Turbo FAQ 101, aftermarket exhausts, and answers to common turbo questions - page 2

More useful turbocharging tips and information

Back to Turbocharging FAQ page 1
Table of contents - Page 2

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

Oil supply and turbo timers

The biggest area of concern in the turbo is the oil supply.  Insufficient oil (especially journal bearing turbos) or excess oil (especially ball bearing turbos) or dirty oil may wear out the bearings, causing wear and shaft play in the turbo.  Because of the high temperatures seen at the turbo, the oil may also break down faster than a comparable non turbo car.  Synthetic oil is recommended for turbo cars because it doesn't break down as quickly as conventional oil.  Because the best engine oils for a diesel engine are synthetics, this is another reason to use synthetic in turbo and diesel applications if you are not already doing so.  

In addition, since the turbo gets hot when running, an engine idling period of 5 seconds before every engine shutdown is enough to let fresh oil circulate to the turbo bearings.  If you were driving hard and hot, a 1 minute idling period or a few minutes of sensible driving before shut down should be enough to let the turbo cool down and receive fresh oil.  If the turbo is too hot and does not receive cooler oil upon shutdown, the oil could become burnt and "coking" may occur.  This is more of an issue with non synthetic oils.

Another issue is letting fresh coolant circulate to the CHRA.  After engine shut down, the coolant heats and expands in the cartridge if the CHRA is too hot.  This creates a natural circulation to drain away the heat and bring in fresh coolant.  The reason it doesn't boil off is the same reason engine coolant doesn't boil off - the engine coolant is a sealed system.  Some cars have auxiliary pumps that circulate coolant after engine shut down.  There would be no benefit to this on a TDI since the turbos are oil cooled only and not water cooled, and because of the lower temperatures that you should see during engine shut down due to a diesel engine and from good shut down practices.  Even on gasoline water cooled turbos, if it didn't come from the factory with an auxiliary pump, I would not add one since the engineers didn't put one there and because there is some natural convection of coolant and oil.  I do not recommend rerouting the oil or coolant lines in your turbo unless you are sure they are routed properly.  I also recommend never using radiator "stop leak" products because they can gum up and clog the turbo coolant lines.

You should also not install any kind of inline oil prefilter upstream of the turbo oil supply line.  Some newer Subaru gas turbo cars suffered destroyed turbos from oil starvation.  These were traced to a design change consisting of an inline oil filters added at the factory - these became clogged, causing oil starvation.  Here is the reference.

The other concern is mounting angle.  If you've seen turbos on engines they're all mounted so that the shaft is parallel to the ground.  I haven't seen specs from VW but Garrett says their turbos must be mounted below 15o tilt.  Beyond this can cause bearing wear and drain issues.

Use of VW approved engine oils in the TDI is also recommended to ensure proper lubrication to the turbo.  The big shift for North American market cars was in 2004 with the introduction of the pumpe duse engine and in 2009 with the common rail engine.  These engines see very high pressures in the head and should use VW approved engine oil to keep your warranty intact.  See 1000q: pumpe duse engine oil and 1000q: non pumpe duse engine oil to see lists of approved oils and reference links to oil manufacturer's websites.  The common rail engine in the 2009 and 2010 VW TDI uses VW/Audi 507.00 spec engine oil only.  At least for warranty purposes, stick to the VW spec, especially since this is a new engine and there isn't any aftermarket engine oil analysis out there yet.

Some people install a turbo timer to keep the engine idling so they can walk away from their car during a cool down period.  I do not recommend these products for a number of reasons.  First, if you have a manual transmission, you should always put it in first or reverse gear when parking in addition to applying the parking brake, so the convenience of walking away with the car idling is not possible.  Also, a turbo timer requires spending money on the timer, cutting wires and introducing an unnecessary failure point.  Lastly, for diesel applications, coking is not as common of a problem due to the lower rpm and cooler exhaust gas temperatures, and you should be using synthetic oil anyways which is more resistant to coking.  If you are truly concerned about turbo care, just make sure that you drive at medium rpms and load when the engine is still warming up and just drive sensibly a few minutes before shutting the engine down.

