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Turbo FAQ, aftermarket exhausts, and answers to common turbo questions - page 1
Introduction
This article contains basic and advanced consumer-level information
about turbos. Some of
the information is applicable to any turbo car and some is VW TDI specific but
everyone should find some new information and views. To keep it brief, more advanced topics
like interpreting
turbo flow graphs or wheel
trim and component selection are not included. Here is a
table of contents for the most frequently asked questions about turbos. If
you are looking for a table of stock and upgraded turbos for the VW TDI and
their specifications, see 1000q:
turbo upgrade chart.
Table of contents
Page 1
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
Page 2
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/gasket 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 a fuel for 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 about 85% nitromethane and 15% methane in engines with a relatively small displacement but with power measured in the thousands of horsepower (hp). Since the fuel is over 50% oxygen, its concentration of energy possible from a given space, its volumetric efficiency, is much greater than an engine burning 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 just 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 difficult to do properly, read more details in 1000 answered questions: turbocharging your own car.
The ultimate goal of turbocharging is to increase air density to make more oxygen available to burn. The energy from combustion, the burning of air and fuel, is what pushes the piston down, creating energy. This increase in air density, or boost, is 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.
Diesel engines are either under boost or ambient atmospheric air pressure because they do not use throttles. Gasoline engines can be under either boost or vacuum (or ambient when the car is off). Here are some examples of boost vs. vacuum and ambient. 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 a gas engine is running at idle, the gauge will normally show a negative reading, for example -7. This means that the intake is under 7 psi 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. Again, 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 air 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 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 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 for more details. In any case, direct injection is superior to port injection used in older diesels and most gasoline cars. Which burns faster: loose thin shredded paper or thick paper tightly wadded into a ball? The higher pressure of direct injection results in finer fuel atomization and allows higher engine compression and higher boost in turbo engines. These also result in cleaner emissions because of a more thorough fuel burn and better fuel economy. When gasoline is injected under very high pressure into the combustion cylinders, it immediately vaporizes and is ignited by the spark plug so fast that it helps prevent uncontrolled detonation of the fuel.
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 very smoky at the rich end. This is because a diesel is throttled by fuel, not by a throttle plate. A turbo gasoline engine's AF is about 13:1 to 11:1 under heavy boost, whereas a nonturbo engine should be at roughly 14:1. A turbo car's computer will change fuel delivery under boost by increasing fueling and making the AF richer. 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. The difference could be from faulty sensors or an air leak that lets out air that has already been measured by the sensors See 1000q: boost and vacuum leak checking for a simple procedure to test for boost leaks.
If the sensors see something wrong or detonation is detected (gas cars), gasoline turbo cars' computer switch to a safety map that retards ignition timing and reduces boost. This prevents any further damage or keeps the condition from getting worse. In a TDI, the car goes into limp mode if it sees something wrong. Two signs that you are in limp mode are: the car feels like it suddenly hit a strong headwind and it is reset by turning off the car and restarting the engine. See 1000q: limp mode troubleshooting for solutions.
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 how the injectors fit into the engine, here is the same
engine with more components shown. 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. Modern TDI turbos even have a one piece exhaust manifold and turbine housing. Earlier and many aftermarket TDI turbos use a separate exhaust manifold and turbo turbine housing.
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, there is natural convection circulation in the turbo's oil and coolant lines from the turbo cooling off. Rerouting these lines incorrectly can reduce this natural circulation, possibly causing long term damage.
| In most cases, the CHRA is fed from the
same oil supply as the engine. Some turbos, like the VW TDI turbos, are
cooled by engine oil only. Most gasoline turbos are cooled by
separate engine oil and water coolant lines.
The turbo pictured at right is a ball bearing turbo and had some damage to the wheels.
|
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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. No modern gasoline car uses this type of turbo except the Porsche 911 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 a gasoline engine, probably causing severe damage.
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.
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 part of a whole system. If the exhaust housing is small but its turbine is large, the airflow will get choked. If the exhaust housing is large but the turbine is 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. Like a water funnel, no matter how much water you put into the top, 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 using a combination of the smallest turbo, the least turbo boost, and the most efficient intercooler, will all reduce engine stresses and maximize engine response.
