, compare an engine that limits boost to 15 psi at the intake manifold. If you have two turbo setups, one with an efficient intercooler with only 1 psi pressure drop and the other with than an inefficient intercooler with 4 psi pressure drop, the turbo with the efficient intercooler only has to make 16 psi at the turbo whereas the inefficient setup has to make 19 psi at the turbo. The turbo making 19 psi is mechanically more stressed and is creating more heat than the turbo that has to make only 16 psi, assuming that they are both operating in an area of normal operation and efficiency. If the turbo is pushed beyond the normal area of efficiency for the turbo, it will create exponentially greater amounts of heat and pressure. Again, pressure does not equal density, you are still creating the same amount of pressure seen at the intake manifold that regulates the turbo, but the air is less dense and hotter, which creates less engine power and efficiency. To detect an air leak, see the FAQ article 1000q: boost leak testing. Also know that a common issue with the VW TDI is the sudden loss of power known as limp mode. The VW TDI ECU has pressure and temp sensors and if the ECU senses a problem, it cuts power to prevent damage to the turbo and engine, preventing damage the turbo from a possible overspeed. See 1000q: limp mode diagnosis for more details on a suddenl oss of power.
| Here is a thumbnail of an aftermarket
off-the-shelf intercooler next to the stock intercooler, click for a
larger view. Note that it has bar-plate construction instead of the
stock tube-fin.
Further flow improvements |
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Another way to increase the efficiency of your general setup is to improve the pre and post turbo and intercooler piping. This will reduce pumping losses. In the VW TDI, this can be difficult due to the turbo, intercooler, and battery locations. The best piping would be smooth on the inside, have a relatively straight path with gentle mandrel bends, and be as short as possible. The shortest, smoothest pipe routing on a transverse 4 cylinder engine would be from a turbo in the front, with a gentle loop to a front or side mounted intercooler and then a loop back to the intake manifold. This is not possible on the VW TDI due to the rear mounted turbo location but you can still improve the existing piping. When dealing with bends in the pipe routing, use piping that has bends built into it with straight silicone couplers instead of using straight pipes with bent silicone couplers. Silicone couplers tend to collapse at tight spots or bend, reducing the cross sectional area. Even if they aren't bent, there is a gap between the piping and it disturbs airflow more than necessary.
Some people also think that larger piping or a larger intercooler increases lag. This is true because it takes longer to fill and pressurize the larger piping and intercooler. However, the difference is extremely small. Everything else being equal, the difference in turbo response will not be greater than 1/10th of a second unless you are going from no intercooler to a huge intercooler. In addition, if the intercooler was the bottleneck in the system, the loss of throttle response is not even ANY factor because the gain of your other upgrades is greater than the extra time it takes to pressurize the greater volume of the piping. Exhaust backpressure and turbo size has a far greater factor in throttle and turbo response than intake piping, so start with the exhaust first. Intake piping makes a difference but on the TDI the priority is low compared to a turbo, injectors, exhaust, and chip.
The one thing to be wary of with the VW TDI is using high flow air filters. The mass air flow sensors (MAF or MAS) on the Mk4+ body seem to be sensitive to the additional dust and debris that a high flow air filter, especially aftermarket oiled cotton filters let into the intake tract. This can lead to a failed MAF, see 1000q: MAF FAQ for more details. The stock air filter and housing was overbuilt and uses the same part as the 240 horsepower Golf R32, so there is little-no gain by switching to a high flow air filter anyways. Lastly, if they do not have a cold air intake snorkel, they will draw in hot underhood air and can actually reduce power.
To make more power, you do not want only more boost. Boost is only a measure of intake pressure. Everything else being equal and within reasonable limits for the setup, more boost makes more power only if the turbo is operating in an efficient range of performance. If you were to increase the boost to the point where the turbo is trying to move too much air, it actually reduces performance. This is because past a certain point of diminishing returns, a turbo is basically blowing hot air. This hot air creates intake air pressure and more boost because boost=measure of pressure. Again, back to the idea of volumetric efficiency, you want the maximum mass of air for the engine. Unless the air can be cooled sufficiently by the intercooler, the density of the air can be less than it would have been at a lower boost level. Modern cars will compensate by using their computer and sensors to adjust the timing. In other words, in this case more boost has reduced the amount of power!