The intercooler

Another component in a good turbo setup is the intercooler.  After intake air passes through the turbo, it heats up partly because of higher pressure.  The ideal gas law states that when all other variables are constant, if pressure is increased, so will temperature.  An intercooler lowers air temps before passing the air into the engine.  (Some other sources of heat are the intake piping soaking heat from a hot engine bay, because the turbo is so close to the exhaust with hot exhaust gasses passing through the exhaust side of the turbo, and mechanical agitation of the air by the turbine wheel.)  Without an intercooler, hot air increases the likelihood of uncontrolled detonation and engine damage.

An intercooler is basically a heat sink that takes away the heat of the intake charge.  Here is a picture of an intercooler in a Jetta TDI.  Cooling ambient air moves through the front bumper, through the intercooler, and through the wheel well in the direction of the arrow.  More air moves through the intercooler as the car moves faster.

You don't see intercoolers on non-turbo cars because the intake air is already at ambient temperature.  An air intake directly connected to an intercooler or anywhere not after the turbo would actually decrease performance by restricting airflow.  Below is a funny picture of an "interfooler", someone who put an intercooler on a non turbo car.  It's there because they want to look cool and are ignorant of what its function is.  Even worse, the air filter is exposed and low enough to suck up water and damage the engine.

The goal of intercooling is to produce the least pressure drop (so the turbo doesn't have to work as hard) and remove the most heat.  Depending on the exact setup, the average well designed intercooler in a car may have .5-2.0 psi pressure drop.  There is always some pressure differential between the turbo and the engine to get air moving from one spot to another.  An intercooler acts more like a heat sink and less like a radiator when boosting because boosting heats up the intake air.  This heat is transferred into the intercooler like a heat sink.  Then the intercooler releases the heat into the ambient air or coolant.  Most of the heat leaves with the ambient air flow (while the car is moving, air is passing through the air ducts) but a little heat can go back into the intake air once air temps have dropped (heat moves from hot to cold).

A good air-air intercooler can cool the air to within 20 degrees of ambient temperature if it has steady airflow to take away the heat.  The advantage of a good air-water intercooler is more consistent intake air temperatures because water is a better heat sink.  Water (coolant) is not as quickly affected by rapid changes in ambient air temperatures and car speed.  But once water is hot, some heat goes out a radiator and some goes back into the air-water intercooler's intake air.  Some cars don't have the routing or space for a good air-air intercooler so they must use an air-water intercooler.  

An air-air intercooler is preferred for diesels because they are normally front engine so there's plenty of space for plumbing.  An air-air intercooler is also easier to fabricate with less chance for leaks.  If there is a major water leak into the intercooler core, it's possible that this could hydrolock the high compression diesel engine.  A air-water intercooler is more appropriate on a mid engine car due to difficulty of intercooler packaging or a car with more peaky temperatures.

In a gasoline engine, the engine is operating at vacuum or low boost most of the time.  Low boost doesn't heat the intake air as much as hard boosting and as a result, doesn't transfer as much heat to the intercooler.  In other words, a larger intercooler is not needed unless you need the extra heat sink capability!  Most modified gasoline cars would benefit a little from a larger intercooler due to higher than stock boost levels.  However, how much it's needed in only lightly modified cars is debatable due to variations between cars, ambient outside temperatures, intended use (street vs. track), desired safety margin and fuel octane, etc..  For example, a large front mount intercooler will cool better than a small intercooler but it may not fit, may be blocked by the bumper, cause overheating problems due to blocking the radiator, etc..  Also check for leaves or dirt blocking the face of the intercooler.

