Turbo FAQ, aftermarket exhausts, and answers to common turbo questions - page 1


This FAQ describes information on turbos and performance with a focus on VW TDI turbos, Audi TDI turbos, and VNT turbos.

Some information is applicable to any turbo car and some is VW and Audi TDI specific but everyone should find something interesting.  If you're looking for a table of stock and upgraded turbos for VW -Audi 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 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

Turbocharger basics

Let's start with the basics: a turbocharger is an engine exhaust gas driven turbine that compresses the engine intake air. This increases the amount of air the engine can consume and increases volumetric efficiency. 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 burned in the engine.  This lets the engine make 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 car manufacturers just don't 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 engine pistons 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.  Ambient air pressure at sea level is about 14.7 (about 1 bar) under standard conditions but boost in North America is expressed as the difference from ambient.  In other words, at 10 psi of boost, the car's air intake is seeing 24.7 psi total pressure but you just say 10 psi.

Diesel engines are normally under boost or ambient atmospheric air pressure because they do not use throttles for controlling engine rpm.  The newer TDI do use an air throttle for EGR gas metering but it has nothing to do with directly controlling engine rpm.  See 1000q: EGR FAQ and 1000q: DPF and Adblue fluid FAQ for more details on the newer systems.  Non turbo gasoline engines have a vacuum when running because they suck air in while they run.  Turbo gasoline engines can be under either boost or vacuum depending on how much the turbo is boosting.  

Here are some examples of boost vs. vacuum vs. 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.  The boost gauge sensor is always downstream of the turbo and in the intake path.

Turbocharging in diesel engines and direct injection

Modern passenger car diesels are all turbocharged for a few reasons.  Diesel engines are naturally very robust because of the characteristics of the diesel cycle and high compression ratio, so they naturally have a stronger design to withstand the additional pressures of turbocharging.  They also use a smaller rpm range vs. a gasoline engine so the turbo doesn't have to perform over the wider rpm range of a gasoline engine. 

Turbos also maximize the benefit from direct fuel 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 TDI diesel, please see 1000q: direct injection, pumpe duse, and common rail for more details.  Older diesels and most modern gasoline cars use port injection where the fuel is injected in the intake manifold before the combustion chamber.  Direct injection means the fuel is injected directly into the engine cylinder at the moment of ignition.  The higher pressure also helps: which burns faster: finely shredded paper or paper tightly wadded into a ball?  The higher pressure produces finer fuel atomization and also allows higher engine compression and higher boost in turbo engines.  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.  Greater control over the moment of injection also results in cleaner emissions because of a more thorough fuel burn, which increases fuel economy.

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 air.  Diesels normally run ultra lean which is one of the reasons why they have great fuel economy.  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 power is restored 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

Basic parts of a turbocharger

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.

The exhaust housing is normally cast iron and the intake hosing is normally aluminum.  Some exciting developments in turbo technology include stainless steel investment cast (like lost wax casting) exhaust housings which give smoother internal flow, reduce thermal inertia, and reduce the amount of weight hanging off the exhaust manifolds.  The exhaust side also holds the internal wastegate, if equipped.

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.  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.  It's also important not to exceed tilt specs because it will cause drain back issues and bearing wear.

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.

On a side note, don't use silicone hoses for turbo oil drain/return or power steering lines because they can sweat and leak.  Some may have a Fluro lining to prevent this but any problems and it will sweat, making a mess and requiring replacement.  Silicone is acceptable for air and most coolant hoses.

Component selection and turbo design

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, 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.

A/R ratio

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 where the flange opening changes to the shape of the volute.  Basically, it concentrates airflow at a point on the turbo wheels through the spiral shape.  Comparing only AR mean much because engine displacement, tune, supporting mods, and rpm, etc.,  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 A could be a 3 inch tall motorcycle tuyrbo and Turbo B could be a turbo 5 feet tall used in a power plant.  When comparing only 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 volutes for four cylinder engines (or 4/bank on a V8).  This is used with a divided exhaust manifold to separate the exhaust pulses from the cylinders for smoother flow and maximize performance.

Internal vs. External wastegate

The exhaust housing may also house an internal wastegate.  An internal wastegate is a passage cast into the exhaust housing with a flap door.  It's common for turbos to spin from 0-100,000+ rpm in a few seconds but if they overspeed they can be damaged.  The flap door lets exhaust gas bypass the turbine and controls the speed and output of the turbo.

Below left is a housing that uses an external wastegate - because it's external it's not part of the turbo.  Below right is an internal wastegate with dual doors from a Mitsubishi Evo.  It has two doors to allow a larger door within the space limits of the exhaust housing.

The wastegate door is is opened and closed by an external lever, visible above, which is moved by a spring loaded actuator.  The wastegate actuator is basically a vacuum diaphragm which normally closes the wastegate because the spring inside the wastegate creates resistance on the lever which holds the door closed.  Once the vacuum diaphragm receives a certain amount of boost pressure, it overcomes the spring and begins to open the wastegate.  Once boost pressure falls back down, it rapidly closes.  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.  On older gas cars this could be done by restricting the amount of boost going to the wastegate which held it shut longer.