It is a common mistake to equate boost, or intake pressure, with denser air. Referring to the ideal gas law as a simplified example, PV=NrT, if you raise pressure, temperature would also have to increase, assuming the other variables remain the same. Also keep in mind the above paragraph about operating a turbo outside of it's islands of efficiency. People often get so caught up with learning about the quick power gains from more boost pressure that they forget that the ultimate goal of turbocharging is increasing air density, not pressure. In designing the engine as a whole system, you want to create the same amount amount of power with the least amount of boost to reduce stresses on the engine and turbo and to keep air moving at an optimum speed throughout the intake tract. Some ways of doing is is to change camshafts to allow more air into the combustion cylinders, changing the combustion cylinders by boring, making the diameter of the cylinders wider, or stroking the engine and making the length of the cylinder travel longer. If the intake piping is too long and bent, this adds air resistance and pressure not associated with creating power. Shortening the intake piping, making the transitions between piping as smooth as possible, and and routing the piping as straight as possible will reduce the amount of required pressure to produce a certain amount of power, increasing reliability and efficiency. A rough rule of thumb is that each 90o bend in pipe adds as much resistance to airflow as 25 ft of straight piping. Of course, this depends highly on diameter, smoothness of bend, etc., but this should reinforce the idea that short straight piping is best for flow.
However, adding a larger turbo or turbo components does not mean the engine will make more power. The turbo is regulated by sensors, computer feedback, and solenoids set to control the boost at a certain pressure. The source of where the computer measures the pressure from is normally the intake manifold or some spot right before the intake manifold. Everything else being equal (load, rpm, etc), one large turbo and one small turbo will flow identical pressures of air at a given psi but remember that psi is just a measure of pressure, density is what matters and is what makes power! In other words, to flow a certain amount of air, where a smaller turbo may have already passed it's maximum efficiency and is blowing mostly hot expanded air, a larger turbo will still be operating in its area of maximum efficiency and is moving cooler air at the same psi. Again, assuming that one turbo is stressed too much and the other is in it's peak efficiency, they are both giving the same psi but not the same density of air. I will state it a third time for fun - 20 psi at 2500 rpm is always 20 psi at 2500 rpm, the difference between an efficient turbo and a turbo blowing hot air is the temperature of the air coming out of the turbo which affects density. There are also other factors that effect this such as the size of the turbo housings, backpressure, etc. You want to select a turbo which balances responsiveness with moving your desired mass of air. Again, this is a consumer level article so do a lot more research before crunching the numbers. Garrett turbo's website has some more info on calculating airflow there. Turbo manufacturers often have published graphs which show where the turbo is operating most efficiently.
Also remember that the control systems and sensors for the turbo are normally located after the intercooler in the intake manifold or piping. If there is an air leak in this section of piping, the turbo has to work even harder to make up for the lost and provide the same reading to the sensors. Because it has to work harder, it may even be operating outside of it's optimum efficiency range and creating excess heat compared to if there was no boost leak. See 1000q: boost and vacuum leak testing for a simple way to test for boost leaks.
All modern diesel passenger car and truck engines are turbocharged, but some readers may be wondering if you could turbocharge an older nonturbo diesel or nonturbo gasoline car. The short answer is yes! The long answer is that for most cars, it is such a large project, requiring such a large amount of custom fabrication, custom tuning, uncertain results, and lots of money, that the same amount of money could go towards buying another car that is already turbocharged and would not require such a large amount of effort and risk. In other words, if you have to ask if it's possible, the project is way over your head!
Some popular nonturbo cars have kits that have already been tried by many other
people. In these cases, the risks are minimal because there are other
people who can give you advice. But remember that it is often easier just
to buy another car that is already turbocharged. The time that you spend
on the project and then fixing all the problems that show up would be better
spent working at a job so you can make more money and just buy the other car. For example, below is
an advertisement by Porsche showing the upgraded parts between a 944 and 944
turbo. See all the extra parts that wouldn't be on your car if you just
added a turbo and parts to make the turbo work? Ultimately, it is your car,
your money, and your responsibility, so FYI, here are some more cautions if you
want to continue.