A diesel engine has a greater need for an effective heat sink vs. a similar gasoline engine because of higher sustained boost levels.  Turbos are also smaller for a number of reasons, for example, the smaller rpm range.  I think that even lightly modified VW TDI could benefit from more efficient intercooling for maximum peak power.  The best way to determine the need is to log pressure and temperature at the turbo and at the intake manifold.  Especially for a front engine TDI, an air-air intercooler (which you already have) is the best option.  The VW TDI naturally puts an oily mist on the inside of the intercooler from the crank case ventilation (CCV) system but trying to keep the inside clean is like trying to keep the oil dipstick clean.  Gasoline cars shouldn't have any oil inside the intercooler.

If you must paint the intercooler to help hide it, use 1-2 light sprays of radiator paint or even better, a heat shedding coating like Swaintech's "BBE heat emitting coating".  I don't know how well it works since bare Al is already very good at shedding heat.  My guess is that because it sells well and measuring before-after intake air temperatures is so easy (assuming equal ambient test conditions), that it probably works.  

Spraying coolant onto the outside of the intercooler is very effective because it can lower the temperature of the intake air below ambient air temps.  CO2 (compressed carbon dioxide gas), N2O (nitrous), and just regular water all work very well at increasing intercooler effectiveness but only work until your coolant runs out.  If you are preparing a short race, placing bags of ice on an air-air intercooler or chilling the coolant in a water-air intercooler works well too.

Keep in mind that in most modern turbo cars, turbo pressure is regulated by how much pressure is seen at the intake manifold, not at the turbo!  Some also measure the air temp at or near the manifold.  Regardless of intercooler efficiency, pressure at the intake manifold should drop only a little.  As an example, assume an engine that limits boost to 15 psi at the intake manifold.  If you have two turbo setups, one with an efficient intercooler with only 1 psi pressure drop and the other with than an inefficient intercooler with 4 psi pressure drop, the turbo with the efficient intercooler only has to make 16 psi at the turbo whereas the inefficient setup has to make 19 psi at the turbo.  The turbo making 19 psi is mechanically more stressed and is creating more heat than the turbo that has to make only 16 psi, everything else being equal.  If the turbo is pushed beyond the optimum area of efficiency, it will create exponentially greater amounts of heat and pressure.  Again, pressure does not equal density, you are still creating the same amount of pressure seen at the intake manifold that regulates the turbo but the air is less dense and hotter, which creates less engine power and efficiency.  This could also happen with an air leak.  To detect an air leak, see the FAQ article 1000q: boost leak testing.  A common issue with the VW TDI is the sudden loss of power known as limp mode.  The VW TDI ECU has pressure and air temp sensors and if the ECU senses a problem, it cuts power to prevent damage to the turbo and engine, preventing damage to the turbo from an overspeed.  See 1000q: limp mode diagnosis for more details on a sudden loss of power.

To the right and above is a thumbnail of an aftermarket off-the-shelf intercooler next to the stock intercooler, click for a larger view.  It features bar-plate construction instead of the stock tube-fin.

Further flow improvements

Another way to increase the efficiency of your general setup is to improve the pre and post turbo and intercooler piping.  This reduces pumping losses and restriction (reduces boost - see the next section for more on this).  On most TDI engines, this can be difficult due to the turbo, intercooler, and battery locations.  The best piping would be relatively smooth on the inside (mandrel bends), have a relatively straight path or gradual angles and transitions, and be as short as possible.  The shortest, smoothest pipe routing on a transverse 4 cylinder engine would be from a turbo in the front, with a 180o loop to a front or side mounted intercooler and then a 180o loop back to the intake manifold.  This is not possible on the VW TDI due to the rear mounted turbo location but you can still improve the existing piping.  When putting together an aftermarket setup, use piping that has mandrel bends with straight silicone couplers instead of using straight pipes with bent silicone couplers.  Silicone couplers tend to collapse at tight spots and can bend, reducing the cross sectional area.  Due to varying fitment, they also tend to have more gaps between the piping, disturbing airflow more than necessary. They economical and easier to assemble but the best system is a simple system.

Shortening the intake piping, making the transitions between piping as smooth as possible, and and routing the piping as straight as possible will reduce the amount of required pressure to produce a certain amount of power, increasing reliability and efficiency.  A rough rule of thumb is that each 90o bend in pipe adds as much airflow resistance as 25 ft of straight piping.  Of course, actual resistance depends highly on diameter, smoothness of bend, etc., but (big surprise here) short straight piping results in the best flow.

