Turbocharger, VNT, and diesel turbo FAQ-page 2
Turbocharger, VNT, turbodiesel FAQ: page 2/4
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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 don’t 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 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 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 7-12° 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. 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.
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.
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