Join Date: Jan 2009
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1st off, boost is just positive manifold pressure. More air in the combustion chamber yeilds more power, a more complete burn and therefore lower egt's. One way off accomplishing more air in the combustion chamber is through more pressure in the manifold, or more boost as it would be. Another way is through cylinder head porting or with a cam, which will increase air flow, but lower pressure.
Turbos are like pumps, they don't make pressure, they flow a substance in the case of a turbo that would be air. The resistance in the manifold and against the valves is what creates the pressure. Larger turbos flow more air, which results in higher boost.
However more must be taken into account. Compressed air equals heat, hot air equals lower density which contains less oxygen and results in a less complete burn, more smoke, less power and higher Egt's. Air flowing throw the compressor housing on a turbo gets compressed and the heat increases. There is a point on a turbo where it is no longer effective or not nearly as effective because the air it is providing is too hot and minimal power gains are noticed at the cost of severly increased EGT's.
So in short, the larger turbo provides cooler air which yeilds more power at the same boost level as a smaller turbo that is out of its effective range, and also has a higher effective range because it is able to flow more air and therefore effectively create more boost. There are many other things that go into a turbos effective map as well, compressor wheel deisgn, size and blade count all play major parts in providing the right set up, and the exhaust side is very important as well. Properly pairing your turbo to your fueling mods is very essential, more cool boost will allow you to effectively burn more fuel which the end result is more power.
Anyone feel free to correct anything you see wrong. Hope this helps,
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