Some people think that larger piping or a larger intercooler increases lag.  This is true because it takes longer to fill and pressurize the larger piping and intercooler.  However, the difference in response is extremely short, especially considering the small, quick spooling turbos on the TDI.  In addition, the loss of throttle response is generally not a factor at all since the larger piping increases overall efficiency and the power gain from other mods offsets any additional lag.  Exhaust backpressure, chip tuning, and turbo size are far greater factors in throttle and turbo response than larger diameter intake piping, so don't worry about piping being too big.  Intake piping makes a difference but on the TDI the priority is lower compared to a turbo, injectors, exhaust, and chip improvements.

The one thing to be wary of with Volkswagens is using high flow air filters.  The mass air flow sensors (MAF or MAS) on the Mk4+ body seem to be sensitive to the additional dust and debris that a high flow air filter, especially aftermarket oiled cotton filters let into the intake tract.  This can lead to a failed MAF, see 1000q: MAF FAQ for more details.  The stock air filter and housing was overbuilt and uses the same air filter as the 240 horsepower Golf R32 so there is little-no gain by switching to a high flow air filter.  Lastly, many cold air filters don't use a cold air intake snorkel.  This draws in hot underhood air and can actually reduce power.

Common turbo myths dispelled

The biggest myth is that every turbo car can make more power just by turning up the boost.  Boost is only a measure of intake pressure.  Pressure can only be created when there is resistance from a restriction.

Everything else being equal and within reasonable limits for the setup, more boost makes more power only if the turbo is operating in an efficient range of performance and if the rest of the setup can benefit from it.  Most turbocharged cars have a little room to safely increase boost.  If you were to increase the boost to the point where the turbo is trying to move too much air, it actually reduces performance.  This is because past the point of diminishing returns, a turbo is basically blowing hot air.  This hot air creates intake air pressure and more boost because boost = measure of pressure.  Again, back to the idea of volumetric efficiency, you want the maximum mass of air for the engine.  Unless the air can be cooled sufficiently by the intercooler, the density of the air might actually be less than it would have been at a lower boost level.  This psi level of diminishing returns is different for every setup and every car and even varies by ambient conditions.  At that point, some modern cars compensate by using their computer and sensors to adjust the timing to prevent detonation.  The TDI engine car computer has air temperature and pressure sensors and a program that will prevent increased power if the only change is increased boost.  You need a chip or other performance enhancement to increase fueling, see 1000q: basic performance upgrades for the TDI for more details.

Again, it is a common mistake to equate boost, or intake pressure, with denser air.  Assuming the other variables are constant, the ideal gas law PV=NrT shows that if you raise pressure, temperature increases.  Also keep in mind the above paragraph about operating a turbo outside of its areas of efficiency.  It's easy to get so caught up with quick power gains from more boost pressure that one can forget that the ultimate goal of turbocharging is increasing air density, not just pressure.  In designing the engine as a whole system, you want to create the same amount of power with the least amount of boost, within a range, to reduce stresses on the engine and turbo and to keep air moving at a reasonable speed throughout the intake tract.

One more time: boost pressure is a measure of intake restriction.  You could put a choke in the intake air path and that would also create boost (but reduce power).  A turbo moving a lot of air but showing relatively low boost on a boost gauge means there is low air restriction in the intake air path.  Remember, the goal in increasing power is to move more air, more efficiently, not just create boost.  Changing camshafts to allow more air into the combustion cylinders, changing the combustion cylinders by boring and making the diameter of the cylinders wider, or stroking the engine and making the length of the piston travel longer, can all increase the amount of air moved.

Adding a larger turbo does not mean the engine will make more power.  In a modern car, the turbo is regulated by sensors, computer feedback, and solenoids set to control the boost at a certain pressure.  The computer measures the pressure with sensors normally at the intake manifold or some spot right before the intake manifold.  Everything else being equal (load, rpm, etc), one large turbo and one small turbo will flow identical pressures of air at a given psi but remember that psi is just a measure of pressure - air mass is what matters and is what makes power!

