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8.9 CR 4AGZE pistons in my AW11

Posted: Mon Jan 15, 2018 12:42 pm
by Alex170984

Im in the process of rebuilding my 4AGZE engine and I have replaced my 8.0:1 pistons with the later 8.9:1 pistons.

Is this going to be a mistake?

Cheers for any advice


Re: 8.9 CR 4AGZE pistons in my AW11

Posted: Mon Jan 15, 2018 3:53 pm
by jondee86
Raising the static compression ratio on a supercharged engine raises the
EFFECTIVE COMPRESSION RATIO as shown on this chart...


So @ 10psi of boost, going from 8.0:1 to 8.9:1 has the same effect on the ECR
as increasing the boost by around 3psi. As the ECR rises, you get a little closer to
the ragged edge, and you need to adjust your tune (fuel and ignition) accordingly.

Cheers... jondee86

Re: 8.9 CR 4AGZE pistons in my AW11

Posted: Fri Jan 19, 2018 2:57 am
by Alex170984
So it's just down to timing?

Re: 8.9 CR 4AGZE pistons in my AW11

Posted: Wed Jan 31, 2018 3:47 pm
by jondee86
Sorry... thread got buried before I notice that you had replied.

Yes, down to timing, or more correctly, to the tune. As the ECR goes up you
get closer to the point where detonation will start to occur. Yout tuner will look
at ignition timing, intake temperature, fuel mixture and octane rating etc, to
determine how much boost you can run before you get into the danger zone.

You will notice that high compression factory N.A. engines can only accept a
small amount of boost when fitted with a aftermarket supercharger or turbo.
Typically 6-8psi without internal modifications to the engine. If you want to run
20-25psi you need to drop the static compression with low comp "turbo" pistons,
thick head gaskets and other mods to keep the engine out of detonation.

The table I posted above shows how the relationship works. Bear in mind that
two otherwise similar engines with different static compression ratios, should
make the same power if they have the same ECR. There is only a problem when
trying to run high boost together with high static compression. It can be done,
but building an engine for that is best left to a race engine specialist.

Cheers... jondee86