For a chart of stock and upgraded turbos for the TDI, see 1000q: turbo upgrade chart.
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. Modified camshafts are more applicable to gasoline engines.
An important term to know when talking about turbo housings is it's aspect ratio (AR). It's not important to know for a non modified car but it's still useful to know what an AR is. AR is the ratio of the area of the cone to radius from the center hub. 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 at 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. It's hard to make a direct comparison since engine displacement and rpm also effect turbo spool up, but 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.
Again, 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. Some VW turbos are called K03, K04, etc., this is only a general spec since there were many different K03 and K04 turbos and most of them are not suitable for a TDI. In other words, don't buy a gasser VW turbo and bolt it onto your diesel because it won't work well.
 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. You may also hear of twin scroll turbos. This is an exhaust housing divided into two separate paths to the turbine wheel. This is used with a divided exhaust manifold to separate the exhaust pulses from the cylinders and maximize performance. |
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Internal vs. External wastegate
The exhaust housing may also house an internal wastegate. An internal wastegate is a passage 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 upstream of the turbo to vent excess gasses. You can also see how the air flow out the external wastegate turbo matches the shape of the exhaust pipe, the internal wastegate has an empty spot where air turbulence can form. Some newer turbos have built in dividers and you can also make or buy an exhaust pipe with a divider to improve exhaust flow. This is in the 3rd picture and expanded on in the "split downpipe" section.
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 costs more in money and work since
you have to buy and route a separate wastegate. This extra exhaust pipe
routing takes up more
space. A good compromise between an internal wastegate and
external wastegate is a split downpipe, see below for more details on downpipes.
In the intake tract there is also a bypass valve or diverter valve that redirects boost away from the engine and back into the intake when you close the throttle. A blow off valve is similar but instead releases boost into the atmosphere. VW TDI do not have these and can't use them because they don't have throttles.
Many passenger car diesel turbos do not have a wastegate. VW TDI turbos, mk4 generation and newer, use a VNT actuator in the same spot as the wastegate actuator on conventional turbos. Once of it's functions is like a wastegate because it limits turbo speed by redirecting the exhaust gases at the turbine wheel instead of dumping gasses out through the wastegate. The other main purpose is to maximize exhaust energy through actuating the VNT vanes. It also uses pressure to actuate instead of vacuum. See the below section and videos for a detailed explanation. 1996-1999 3rd generation (mk3) VW TDI turbos use a conventional turbo with an internal wastegate, all later generations used a VNT turbo. 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. It can also freeze in one position, causing low power. If you have constant low power, see 1000q: constant low power.
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.
How much power is released by putting a straight pipe exhaust on a TDI? First, remember that total power is the area under a power curve, not just peak power. You may not gain much peak power with only an exhaust change but the total amount of power will increase. Also remember that an exhaust is a basic supporting mod for any future modifications such as a chip, larger turbos, fuel nozzles, etc. A TDI diesel is throttled by fuel and uses a relatively small turbo with computer controlled fueling. Peak power may not go up much but combined with chip tuning, it should increase response and area under the power curve, and let any further mods reach their full potential.
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, sharp 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. 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, vibration, the weight of
the turbo, 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 showing equal length exhaust runners. Since this article was
written, vendors have come out with aftermarket manifolds for the TDI.
Unless you have access to a high quality fabricator nearby, I would contact the
vendors directly to see what options they have. Here are direct links to
compare cast manifolds from from JSperformance
or Kermatdi. Each appears slightly different in coatings or other
modifications so contact the vendor directly to see what they offer. 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 assuming that it is not an integrated manifold housing and 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. With a turbo engine, 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
quantitative way to know how much. Even two otherwise 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 an OEM
parts supplier making thousands of factory parts. All you can know for sure is that it
should be an
improvement over your OEM exhaust as long as the replacement parts are quality
pieces. One example is the new Hyundai Genesis 2.0T exhaust. There
is a big crimp in the exhaust for support clearance, pictured below. Replacing
this piece would result in a small power and response gain with no other
modification. Because it's between 2 catalytic converters, any gain in
flow would be small but it adds up.