Here is an example: to flow a certain amount of air, where a smaller turbo may have already passed its maximum efficiency and is blowing mostly hot expanded air, a larger turbo will still be operating in its area of maximum efficiency and is moving cooler denser air at the same psi.  Again, assuming that one turbo is stressed too much and the other is in its peak efficiency, they are both giving the same psi but not the same density of air.  20 psi is always 20 psi, the difference between an efficient turbo and a turbo blowing hot air is the temperature of the air coming out of the turbo which affects density.  20psi of 50oC air is not the same as 20psi of 14oC air.  There are also other factors that effect this such as the size of the turbo housings, backpressure, etc..  You want to select a turbo which balances responsiveness with moving your desired mass of air.  Do a lot more research and consult your performance and parts vendor before crunching the numbers and selecting a turbo setup.  The same turbo on a 4.0L engine will respond totally different than on a 2.0L engine.  Garrett turbo's website has some more info on calculating airflow.  Turbo manufacturers often publish graphs which show where the turbo is operating most efficiently.

Also remember that the control systems and sensors for the turbo are normally located after the intercooler in the intake manifold or piping.  If there is an air leak in this section of piping, the turbo has to work even harder to make up for the lost and provide the same reading to the sensors.  Because it has to work harder, it may even be operating outside of its optimum efficiency range and creating excess heat over a leak free car.  See 1000q: boost and vacuum leak testing for a simple way to test for boost leaks.

What does this mean for detonation in a gasoline car? Detonation or engine knock is the explosive ignition of fuel. It often occurs from preignition on hot spots inside the engine cylinder which can be caused by pitting from earlier detonation. If an engine starts to knock at 20 psi, it will always knock at 20 psi, everything else being equal (ambient conditions, same octane and fuel quality, same exact engine). In this example, a more efficient turbo will move more air mass at the knock limit of 20 psi than a less efficient turbo at 20 psi. Therefore, the larger turbo can move more air (and make more power). This goes back to the last section on flow improvements: move more air, more efficiently to make more power.

Turbocharging your own car

All modern diesel passenger car and truck engines are turbocharged, but some readers may be wondering if you can turbocharge an older nonturbo diesel or nonturbo gasoline car.  The short answer is yes!  The long answer is that for most cars, it is such a large project, requiring such a large amount of custom fabrication, custom tuning, uncertain results, and amount of money, that I'd rather just buy a car that is already turbocharged and skip the effort and risk.  In other words, if you have to ask if it's possible, the project is way over your head!

Some popular nonturbo cars have kits that have already been tried by many other people.  In these cases, the risks are minimal because there are other people who can give you technical advice or the business who sells the kit will also install and tune it.  If you do your own kit, the time that you spend on the project and then fixing all the problems that show up would be better spent working at a job so you can make more money and just buy the other car.  To the right is an advertisement by Porsche showing the upgraded parts between a 944 and 944 turbo.  See all the extra parts that wouldn't be on your car if you just added a turbo and parts to make the turbo work?  Buying the other car would be cheaper than buying all those parts and retrofitting them.

With some turbo cars, they already sell higher end models with everything you want already on it, so it's not economical at all to spend too much money on increasing the performance of the base car.  For example, the Subaru WRX and Mitubishi lancer ralliart have less power, simpler suspension and all wheel drive systems, different interior and trim levels, etc., compared to the STI WRX and Evolution.  With the money and time upgrading the base turbo car to the high turbo car, it makes more sense to sell your car and just buy the higher end model.  Ultimately, it is your car, your money, and your responsibility, so FYI, here are some more cautions if you want to continue.

