It is the effective compression ratio when supercharged that is important. This should not be higher than about 9.5:1 for most cars. If the boost pressure is say 6 psi, then the un-supercharged compression ratio should not be greater than 8.0:1. For 8 psi blower pressure it will need to be no more than about 7.7:1, and so on.
Taking the case of a blower pressure of 6 psi , the maximum 8.0:1 un-supercharged CR, plus the engine capacity, dictates the minimum head volume which is acceptable. From this, assuming flat-top pistons and a standard head gasket, you can quickly determine what minimum head thickness is required.
It will not be possible to use domed pistons with a blown XPAG.
Mike Card
TC8233, TC9477
----- Original Message -----
From: Chip Old
To: MG-TABC
Sent: Monday, September 23, 2002 5:46 PM
Subject: Re: [mg-tabc] Parts for sale
The deciding factor when fitting a supercharger is not whether or not the
head is late or early, and not whether the pistons are flat or domed.
What matters is what the compression ratio is. Back in the bad old days,
racers trying to get maximum horsepower out of the XPAG ran supercharged
engines with insanely high compression ratios. For a reliable
supercharged street engine you want to keep the compression ratio fairly
low.
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