The turbocharger is a pretty magical device; it takes the seemingly useless exhaust gasses and uses them to drive a turbine, which then compresses extra air for the engine. If one is willing to put up with the turbo lag, they are quite a nice way to pick up some extra horsepower. Some turbos are very high-tech, with variable-geometry exhaust turbines, made out of trick ceramic materials.

Turbochargers, it is worth noting, require a certain amount of time to spin up and begin supplying the extra air to the intercooler or intake manifold. The cool noise they make is the high-pitched whine produced by the turbine.

Why aren't many race cars turbocharged? Because the rules for many classes stipulate that the engines must be normally aspirated, as opposed to using a turbo or supercharger.

Starrynight: Many rally cars are turbocharged. However, your mentioned turbo-lag is a real pain in the ass for them as they are constantly speeding up and slowing down. Someone came up with a neat workaround for this: as the car slows down, the turbocharger is kept up to speed by the injection of fuel directly into the exhaust pipe, which is then ignited. The resultant explosion mimics the effect of exhaust at high speed, driving the turbine. This is why you hear a lot of banging noise as a rally car slows down to take a corner.

To do this on a normal road car would be outright insanity. Not only would you probably cause severe injury to anyone walking past your car, but the exhaust pipes wear out really fast; I don't know exactly how long they last, but I think its tens of miles.

I don't actually know anything about this sort of stuff. I was told all this by an "engines-hacker" friend of mine and thought it was cool. /msg me with corrections.

If you own a turbocharged car, it's important to take proper care of your turbo. This will keep you from having to get it replaced after a few thousand miles (and most limited warranties don't cover turbochargers).

The first thing to remember is that turbochargers need time to warm up and cool down. After starting your car when it's been sitting for a while, don't drive it hard. This is actually good advice for naturally aspirated cars as well. You need to give your engine and turbocharger time to warm up and get the oil circulating. After 10 to 15 minutes or so of easy driving, your car should be sufficiently warmed up. This gives the engine time to adjust to a fairly constant operating temperature, which keeps parts from getting too stressed. Remember, metal expands with heat and contracts with coolness.

Turbochargers get very, very hot. Not only is the turbo circulating extremely hot exhaust gases, it is also spinning at insanely high RPMs. This creates friction, and friction creates heat. Thus, turbochargers need a cool surface on which to spin with a minimum of friction. Most turbos simply use engine oil for this -- the friction is very low and the oil has the added benefit of cooling the turbo down a bit. Higher-end turbos use ball-bearings, which are more efficient.

If you don't have a ball-bearing turbo, it's very important to let your turbo cool down for at least a minute or two before you turn off your engine. The reason for this is that shutting down the engine while the turbo is hot can cause the engine oil in the turbo to "coke up", or become sludgy and sticky, coating the turbo. It's kinda like cholesterol in your arteries -- if too much builds up, the turbo will stop working and you'll be very, very unhappy. Letting the car idle for a few minutes allows the oil to circulate and cool the turbo down, thus preventing coking. Furthermore, if you use synthetic oil, it will not coke up at all. Synthetics are good.

If you don't have time to sit in your car for several minutes before shutting it down each time, you should invest in a turbo timer. Turbo timers are devices that connect to your car's ignition system and allow the car to run for a few minutes, unsupervised, then shuts it off. If anyone tries to drive off with the car during this time, the turbo timer automatically shuts off the engine. A lot of high-end alarm systems come with built-in turbo timers.

If you take good care of your turbocharged car, it should last you a nice long time. And you'll get the benefit of a blazingly fast car as well. Enjoy.

As has been stated previously, a turbocharger is a two sided device primarily comprised of a turbine wheel fed by exhaust air connected to a separate compressor wheel via a spinning shaft that compresses intake air. More air in the cylinder allows more fuel to be burned by power stroke, producing more power.

It is a common misconception that the exhaust turbine of a turbo is driven purely by the kinetic energy of the exhaust gases. While the kinetic energy of the exhaust flow does contribute to the work performed, the vast majority of the energy transfered comes from gas expansion. As the exhaust pulse exits the cylinder, it is at high pressure and temperature. The relatively high energy exhaust pulse enters the small volume turbine inlet. Passing through the diffuser and into the turbine housing, the exhaust gases expand and cool. The gas expansion pushes against the turbine blades, producing that work needed to drive the compressor wheel.

Turbines are incredibly efficient devices, but only when operated in a rather limited RPM range (turbine shaft RPM, not engine RPM). That is, a turbine/compressor is very efficient at a certain RPM/flow capacity, but once the shaft RPM deviates much, the efficiency drops off significantly. Spun too fast, the turbine blades cavitate and stall aerodynamically. Spun too slow, and the blades don't have enough air volume to compress. As a result, flow drops. This is why it is essential to match the size of the turbine and compressor wheels to the size of your engine. If your engine is too small, it will not produce enough exhaust gas energy to spin the turbine wheel fast enough to make boost. If your engine is too big, the turbo will be too small to produce the required air flow.

As one might expect, the compressed air at the outlet of the compressor housing of the turbo will have an increase in temperature (pv = Nrt). An intercooler is typically employed in the air stream past the compressor to cool the air before it is injested by the engine.

