Turbo boost versus compression ratio ?

[Jörgen Karlsson]

> is there a way to explain
> this issue clearly enough for the layperson,

I can make a try.

The colder the intake charge is the harder it has to detonate.

	Better intercooler = good
	Another benefit is that for every 27degC? you lower the intake temp you get
10% more air into the engine.

A lower effective compression ratio mean less risk of detonation.
Effective compression ratio can be lowered by decreasing boost or static
compression ratio. Lowering static CR takes a little power but raising boost
adds a lot of power. For max power the static compression ratio should be as
low as possible with as high boost as possible to maintain the same
effective compression ratio. BUT Lowering CR the wrong way can make the
engine detonate even easier then before at the same boost! One example is to
make custom pistons that is a copy of the stock piston but with a shorter
distance between the pin and the top or a thicker head gasket. Even if a
thicker head gasket works good on a Honda B16A, that can run .5bar of boost
with a static CR of 10.2:1!

But the smartest mod that I know of is water injection, it cools the intake
charge and continues to cool the combustion after that. It is very effective
at suppressing knock.

I think that on the 200SX the proper order of detonation suppressing mods
should be. Water injection, intercooler, lower compression ratio. When you
need to lower the compresion ratio you probably need stronger internals
anyway.

Jörgen Karlsson
Gothenburg, Sweden

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