Compression ratios

[Jorrocks]

Can you chaps help me out here?
I am in a state of confusion as regards compression ratio and how it is affected by using aluminium cylinder heads.
My understanding was that if, say, you measured your static compression at 11:1, then with ally heads your actual compression was 10:1. Is that correct?
My engine builder is telling me to go for low to mid 9’s, but surely with ally heads then I’ll be no better off than with the standard 8.2 comp of the factory spec?
The -6cc pistons I have will apparently yield 11.9:1 (10.9:1 when taking into account my heads, no?). That, I assume’s with zero deck height and .040 gaskets. Surely I can leave the pistons “down the hole” by 0.02 (just squaring the decks rather than zeroing them) then with 0.40 gaskets I will be at 11.16:1 (10.16 actual?) with squish still at 0.060 which I understand is still within acceptable limits re. detonation. That should be ok with v-power 99 fuel shouldn’t it, without having to retard the ignition?

[Steve Payne]

That is a lot of compression on modern fuel( even super unleaded) and your engine builder is correct about where you need to be.

Personally I run 9.7 and with standard heads and a mild cam, at high throttle openings I was surprised at how little advance I could run.

With ally heads you can go higher but I don't think that high.

A big cam with a lot of overlap will reduce the compression but I would take your engine builders advice and aim for about 10 to 1 even with the ally heads.

In some cases more is not always better.

You will probably have to run a mapped ignition device to get the best from the engine you are having built.

Steve

[Jorrocks]

I've taken his and your advice! Ordering a set of -24cc slugs which will mean sensible compression with tight squish so "more betterer"!