The biggest problem is that a nonturbo car was not engineered for turbocharging and that people generally do not know the full consequences of turbocharging. For example, the transmission may only be designed to hold the amount of power from the nonturbo engine. If you were to increase the engine's power, the transmission could be more easily worn out and break. The clutch may not hold the amount of increased power, so you would have to replace the clutch and pressure plate with one that could withstand more power. But then, the clutch hydraulic system may not be able to handle the increased pressure required to actuate the clutch so you might have to change the components or rebuild them. The clutch pedal's metal may be designed for light pressure, and having high clutch pedal pressure could deform or wear out the clutch pedal levers and bushings. Some cars are susceptible to thrust bearing wear on the crankshaft from a stronger clutch pressure plate. The intake tract, including the various throttle gaskets and seals, piping, and vacuum lines may not be designed for positive pressure. Putting these components under boost can pop them off or cause small leaks that only show up under pressure and blow various seals. The engineers who built your car can't overbuild everything that they want to, otherwise your car would be as heavy as a tank and cost $100,000. So even if "x" is reliable at higher power levels, "y" breaks. Again, each car model is different.
The compression ratio is also higher in nonturbo cars. This is true for both diesel and gasoline cars. Because of the higher compression ratio, it limits the amount of pressure and boost you can put into it. This pressure also creates the need for stronger pistons. The pistons in turbo cars also tend to have oil squirters that direct oil at the inside top of the piston which help carry away the additional heat of combustion.
Lastly, the typical nonturbo engines is not built as robustly as turbo
engines. This includes the seals and gaskets, the moving metal
parts of the engine, the bearings, and the engine block itself. For the
same reason that you can't take a gasoline engine and turn it into a diesel
engine (and expect it to last a long time), most nonturbo engines are not
designed to stand up to the stresses of turbocharging. For example,
picured below is a girdle or cage around the crankshaft bearings on a turbo car.
It depends on the car and the turbo kit, but if you want to turbocharge a nonturbo car and maintain
the same reliability, your best bet is an engine rebuild
with more robust components and a change of the compression ratio. You can
add a turbo to your exiting engine but it will not be as durable and you will
not get the same results without an engine rebuild with different
components. Here is a picture of what can happen if you try to
boost too much on an engine not originally engineered for turbocharging. Of
course, this can also happen if you boost too much on a turbo engine, but turbo
engines are normally engineered to be more resistant to abuse. Hint: the engine
rod is
supposed to be straight.
| If you think I am against turbocharging your own car, you are right. This section is written for the person who asks, "I saw a turbo kit on ebay that said it supports 500 horsepower and costs only $500". Even worse, "my ebay electric turbocharger is even better than your kit". Because most people run out of money or don't know how to do the job right, pictured right is what I think of when I see a DIY job . Oddly enough, the CRX is a car which a lot of people have successfully turbocharged with great results! There are many successful turbocharging jobs, but it requires either a lot of cash to pay someone else to do it, or a certain level of turbo and mechanical knowledge and experience. |  |
Port matching is a simple technique that improves flow on components that are not perfectly matched. This basically means that all of the casting flaws or abrupt transitions between components should be matched to it's gasket and whatever else it's mated to. The rough edges inside the engine that disrupt airflow, coolant flow, etc, are normally due to casting flaws and casting flash or gaskets that are slightly off. Due to the difficulty of casting metal, there may even be some areas that can be removed to improve the efficiency of the engine. But keep in mind that some components may have a ridge or step to improve air swirl or for anti-reversion, so make sure you identify what is casting flash and what is intentional. This tip can be applied to cylinder heads, intake manifolds, exhaust manifolds, turbo housings, wastegate ports, exhaust piping, almost any part of an engine that requires good flow. Keep in mind that this is NOT the same as porting heads! There are 1000 fine points to learn about porting, this is only a tip to fix casting flaws and improve fitment.
