Turbocharger, VNT, and diesel turbo FAQ-page 4
Turbocharger, VNT, turbodiesel FAQ: page 4/4
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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 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 another car. Below is an advertisement from 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 kit?
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't know what's going to break when you turbocharge the car unless there's already experienced installers who can help you. 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 which have cracked just from heavy track use. Crankshaft thrust bearing wear also 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 or using X amount of fuel or air, 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 without special engine tuning. 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, which means "If you have to ask, it's way over your head" isn't really correct...you can also pay someone else a ton of money!
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 a 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 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 fine aspects of heating porting; this is only a tip to fix casting flaws! 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 could be designed to 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, otherwise leave it be!
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.
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.
Thanks for reading the myturbodiesel turbo FAQ, please feel free to expand or correct this wiki as needed!