Hi all,

It's been a while! I've been looking at a 347 short motor that had AFR185s on it with 72cc combustion chambers. With that setup it had compression of 10.8:1. Now, if I put my 61cc chambers what would the comp be then? I found a compression calculator and had a few goes to get compression of close to 10.8 with 72cc chambers. I then changed it to 61cc and the result was 12.5:1. Am I correct in what I've done? Would 12.5:1 be too steep? This is to go into my EL XR8 which gets a fair bit of normal street use, although it never sees anything less than 98 octane fuel.

Can anyone help, please?

Cylinder swept volume + Combutstion chamber volume = Total cylinder displacement

Total cylinder displacement / Combustion chamber volume = Compression ratio

You could run with the 12.5:1, but be careful. Personally, Id say 10.5 is plenty for a drive car.

But you have to also think about the volumemetric efficency of the engine. Reason I say that is that its highly unlikely that you will get 100%, even with all the top kit. Id work it out on 95% which is a bit more realisitic. This would give you C/Rs of 10.3:1 & 11.85:1.

But at the end of the day, your car, your money, your call.

I would stay under 10.8 Scaff.
BTW the best comp calculation formula is found in the Ford racing catalog along with lots of other excellent info on things like injectors and fuel pumps.

Thanks guys. Well, it looks like I'll have to can that idea. Cheers fellas.

This is correct, but you should also take into account gaskets, notching in pistons, and the volume (if any) depending on how far down the bore your pistons sit at TDC.