The biggest problem is that a nonturbo car was not engineered for turbocharging and that people generally don'tknow what's going to break when you turbocharge the car.  This also assumes that there's even space under the hood for the piping and turbo.  For example, the transmission may only be designed to hold the amount of power from the nonturbo engine.  The clutch may not hold the amount of increased power, so you would have to replace the clutch and pressure plate with one that could withstand more power.  But then, the clutch hydraulic system may not be able to handle the increased pressure required to actuate the clutch so you might have to change the clutch system.  The clutch pedal may be designed for lighter pressure, and having a hard clutch pedal (from using a stronger pressure plate) could deform or wear out the clutch pedal levers and bushings.  Some newer cars use plastic clutch pedals and they have cracked just from very heavy track use.  Crankshaft thrust bearing wear increases from using a heavier clutch pressure plate.  The intake tract, including the various throttle gaskets and seals, piping, and vacuum lines may not be designed for positive pressure.  Putting these components under boost can pop them off or cause small leaks that only show up under pressure and blow various seals.  The engineers who built your car can't overbuild everything that they want to, otherwise your car would be as heavy as a tank and cost twice as much.  So even if "x" is reliable at higher power levels, "y" breaks.  Again, each car model is different and will have different problems that show up once you start modifying it.

With modern traction control and stability controls, the car can also restrict power if it senses the car moving faster than it was designed to.  As an extreme example, assume a stock car that, even under the most favorable conditions (going downhill), accelerates 0-60 in 7 seconds.  If the car's computer sees your modified car accelerating to 0-60 in 3 seconds, it knows that something is wrong, thinks the tires are spinning on ice, etc., .  (If your car really does go 0-60 in 3 seconds, you probably are spinning the tires).  The stability control will do what it was designed to do and reduces power or applies the brakes to regain control.  This is not a problem with VW or VW TDI, but this condition has happened on a few cars involving OEM engine swaps.  Hopefully this won't become a trend as these systems mature.

The compression ratio is also higher in nonturbo cars.  This is true for both diesel and gasoline cars.  Because of the higher compression ratio, it limits the amount of pressure and boost you can use.  This pressure also creates the need for stronger pistons and engine construction.  The pistons in turbo cars also tend to have oil squirters that direct oil at the underside of the top of the piston which helps carry away the additional heat of combustion.  Turbo engines generally also have more robust construction.  This includes the seals and gaskets, the moving metal parts of the engine, the bearings, and the engine block itself.  If you're lucky, the engine's setup will result in cascading failures starting with easy to fix problems appearing first.  If you're not lucky, the engine will be totally destroyed.  For the same reason that you can't take a gasoline engine and turn it into a diesel engine (and expect it to last), many nonturbo engines are not designed to stand up to the stresses of turbocharging.  For example, pictured right is a girdle or cage around the crankshaft bearings on a turbo car.  All these differences limit the amount of boost that you can use on a turbo conversion car.  

It depends greatly on the car, the turbo kit, intended use, engine condition, etc., but in general, if you want to turbocharge a nonturbo car and maintain the same reliability, your best bet is an engine rebuild with more robust components with a compression ratio change.  Again, it varies by so many factors and so many cars are successfully turbocharged with aftermarket kits, but my opinion is that if you want a turbocharged engine, get a car that came with it stock because the bang for the buck, potential for tuning, and performance are all much greater.  Once again, many non turbo cars out there have good kits but if you have to ask for details you need to do much more research.  Here is a picture of what can happen if you try to boost too much on an engine not originally engineered for turbocharging. Of course, this can also happen if you boost too much on a turbo engine, but turbo engines are normally engineered to be more resistant to abuse.  Hint: the engine rod is supposed to be straight.

If you think I am against turbocharging your own car, you're right.  This section is written for the person who asks, "I saw a turbo kit on ebay that said it supports 500 horsepower and costs only $500".  Even worse, "my ebay electric turbocharger is even better than your kit".  Because most people run out of money or don't know how to do the job right, pictured right is what I think of when someone asks a about DIY kits.  Ironically, the CRX is a car which a lot of people have successfully turbocharged with great results!  There are many successful turbocharging jobs and many good kits, but it requires either a lot of cash to pay someone else to do it or a certain level of turbo and mechanical knowledge and experience.