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 90% efficient, you might end up spending $$$. As a result, work with your budget to reach your realistic power goals. 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 90% efficient exhaust? If all the parts on your car were one level better, it might otherwise sell for twice as much. If they put all 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. 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 car setups, 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, the effect of these improvements will be lessened. 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.
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. VW diesels did not use an O2 sensor in the exhaust except 2004-2006 pumpe cars and newer TDI. If removed and not worked around with a chip or resistor, it sets 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 a greater pressure differential across the turbo and let it work "easier". 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 with test pipes.
Another factor is that while the catalytic converters act as a restriction in exhaust flow, they do 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 in structure and allows exhaust to flow through it.
All in all, especially for diesels and daily drivers, 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 than making custom piping. It's almost impossible for a car to pass emissions testing without catalytic converters. 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 power and turbo response. Anyways, chip tuning is a bigger factor in throttle response in a TDI because of the electronic throttle and fueling. 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 or don't know what a real sports car is like. In the end, it's your car. 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. Up to 5% biodiesel is allowed by the TDI warranty. Some diesel filters, especially the Bluetec filter system sold in Mercedes, upcoming Audis, and 2009 and newer VW TDI with US emissions, use a series of filters to catch diesel particulate emissions. At about 400-500 miles or when exhaust backpressure exceeds a set value, the car's computer dramatically raises the exhaust gas temperatures with post combustion injection at the cylinder to burn off the particulates and clean the filters. Hard and hot runs will reduce regen cycles by naturally increasing the burn off and short trips/cold starts will increase the regen cycles. Biodiesel could potentially cause excess regen or filter clogging. Long term experiences with real world TDI drivers and homebrew are not yet known. Homebrew biodiesel may put excessive byproducts and unreacted chemicals into the filters and cause them to clog. 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, like Mexico. 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. Some curves are necessary due to packaging but it should be relatively straight with gradual curves. 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.
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 or actually like it. Some people like
loud neon green paint jobs too, but at least bystanders can look away. Resonation differs
from loudness because it has a certain boominess, rattling, buzzing, or hollow
vibration sound 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 installing a catalytic converter (also makes overall sound quieter).
Pictured below is a venturi - splicing it into the exaust at the correct spot
can reduce resonation noise.
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's normally a single pipe that collects the exhaust from both the turbine output and wastegate output. From the above pictures, 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 area of turbulence saps power because the air downstream of the turbine isn't moving smoothly and as fast as it could be. This problem is not a factor for housings without an internal wastegate, like external wastegate turbos and VNT turbos. Also note that some turbos have the initial section of downpipe as part of the exhaust housing.
An example of a horrible downpipe is the mk3 TDI's piece. The wastegate
exhaust flow hits a solid plate and crashes 90o into the exhaust stream leaving
the turbine. Since TDI turbos are small and spool up quickly, the
wastegate opens pretty early and causes turbulence for much of the rpm
range.
Another difference between your TDI downpipe and gasoline downpipe is that your TDI downpipe is just a pipe while gasoline car downpipes have a small catalytic converter immediately downstream of the turbo because of emissions. The cast iron manifold and turbo absorb heat and can quadruple the time for the catalytic converter to heat up and start cleaning emissions. 90% of a modern car's emissions are during cold start and the small catalytic converter is needed to take care of these emissions. While removing it is illegal and will make your car's emissions much worse, removal will make a big difference in how the turbo spools up.
Since this article was written, the 2009 TDI introduced regenerating exhaust filters for US emissions. Earlier TDI had no emissions sensors at all. Pumpe duse TDI used an O2 sensor for EGR metering only. 2009 and newer use sensors and a valve that produces backpressure for the EGR system. When the exhaust filter is near full, the car's computer uses post combustion injection to raise the exhaust gas temperatures and heat up the filters. This burns off the build up in the exhaust filter and lets the car meet emissions without using a wet system like urea injection. Hard driving will reduce regen cycles since it naturally burns off some of the build up, short trips and cold starts increase regen cycles. V6 VW/Audi TDI, BMW, and Mercedes all use a wet urea system for emissions.