One interesting thing about the turbos shown above is that their housing outlets match the shape of their exhaust pipes.  This is best for good exhaust flow.  Most turbos look like the one on the right - internal wastegate with a single door and not matched.  This space creates an empty spot where air turbulence can form.  Many newer turbos have built in dividers and you can also make or buy an exhaust pipe with a divider to improve exhaust flow.  More on split downpipes in the sections below.

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 also 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).

Some new Borg Warner turbos have a new type of wastegate which is built into the exhaust housing at an angle.  This saves space and lets it approach the efficiency of an external wastegate turbo.  Below you can see how it looks on the inside.

Here you can see the angle that the door is placed on this new line of Borg Warner EFR turbos.  You can also see the wastegate actuator and lever.

Most modern engines also have 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 releases boost into the atmosphere instead.  TDI don't have these and can't use them because they don't have throttles that work like that.

All modern TDI and many modern diesels don't even wastegates at all!  All VW TDI and Audi TDI turbos, mk4 generation and newer, use a VNT mechanism and actuator instead.  On the outside a VNT actuator looks similar to a wastegate actuator but it works off vacuum instead of pressure.  The outside of the exhaust housing is also thicker to house the VNT mechanism inside the turbo.  The VNT also limits turbo speed but instead of opening a bypass door, it redirects the exhaust gases at the turbine wheel instead.  The other main purpose is to maximize exhaust energy through actuating the VNT vanes to the optimum angle for a given amount of rpm and flow.  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.  To test or adjust the VNT actuator, see 1000q: VNT check.

Turbo exhaust flow

You want the least backpressure in the exhaust downstream of 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..  A non turbo or supercharged cars may not have maximum performance with the least backpressure because some backpressure may be produced as a byproduct of the best performing exhaust. Pipe diameter and design affects performance mostly because of how it affects exhaust pulse wave tuning and the scavenging effect from cylinders. A "tuned" exhaust can help pull the next exhaust pulse out of the engine which increases performance. This tuning can sometimes result in backpressure as a byproduct. This is why an exhaust that's too large can penalize non turbo cars, depending on the application. Again, backpressure itself is never desirable.

With turbo exhausts, there is no wave tuning because the turbo chops up the exhaust and it's making plenty of it's own energy waves. A turbo gets its energy by a pressure ratio differential so you want the highest energy differential across the turbo (exhaust gasses have energy in both velocity and heat).  This means you want the least backpressure and highest velocity exhaust gases after the turbo.  Image a waterwheel: you want the pressure highest before the waterwheel and lowest after the waterwheel to transfer the most energy to the waterwheel.

Manifold, turbo, and downpipe exhaust wraps and coatings may help performance. These serve two main purposes: to keep energy in the exhaust stream instead of passing it to the cast iron exhaust and to lower underhood temperatures, both of which can increase performance.  However, coatings also reduce radiation and external convection which results in more heat being passed into the CHRA and cylinder head.  Some heat is carried away by cooler EGT when the engine is off boost but it doesn't take a genius to know that insulation on something hot makes it stay hot longer.

The easiest way to reduce backpressure is with a straight pipe exhaust, meaning that the exhaust is a simple pipe after the turbo (not street legal).  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 should slightly increase.  A TDI diesel is throttled by fuel and uses a relatively small turbo with computer controlled fueling.  Increasing boost will only cause limp mode because the car's computer thinks something is wrong.  However, less backpressure will let the system work to their full potential so consider an exhaust as a basic supporting mod for any future modifications such as a chip, larger turbos, fuel nozzles, etc.

Below are some more details on individual components of exhaust systems.  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 many cases.

Mandrel bent exhausts are 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, a sharp 90o bend in the piping could have 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 and take up less room. It can also be more durable because it's cast in one piece of iron - without weak welds they can't easily crack with repeated heat cycling, expansion, vibration, the weight of the turbo, and stress.  In my opinion, the amount of custom fabrication is high (expensive) so you'd see better results spending your money and time improving other areas first.  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.  Here's 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 Hyundai Genesis 2.0T exhaust.  There's a big crimp in the exhaust for support brace clearance, shown below.  Replacing this piece would result in a small power and response gain with no other modification.  Because it looks like it's between a catalytic converter or a muffler, 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 would be a lot more expensive.  If they put all luxury car parts or premium sports car parts on an economy car, it wouldn't be an economy car would it?

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 for performance applications, you donít want more than a 1:1.5 ratio of boost to backpressure.  Practically all street cars will make more backpressure than boost. As a rough 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 youíll 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 free flow exhaust.  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 and the manufacturer has spent lots of time engineering optimal exhaust systems given their constraints.

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 the kind of restriction you were imagining.  A catalytic converter is actually honeycombed or grid-like in structure and allows exhaust to flow through it.

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 (DPF) to become clogged with particulates.  Up to 5% biodiesel is allowed by the TDI warranty.  Unlike a catalytic converter which lets gasses and particulates pass through it, DPF are block off filters which let gases pas through but trap particulates and solids.  When exhaust backpressure from particulates clog the filter, the car's computer dramatically raises the exhaust gas temperatures with post combustion injection at the cylinder to burn off the particulates and self 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.