Keep in mind that some gaskets
act as restrictors! Some engines have head gaskets that restrict the
return line of coolant or oil to regulate and restrict the flow of liquid to
ensure uniform pressure and cooling, as shown below. You can see
how the coolant has stained the head gasket around the orifice that acts as a
restrictor. Always know exactly which port or gasket you are modifying!
Sequential twin turbos vs. symmetrical twin turbos vs. single turbo
Some cars have twin turbos instead of single turbos and some cars that came
from the factory with twin turbos are aftermarket converted to single
turbos. The main types of twin turbos are parallel/symmetrical twin
turbos, or asymmetrical sequential twin turbos. Parallell/symmetrical twin
turbos are found mostly on V-configured engines such as the 300zx twin turbo or
Audi S4 twin turbo. They are most appropriate for
V configured engines because each side of the V engine feeds one turbo and all
the piping is kept equal. Both turbos should be equally sized to keep
the engine balanced. Factory setups that use this configuration generally provide more low end power
because the twin turbos will generally be smaller than one large turbo but a V
engine can also produce more torque, so it really depends on the engine and
design. Symmetrical twin turbos can also be found on the BMW
335i inline twin turbo gasoline engine but in a different alignment. Below
is a cutaway picture of the 335i engine. Note that each turbo is fed from 3
cylinders only and lead into a shared outlet pipe before the intercooler
(pointing to the right).
Inline engines can also be fitted with a another type turbo setup. A twin or asymmetrical sequential
configuration is used in the supra or RX-7 twin turbo gasoline cars or the BMW 535d twin turbo
diesel. Sequential twin turbos are most
suitable for an inline engine because the exhaust stream is coming out only one side
and the piping is simple and short. If you tried to use sequential twin
turbos in a V engine, the piping would have to be routed all the way around the
engine, creating piping and space problems. Asymmetrical tubos use one smaller turbo for lower
rpms and one larger turbo for
higher rpms. The exhaust gasses are normally diverted to the smaller turbo until
a certain air flow is achieved, then the exhaust gasses are diverted to the
larger turbo to provide top end power. Sometimes the gasses go to both
turbos at the same time. Mercedes Benz and Audi are working on asymmetrical
twin turbo diesels that use one small and one large turbo. Below are some
diagrams of their systems.
A single turbo is most suited to inline engines instead of V engines mainly because of packaging and exhaust routing obstacles. Some turbocharged Saab gasoline cars use inefficient exhaust routing on a single turbo V engine that placed the turbo off to one side of the engine. They experimented with placing the turbo in the middle of the V engine on the top, but this actually melted the paint due to the red hot exhaust. Mercedes Benz's latest Bluetec turbodiesel engine do place the turbo near the top of the engine, but they must have found a solution for excess heat control.
Pictured below are top and bottom pictures from the sequential turbo on a
Mazda rx-7. It has one large and one small turbo connected by a shared exhaust
manifold in the middle. Although the turbos might not look small/large,
they are different sizes and the difference is very noticeable when the larger
turbo kicks in.
These operate in almost the same way as a turbo but instead of being driven by exhaust gases, they are driven by a belt or shaft, normally by the front serpentine belt along with the rest of the engine accessories like the AC compressor, alternator, etc. It is basically the compressor side of a turbo attached to a pulley and clutch instead of an exhaust side. It is geared to increase the rpm of it's drive belt to much higher speeds. They tend to not be as efficient because they drain net output energy from the engine to achieve an overall gain in power instead of using the exhaust gases. They also deliver power in a more linear fashion instead of a turbocharger's steeper "boost" curve. Because they are belt driven instead of exhaust driven, the exhaust theory for turbo charged cars does not apply.
Are centrifugal superchargers better than turbos? There is no short answer, the long answer is that it depends on what car you are using it on, packaging demands, power goals, etc. Ignoring cost and packaging, everything else being equal, turbos are better. I remember seeing a dyno chart which showed a Chevy Cobalt SS ( a stock supercharged car) which had the supercharger removed and an aftermarket turbo kit added. The turbo car had a much better powerband everywhere.
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