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 downpipe pictured 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 about 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 because of exhaust noise and the exhaust fumes that would surround and leak into the cabin. You want the highest exhaust velocity after the turbine, and while bigger normally equals better, too large of an exhaust or bad routing could cool the exhaust, reduce its velocity, and create excess backpressure. A side effect of a more gradual expansion and wastegate pipe is that it sounds much smoother than a some pipes which have resonation at certain rpm due to the fluttering of the exhaust.
These downpipes all have O2 sensor bungs welded in them because they are for
gasoline cars, but the same ideas apply to diesel cars. Keep in mind that many diesel
turbos and all newer TDI do not have wastegates. They use variable nozzles
(VNT) within the exhaust housing
to control turbo speeds. Without a wastegate, an excellent downpipe would
be a straight tube or trumpet like the below picture except without the smaller
wastegate pipe. 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 wastegate doors and a
divorced exhaust housing matched to the downpipe.
Some exhaust housings don't fully separate the exhaust and wastegate streams and leave a small gap. This is fine because it still separates most of the air and allows some spillover. |
|
Many turbodiesel engines feature variable vane, variable geometry, or variable nozzle technology that is only now being used in gasoline engines. A gasoline engine's sustained exhaust temperatures are higher than in a diesel, which resulted in damage and short lifespans for early gasoline variable turbos during 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, currently uses it.
Variable vane, variable geometry, or variable nozzle technology change the angle which the exhaust pushes against the exhaust turbine. This greatly reduces lag while keeping top end power. It combines the fast spool of a small turbo with the flow capacity of a larger turbo. 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. This VW turbodiesel engines mk4 (4th generation) and newer, starting in 1998 with the New Beetle all use a variable geometry turbocharger.
Below are 2 youtube movies showing how it works. There's a vacuum can
which moves a lever in the exhaust side of the turbo hosing. Vacuum is
being applied to the can, not pressure.
The lever moves a ring and the ring moves the vanes. These vanes change
the angle and speed of exhaust hitting the turbine wheel.
Here is another animated picture of a variable geometry turbocharger vane, from a VW TDI with VNT. I didn't put the picture directly on this page because file size is 500kb, if you are on 56k connection please be patient - it shows the same thing as in the above movies.
Here is an animation showing VNT turbo speed vs angle
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 boost. Just remember that although it changes the feeling of the power curve, a turbo car usually makes more power over every part of the power curve compared to an identical non turbo car's engine.
Lag can be reduced by lowering the rotational inertia of the turbine or by use of ball bearings. Manufacturers may use lighter parts such as ceramic turbo wheels to allow faster spool-up. 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. The best way to reduce lag in a VW TDI is through the chip tuning and reducing pumping losses through improved intake and exhaust piping. Moving from a VNT turbo to an aftermarket non VNT will increase lag since the non VNT TDI turbos are all large compared to the VNT turbos. Click 1000q: TDI turbo upgrade chart to see what's available.
The center housing rotating assembly (CHRA) is the center section that contains the bearings which hold the main 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. 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 fastest 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 to provide adequate lubrication than a journal bearing turbo. This lower oil volume also reduces the chance for seal leakage.
| But if they receive too much oil, ball bearings will actually skid in their races, creating wear in one spot which damages the bearings. 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 significant difference! |
|
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 and cracks it. The turbine blades
generally do not get break unless a foreign object falls into the
turbo or air intake. But a worn or damaged CHRA can allow shaft play and
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 engine. The turbo runaway is a variation of the diesel
engine runaway. Older turbos 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. Some older 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, excessive thrust movement and pressures (caused by manufacturing issues and worn
bearings) can cause excessive wear and play and can let oil leak out. Both compressor and turbine sides of the turbo can
respectively leak oil out the
intake or exhaust sides. If oil goes out
the exhaust, it will cause black or "blue" smoke and soot and may shorten the life of
the catalytic converter due to melting or clogging. If too much oil goes into the intake,
it can cause a more serious problem for turbo diesels: the engine runaway.
