Sadly, there are alot of people in the tuning world that don't know how this stuff works. I decided to do a quick rundown on forced induction and exactly what this entails. If you are new to the tuning world, I would highly recommend that you read this, the whole way through. And if you're not new, you still might want to read this. You might learn something new.

Turbocharger's are a method of increasing the power of an engine via forced induction. A engine makes a certain amount of power, and with each stroke draws in air which is then compressed, ignited and exhausted. This is how your car makes power. With the addition of a turbocharger, the power level can rise a huge amount, but only if it is done right. Turbochargers "boost" It compresses air and then pushes it into the engine. When it comes time for the ignition, there will be more air in the cylinder, and (if done properly) more fuel to compensate. More fuel and air in the cylinders will create more powerful explosions and give the engine more power. But how do we get this extra air?Turbocharger does it for us. The turbocharger is a fairly simple device, and has only a few parts. The housing, err...houses all the internal parts. A turbo is bolted up to your exhaust manifold, and then to a downpipe. Why the hell would you want a turbo in the way of your exhaust? Because it gets its power from the exhaust. A turbine is affixed to a rotating shaft and is positioned so that all the exhaust gases coming from the engine have to go through the turbo. Imagine how a steam boat works, hot air pushes against the wheel which moves the boat. The turbine moves in a similar way, the exhaust gases all travel through the turbo, and in the process spin the turbine over 100,000 RPM. After the gas has pushed through the turbine, it exits into the downpipe which is just a....downpipe connecting to your cat (if you run one). Now, spinning the exhaust turbine alone does nothing for power. This is where the other side of the turbo comes into play. The other side of the turbo has a compressor wheel, which compresses incoming air. The compressor wheel is attached to the same shaft as the exhaust wheel, but on the opposite side. So what happens when the exhaust wheel is spun? The compressor wheel will have to rotate along with it since it is affixed to the same shaft. Now, the compressor rwheel will take the incoming air (the "cool side of the turbo gets the air from your intake. Your intake runs directly to the turbo.) and compress it. The compressed air is then sent through your piping and one way or another ends up in your cylinders. As you can recall, the power is made by air in the engine, and the turbo forces air into the engine. The air is considered forced because it is more than your engine would take in with natural aspiration.
With the expansion of any gases, heat will rise, and this stays true for turbochargers, too. After the air has been compressed, it will exit the turbo into the piping to your engine and will expand in the process...the hotter the air the less dense your flow will be. This is where a wastegate and intercooler comes into play. A wastegate is essential for a modern turbocharged car. A wastegate regulates the amount (measured in PSI (pounds per square inch) or in bar (a ratio to atmospheric pressure. 1 bar is ~14.7 psi at sea level))
As long as there is sufficient exhaust to spin the turbines, the turbo's shaft accelerates, building more boost. A wastegate will open when he boost reaches a preset level. On DSM's it is 10-12 stock. The wastegate triggers, and a connecting arm will open a flapper, imagine if you will a small hobbits door or something similar. With this flapper open, some of the exhaust bypassed, which prevents the turbine from gaining speed, thus regulating boost.
Intercoolers are essential for most turbocharged cars, but are not fully NECESSARY for cars boosting very little (eg, Honda pushing 5~6psi doesn't need one but it would help). Since the gas has expanded due to the heat created in compressing the air, it will be very hot. If the air is too hot when it enters the cylinder it can ignite without the spark plug firing. This is bad for your engine and bad for performance. A intercooler is placed somewhere in the tubing from the turbo to the engine, usually midway. The intercooler then cools the charge by radiation of heat energy through the fins of the intercooler and with the cool air from the outside passing through the fins.. As the air goes through, it cools off and contracts, becoming denser. Denser air means you can squeeze more air into a cylinder, so when the air fuel mixture ignites, the power from expansion is bigger.

When you shift gears, the throttle body slams shut, not allowing any more air to enter the intake manifold for the duration of the shift. However, your turbo is still boosting because you still have exhaust flow. If the air were to exit the turbo, rebound off the throttle body and travel back towards the turbine, it would attempt to force the turbo's compressor wheel in the opposite direction that is was spinning. If air does make it back to the compressor wheel, it can cause damage to your wheel, or break it. The wheels are made of alloy's that are made to widthstand extreme temperatures, but are not fracture proof. The cure for this is a Blow off valve, or compressor bypass valve. A blow off valve is placed somewhere after the intercooler and before the throttle body. When you shift, the blow off valve opens and redirects any air heading back to the compressor wheel to the intake. BOV's can also be vented, which means instead of returning the excess air to the intake, it is simply vented to the atmosphere. On a stock car this can cause problems like stalling. The best fix is a maf translator and gm maf, but I will not go into these here. If you are stock, don't vent but if you do you may some problems.

This concludes the basics of turbocharger section of this article. Great sites to find out more on how turbos work.
www.howstuffworks.com
www.vfaq.com ----great site. It's a must
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