A final (or first, depending on your view) consideration for DIY turbocharging is emissions and emissions testing.  Catalytic converters need to heat up from the exhaust energy before they start to work well.  Modern cars are so clean and catalytic converters so good that the majority of emissions released are during cold engine starts.  Adding a turbocharger between the engine and catalytic converter will result in much greater emissions during cold engine starts because the turbo (a heavy cast iron lump) absorbs heat energy instead of warming up the catalytic converter.  It also takes away heat energy to spin the turbine wheel.  Factory turbocharged cars are engineered from the factory to meet emissions and adding a turbo will result in significantly greater emissions during cold starts and the possible failure of emissions testing.  Ignoring the possibility of failing the visual inspection, a gasoline car with a DIY turbo that is warmed up, in good working order, with catalytic converters, and is tuned well, should pass the average emissions sniffer test.  If the car is cold and had to wait in line at the emissions testing facility or is poorly tuned, the chance of failure is much greater.  Without catalytic converters, there's no way any gas car can pass emissions.

Port/gasket matching

Port/gasket matching is a technique to fix casting flaws and improve flow on components that are not correctly matched.  This basically means that all of the casting flaws or gaskets that are slightly off can be lined up or smoothed out.  Casting flash, or ridges left by casting metal, create rough edges that disrupt airflow, coolant flow, etc..  Some engines just run hotter on some cylinders and casting flash in the coolant passages can prevent the proper flow of coolant, making the problem even worse.  Gaskets that stick out can be cut to match their openings.  This is called gasket matching.  This tip can be applied to cylinder heads, intake manifolds, exhaust manifolds, turbo housings, wastegate ports, exhaust piping, almost any part of an engine that is cast or uses a gasket.  

This is NOT porting cylinder heads!  There are many many fine aspects of porting; this is only a tip to fix casting flaws and improve fitment.  Some engine components have a ridge or step to improve air swirl or flow, so make sure you identify what is casting flash and what is intentional.  For example, an exhaust manifold opening slightly larger than the exhaust head port can sometimes help with anti reversion.  Removing material on the head to improve flow is head porting, removing a ridge left by casting or cutting the gasket to fit correctly is fixing casting flaws or gasket matching.

Some gaskets "fit poorly" because they are restrictors!  In the picture below, you can see how engine coolant has stained the head gasket around the coolant orifice.  This is done to restrict the return flow of coolant.  This is not a mismatched gasket.  The restriction raises the pressure in the head and ensures more uniform pressure and cooling which reduces hot spots.  Some oil lines have restrictors for the same purpose and in the previous section about CHRAs, you can see pictures of BB turbo oil line restrictors vs. journal bearing turbo oil lines.  Always know exactly which port or gasket you are modifying and know the consequences.

Sequential twin turbos vs. symmetrical twin turbos vs. single turbo

Some cars have twin turbos instead of single turbos and some cars that came from the factory with twin turbos are aftermarket converted to single turbos.  The main configurations of twin turbos are parallel/symmetrical twin turbos, or asymmetrical sequential twin turbos.  Parallel/symmetrical twin turbos are found mostly on V-configured engines found in the 300zx twin turbo or Audi S4 biturbo.  They are most appropriate for V configured engines because each side of the V engine feeds one turbo and all the piping is kept equal.  Both turbos should be equally sized to keep the engine balanced.  Factory setups that use this configuration generally provide more low end power because twin turbos are generally smaller than one large turbo but a V engine can also produce more torque. It really depends on the engine and setup.  Symmetrical twin turbos can also be found on the BMW 335i inline engine but in a different alignment.  To the right is a cutaway picture of the 335i engine.  Each turbo is fed from 3 cylinders only and lead into a shared outlet pipe before the intercooler (pointing to the right).