In a gasoline turbo engine, oil in the fuel effectively reduces the octane of the
fuel
and makes the engine more likely to detonate. In a diesel engine, it can
result in a runaway engine. Both conditions can result in damaged or
destroyed engines. Since your diesel engine will
run off engine oil, it actually increases the rpm by increasing the amount of
fuel consumed (the engine oil). This is a diesel runaway. The line
between a leaky turbo and an engine runaway is when the
engine suddenly increases in rpm and draws the engine oil out of the turbo
seals and feeds off that oil, raising the rpm, drawing even more oil out. 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. It feels like someone just stomped on
the gas pedal. 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. An engine runaway from eating oil occurs in diesel
engines only since gasoline engines can't run off oil.
A runaway engine can also be caused by a number of other problems such as excessive crankcase pressurization - older VW diesels had other conditions that could cause a runaway. Because this a turbocharging article focusing on modern TDI, a leaking intake turbo seal is among the most common reasons for a runaway engine on these modern engines. Although this is a pretty dramatic failure that could wreck the entire engine, this is a rare failure. It's normal to have a mist of oil on the inside of the intake piping due to the EGR and CCV system and because diesels have a lot of piston blowby. A little oil pooled in the bottom of the intercooler is also normal. A lot more TDI engines are ruined by faulty or neglected timing belt changes than runaway engines. Also remember that like most cars, the brakes in the TDI can overpower the engine.
If this failure ever does happen to you, first and foremost, concentrate on safe driving and keep the car under control. If you feel the engine runaway, don't risk getting rear ended on the freeway and personal injury to yourself and others, pull over only as soon as is safe and practical. It's not worth getting into an accident to save the engine.
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 first step to stop a runaway engine is to shut off the ignition key and pull over as soon as practical and safe. To test this idea, if you have a 98-03 engine, manually close the valve with the car idling. Make sure you actuate the valve all the way. The engine should shut off. If it doesn't, there is an leak downstream of the valve or the valve isn't fully closing. You won't hurt anything by moving the valve manually to choke the engine. 1998-2003 engines use a vacuum operated valve, 2004-2006 engines use a more robust valve that may be better at stopping an engine runaway. Although the valve can't stop a very strong runaway engine, I would still leave the valve in place since every little bit helps. An advanced technique is to shut off the ignition key, cycle back to on, then shut it off again. The valve only closes for a second before it opens again. Cycling the key will close the valve as soon as it opens again. You must practice these techniques under safe controlled conditions before attempting it in the real world!
Also remember that the brakes are vacuum boosted and that you will only have power assist on the brakes for about 2 full pumps of the brakes. If you lose the vacuum boost on the brakes, it becomes harder to step on the brakes but they are still working. You can test how the brakes feel without vacuum boost. With no traffic or people around, find a small hill that you can roll down safely. In neutral gear, shut off the engine but quickly turn the key back to "on" so that you can still steer the car. (This is for cars up to 2005 only, all 2005.5 mk5 and newer use electronic steering and I don't know how they react). Step on the brakes. You'll find that the first pump and maybe the second pump feels normal. After that the brake pedal will get much harder to press but you'll find that the brakes can still stop the car.
Another way to stop the car (manual transmission) is to quickly shift into the highest gear and firmly step 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. If you have an automatic, firmly step on the brakes to slow you down and try to stall the engine. Again, only stop when it's safe and practical, it's not worth getting into an accident to save the engine.
Once the engine is stopped after a runaway, do not start it again. Have it towed to a diesel mechanic and explain that the engine had a diesel runaway. If you stopped it successfully, you should remove the piping around the intercooler. A little oil is normal but a lot could be a symptom of a runaway. You should do further diagnosis to make sure where the oil is coming from. If you let the engine runaway for a while and it stopped on its own, it's likely that something was damaged. It either sucked enough oil that the engine seized from lack of lubrication or the engine internals were damaged from hydrolock. Further diagnosis is needed, don't try starting the engine again just to see if it starts. Although a runaway diesel engine is rare, it does happen so be prepared.
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FAQ page 2
Do you have information or corrections that should be added to this article? Would you like to know more information? Post your comments in the myturbodiesel.com forums Also see 1000q: turbo upgrade chart for TDI.
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