Turbocharger, VNT, and diesel turbo FAQ

Dec 23, 2013
Turbocharger, VNT, and diesel turbo FAQ
  • Turbo FAQ, aftermarket exhausts, and answers to common turbo questions - page 1/4

    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.

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    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.

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