How does cylinder bore affect compression



Something just dawned on me today.

When I had my '73 250 head milled for my 65 block, they milled it to 53cc's (this was less than I asked them for, BTW). They told me that it was milled an additional .25 for gasket, but I don't believe them. I'm just assuming they went to 53cc's.

Anyway, when they pulled my old head off the bottom end had already been rebuilt with .40 over pistons.

Do those pistons increase or decrease my compression?

Assuming that everything else is on the block is the same as stock, what inputs shoud I use on the compression calculator?

The over bore increases compression.

You need to go to my web site in my signature and click on "Compression Calculator" Go to the bottom and click on "Calculate". The engine presets will show 199 cubic inches and 8.7 compression. Now add .040 to the 3.68 bore. (3.72) Click on "Re-Calculate" and you'll see 204 cubic inches and 8.8 compression.

Good Luck
Thanks Dennis.

Assuming that he did not mill any more to compensate for gasket thickness and I used a fel-pro, then I should put .45 for gasket thickness, correct?

Last question - my engine builder mentioned that increasing the intake valve size would also increase compression. I may have misunderstood what he said, but is that true and if so, how would that work? I don't know where in the process he cc'd the head (before or after the installation of the larger 1.73 intake valves)

Alos -- see my post below regarding hardened seats on my D3 head.

Yes, use .045 for the gasket.

When you put in new seats and valves they tend to sit higher than stock, thus taking up more room in the combustion chamber and making the cc number smaller.

Don't ever worry about asking questions. If you ask it, there are a bunch of folks or there reading your questions and wondering the same thing.

Good Luck
A little applied physics,
There is static CR and there is dynamic, what you're calculating is eqv to static. When the auto mach said bigger valves increased CR he may have been referring to dynamic, the actual CR when running. With bigger valves it would increase cylinder filling during the intake cycle. Same with cam design/ duration/ timing. When speaking of an engine volumetric effeciency that is referring to % of cylinder filling when running. It is usually around 80% but it is possible to be > 100% at a specific point where the optimum cam, intake ram effect and header scavenging concide.
Then there is superchargering, static could be i.e. 7:1 but on boost the dynamic could be 11:1. ;)