Inline engines can also be fitted with a another type of turbo configuration, the "twin" asymmetrical and/or sequential configuration.  Cars like the Supra or RX-7 twin turbo gasoline cars or the BMW 535d twin turbo diesel use this setup.  Asymmetrical twin turbos use one smaller turbo and one larger turbo.  Sequential setups have a smaller turbo for low end power and a larger turbo for higher end power.  Exhaust gasses are diverted to the smaller turbo until a certain air flow is achieved and then the exhaust gasses are diverted to the larger turbo to provide top end power.  Sometimes the gasses go to both turbos at higher rpm and sometimes they are switched between small and large.

Mercedes Benz, Audi, and BMW use asymmetrical twin turbo diesels that use one small and one large turbo.  Below are some diagrams of their systems.  The BMW 335d uses a similar setup.  Sequential twin turbos are most suitable for inline engines because the exhaust stream is coming out only one side and the piping is simple and short.  If you tried to use asymmetric sequential twin turbos in a V engine, one cylinder bank would be pushing a large turbo and the other would be pushing a small turbo, creating an imbalance.

A single turbo is most suited to inline engines instead of V engines mainly because of packaging and exhaust routing obstacles.  A few older turbocharged Saab gasoline cars used a single turbo V engine that placed the turbo off to one side of the engine.  They experimented with placing the turbo in the middle-top of the V engine but this actually melted the hood paint due to the red hot exhaust.  Mercedes Benz's latest Bluetec turbodiesel engine places the turbo near the top/rear of the engine, but they have a solution for heat control.  I suspect it's also due to lower sustained temps in a diesel, engine bay ducting, and better heat shielding.

Pictured below are top and underside pictures from the sequential turbo on a Mazda Rx-7.  It has one large and one small turbo connected by a shared exhaust manifold in the middle.  Although the turbos might not look small/large, they are different sizes and the difference is very noticeable when the larger turbo kicks in.

Centrifugal superchargers  

These operate in almost the same way as a turbo but instead of being driven by exhaust gases, they are driven by a belt or shaft, normally the front serpentine belt.  This belt usually powers engine accessories like the AC compressor, alternator, etc.  This type of supercharger is basically the compressor side of a turbo attached to a pulley and clutch instead of an exhaust side turbine.  It's also geared to increase its rpm to much higher speeds than the drive belt.  They tend to not be as efficient as a turbo because they drain energy from the engine instead of using the exhaust gases for a source of power.  Because they are belt driven instead of exhaust driven, many rules of thumb for exhausts on turbo charged cars do not apply.  Since there isn't a turbocharger (large lump of iron) in the exhaust soaking up heat, adding a supercharger shouldn't effect emissions much.  (see the above section on DIY turbocharging)  Since this is a turbocharging article, I only showed a centrifugal supercharger below since it is looks sort of like a turbo.  The other types look like gears or screws and are more likely to be found on factory supercharged cars because they are usually more efficient than this type.  The problem with adding those is that they must be located on the top of the engine which can require a bulging replacement hood.  Centrifugal superchargers are popular in aftermarket kits because it usually takes less effort and cost to add one to the front of the engine than the top.

Are centrifugal superchargers better than turbos?  The short answer is no.  The long answer is that it depends on what car you are using it on, packaging restrictions, budget, power goals, etc.  If your goal is top end power, maximum power, or if the car is used in a racing environment, turbos are almost always better.  I remember seeing a dyno chart of the older Chevy Cobalt SS (a stock supercharged car) which had the supercharger removed and a custom turbo kit added.  The turbo car had a much better powerband everywhere.  In fact, the newer Chevy Cobalt SS engine (excellent budget bang for the buck turbo car) switched to a factory turbocharger and gained a lot more power.  There are many other reasons why it got a new engine but everything else being equal, a factory turbocharged car can make more power than a factory supercharged car.  Superchargers in general are coming back in cars like the Corvette ZR1 and Audi S4 due to more efficient roots type 4 lobe supercharger designs but these are not centrifugal superchargers, they're roots blowers.

Would you like to know more information about VW TDI performance?  Ask in the myturbodiesel.com VW TDI forums  Also see 1000q: turbo upgrade chart for TDI.

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