For the 2.0L engine system used on VW Jetta TDI, Golf, and Audi A3, see 1000q: DPF FAQ.  For the Adblue equipped systems used on VW Passat TDI and the VW Touareg TDI and Audi Q7 TDI, see 1000q: Adblue and 3.0L DPF system explained.

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 vs. just being loud.  There are many possible causes but the most common ways to get rid of it are to install a venturi and 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 exhaust at the correct spot can reduce resonation noise.

The DPF diesel particulate filter

All TDI after 2009 (and 2006-2008 VW Touareg TDI) have diesel particulate filters (DPF).  This is a soot filter downstream of the catalytic converter that captures soot and burns it out during a self clean cycle.  It's not possible to bypass the DPF on your VW TDI or remove the DPF on TDI engines because there are a number of pressure and temperatures sensors that expect to see proper operation of the DPF.  For detailed information on the DPF system, see 1000q: DPF FAQ.  V6 VW/Audi TDI, BMW, and Mercedes all use a "wet" urea system with Adblue fluid for NOx emissions.  See 1000q: Adblue w/DPF FAQ for information on the wet systems. 4 cylinder TDI have a slightly different no-Adblue fluid system except for the heavier Passat TDI.

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 earlier 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.  Some turbos even 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.  You can see the soot marks where the turbo exhaust flows.

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 turbo response.

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 7-12į 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.  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.  

Variable geometry VNT turbo vs. fixed geometry turbo

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.  To see disassembly of a VNT turbo on a ALH engine TDI, see 1000q: VNT vane removal and cleaning.  Some newer TDI use an electric motor to move the rod instead of a vacuum can.

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 so 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

Here's a newer video showing a Borg Warner VNT turbo.

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.

A higher compression engine reduces lag.  Lag can be reduced at the turbo 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 current trend for high performance turbos are billet turbo wheels instead of cast wheels.  Everything else being equal, this does not increase performance.  Here is the reference from Garrett.  The reason turbo wheels are machined instead of cast is because it's cheaper to make low volume parts by machining them instead of making the casts and because the wheel can be made stronger.  One reason why most billet turbos in the car aftermarket are an improvement is because they use newer wheel designs.  If the wheel can be made lighter, it will increase turbo response.

Gamma Titanium Aluminide turbine wheels are showing up on turbos from cutting edge applications on jet engines.  Lighter turbine wheels mean up to 50% lighter weight vs. inconel.


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.  There are some new turbos which can handle greater mounting angles but almost all turbos are mounted with the shafts parallel to the ground so that there are no excess loads and that the oil drains properly out of the housing.  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 which 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.  As a generic recommendation, if you exchange your old journal bearing turbo for a new BB turbo you must change the oil lines or install a restrictor to prevent smoke due to excess oil leaking out the exhaust side.  To the right is a picture of a journal bearing banjo bolt vs. replacement banjo bolt oil feed restrictor for a ball bearing turbo.  The journal bearing oil line is the larger diameter one - as you can see, there is a massive difference in the required oiling vs. the small hole on the ball bearing!

On a side note, excess crankcase pressures from clogged vents and too little backpressure can cause some turbos to smoke, especially with worn seals.  Obviously worn seals will cause smoking but it can be made worse if there's pressure in the crankcase.  Without the catalytic converter and other things that can create some backpressure to counteract the crankcase pressure, it's easier for oil to get past the seals.  Crankcase pressure also causes the oil drain line flow to slow which makes the problem worse.  TDI have a vacuum pump to run the various car systems which exits into the crankcase so it's very important for the crank case vent (CCV) system to be clear.  Unlike gas cars which have a PCV valve, the CCV system is pretty much always pumping out air.

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.

To see removal of a CHRA from the housings, see 1000q: VNT vane removal and cleaning.  Most Garrett turbos use a machined fit on the housings.  Under very high turbo boost, some turbos will leak at the machined fit.  Removal of the housing, machining of the compressor back plate, and installation of an o-ring can help fix a leak there.  

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 is a little better because they distribute the load across a wider area - see below for a picture comparing them (not a TDI engine).  Most 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, bad oil, or worn bearings) can cause excessive wear and play and can let oil leak out.  Because diesel engines can run on engine oil, and diesels are throttled by fuel, too much oil in the intake will cause the engine to race and suck out more engine to burn, resulting in a nasty feedback cycle.  Please see 1000q: runaway engine FAQ and suggestions for details and more explanations.

If this or any other unintended or unexplained acceleration occurs, the driver's first priorities are to concentrate on safe operation of the vehicle, keep it under control through braking and steering input, and to regain control over engine power or shut the engine off as soon as is safe and practical.  Pull the car over to the side of the road only as soon as it's safe to do so.

Continue to turbocharging FAQ page 2


Do you have information or corrections that should be added to this article about performance turbos for VW and Audi TDIs?  Please post your comments in the myturbodiesel.com TDI discussion forum.  Also see 1000q: turbo upgrade chart for TDI.