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      06-30-2011, 09:15 PM   #107
Kinzer S13GT
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Originally Posted by Needsdecaf View Post
What are you talking about gasoline engine? I'm not sure exactly what your question is?

Self detonation,as was said, is not only about manifold air pressure. You can have self-ignition in a naturally-aspirated engine. It's all about the heat of the fuel, and the fuel igniting from temperature vs. spark. Temperature can change with increased intake air temp, increased fuel-air ratio, changes in compression ratio, changes in manifold pressure, etc. There is just too much going on to look at one component - otherwise anyone could tune an engine.

Take this gross oversimplification with a grain of salt, but it will help you to unerstand: the engine is a pump. The greater volume of fuel/air mix it can pump through at a given rate, the more power it can make. Now, you can increase that pumping rate several ways:

1. Speed up the pump.
2. Increase density of fuel / air mix (i.e. more fuel / air per given work cycle of the pump).
3. Increase the physical capacity of the pump.

#1 is accomplished by simply adding more RPM. Obviously there is a limit to how fast all the whirly bits can spin around without coming apart.

#2 is accomplished by forced induction. Supercharging and turbocharging (really exhaust-driven supercharger to be correct) increase the density of the fuel / air mix in a given volume of intake gas by compressing it. Therefore as the engine moves it's fixed volume of gas per cycle, more fuel / air mix is passed through.

#3 is a fancy way of saying increasing displacement of the engine.

Now, there are a lot of factors that influence design decisions. One example is bore vs. stroke, which will dictate the usable RPM range and the torque characteristics coming off the engine. That's why a ship diesel has connecting rods the length of a broom handle and an F1 car's is the length of a toothpick. There are literally hundreds of other parameters that will influence the design and the outcome. '

Hope this helps!
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