Turbochargers are positive feedback devices. As more boost is made, more exhaust is created, which in turn, makes more boost. Typically a device called a wastegate is employed, manipulated by a mechanical or electronic boost controller to bypass the exhaust gas around the turbine and directly into the exhaust pipe. This regulates the speed of the turbine so that a costant level of boost is produced.

As the turbine housing of a turbocharger is typically connected directly to the exhaust manifold of an engine, it operates at a very high temperature. The shaft housing of the turbo is usually connected to the vehicle oiling system as the shaft bearing rides on a thin film of oil. Engine coolant is also routed through the turbocharger housing to keep it cool.

Turbochargers are conceptually very simple, but in practice are extremely complicated devices. Typical turbine shaft speeds range between 80,000-100,000 RPM. The turbine and compressor blades, as well as the rest of the components and housings, must be precisely machined and balanced. Or the turbo will go BOOM.

Since turbochargers and superchargers cost about the same (around US$3000-4000 for a good kit), other merits and problems must be considered when choosing whether a supercharger or turbo is better for your car. The following are some of the most common pros and cons that turbochargers have:

Pros:

Turbochargers utilize exhaust heat which would otherwise be wasted. This reduces parasitic loss compared to superchargers, which directly derive their power from the engine crank.

Once the turbo lag subsides, turbochargers generally have a flat torque curve until the turbine RPM's get too high

Turbochargers are lighter and more compact than most superchargers

Higher boost levels are possible with turbochargers



Cons:

Turbo lag (the period in which the turbo cannot make boost because the exhaust pressure is too low, as is the case in the moment after starting)

Creates an exhaust restriction

Effectiveness of turbo is highly dependent on cam profile

Are viewed as "import" modifications in some domestic circles (which is part of the reason why muscle cars use superchargers much more than turbos)

Extra plumbing and modifications to the car are needed because engine oil is shared, and the exhaust is used

Engine must idle a minute or so before and after using the turbo to prevent "coking" and excessive turbine wear

Are harder to install

Almost all turbochargers require intercoolers, because they run hotter than superchargers

Miscellanious notes and recollections about turbocharging and forced induction.

As echoed by previous posters, turbo's add what my dad deems unecessary complexity to a car. The main reasons are outlined above, but I feel it necessary to add more.

Notes about turbo'd cars:


note: I consider this last note to be the minimum level of modification you should apply to your turbocharged vehicle. It will cost you aprx. $1000, but the longevity and horsepower gain will more than make up for the initial outlay.

Parable: I own a turbocharged, 1990 Ford Probe GT. This car has been one hell of a learning experience. I comitted the 3 cardinal sins of used car buying, all in the same car! 1) I bought a turbocharged car used. 2) I bought it from a mechanic. 3) It was a salvaged vehice. It should be fairly obvious why these are classified as sins. If not, I have a car I'd like to sell you. :)

Anyhow, the car provided decent performance stock (rated at 145HP @ 4800 RPM, 190Ft/Lbs of Torque @ 3500 RPM, according to my Chilton's manual), and after the addition of the above mentioned components, I felt a marked increase in low end torque and horsepower. (I did not have access to a dyno to confirm this, but going from a 15.5s 1/4mi to a 14.2s 1/4mi, I'd say that's good enough.) Also, I started using synthetic oil. All was indeed fine for about a year after I bought the car in 1997.

Around late 1998, I noticed an increase in problems with blown coolant hoses. I was replacing hoses on a monthly basis, during both summer and winter months. (As anyone who's ever had to shop for parts knows, the odd-ball parts on odd-ball model cars gets harder to find as years go by. When you do find them, they become expensive. Case in point: $17 for a small 3" long coolant bypass hose!) After replacing almost all the hoses, a strage, white, acrid smelling smoke was billowing out of my tailpipe.

An independent mechanic inspected the car, and determined that the bearing seals in the turbo were shot. He readily admitted to avoiding turbos, and suggested I see the dealer. I did, and they replaced it to the tune of $1500 for a used turbo, plus labor.

Not sooner than i drove off the lot, did the engine start smoking again. I took it back that second, and they said I should just let the oil burn out of the catalytic converter. (If you're not shrieking at the horrible mistake of that advice, read on.) So I kept driving it. At the time, I was working out of town, and commuting every day. And no sooner than 2 days after I got back to work, I noticed the smoke again.

I took it back, they had to replace the turbo for the second time. This time, they cleaned out all the oil from the rest of the parts (like the intercooler, intake plenum, etc.), but suspiciously, not the catalytic converter.

After a while, I couldn't figure out why my car kept overheating. I took it to an exhaust shop recommended by a good friend. Upon inspection, the pressure at the top of the cat was 3psi under no load acelleration. This is abnormally high, it should be anywhere from 0-1psi, and no higher. And under load, this kind of back pressure was causing my turbo to glow cherry-red under even moderate to light driving conditions.

I replaced the cat, muffler, and down pipe, and the overheating slowed significantly. But it wasn't gone. As it turns out, the fuel mixture was far too lean, causing abnormally high exhaust gas temperature, and the water pump had started leaking. Combine that with a faulty oil pump, and you can see where this goes.

Moral of the story: Buying a used, salvaged, turbocharged car is a very bad idea. Taking such a beast to a dealership, is an even worse idea. Most of the low-grade tech's that work on cars at dealerships wouldn't know a compressor from an engine block if one fell on their foot.

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