750 compression ratio

Hello Guys,
I'm in the middle of a engine rebuild on my 66 Atlas. I am shooting for a final compression ratio of 9.5 to 1 and trying to figure out if simply not using the cylinder base gasket will put me where I need to be or if I will have to skim the head as well. I'm using Commando 8.9 to 1 GPM pistons but as mentioned looking for a final compression ratio of 9.5 to 1. Does anyone know what the relationship of head material removed to increased compression is, or what removing the stock base gasket on a 750 barrel increases the compression by? Also I seem to recall someone mentioning that the compression ratios given by piston manufactures aren't always accurate (less compression than claimed) has anyone experienced this? Thanks in advance for any help!

At some point it may behoove you to measure the compression, if you want the most accurate number.

If you want an accurate answer, its best to start with what you have now, i.e. the head may have been skimmed in the past. A good auto engine shop should be able to measure your TDC volume and calculate the actual achieved compression ratio and advise the neccesary deck height to alter it. Another alternative which may be quite applicable is to assemble the engine and do a compression test. It will give you the PSI in each pot and if its over 150PSI, you're in good shape and it can be reverse calculated to estimate the compression ratio. You may be able to find an indicative calculator on the web which may help, but I'd stick to going to an auto head shop that has the right equipment to properly measure volume and make any neccsaryy adjustment, such as equalising combustion volumes and deck heights.

Mick

I have an Excel chart I did with various gaskets, base spacers etc. It's about 0.3:1 drop with Combat head on 750 and flat topped Hepolite pistons. That's with std chamber volume and head gasket. Very roughly on mine it's a 0.1:1 reduction for every 005" increase in gasket/spacer thickness etc. so for 0.6:1 increase roughly that's lose the base gasket (0.016" appx) and take 0.016" off the head. I can send you the file (PM) if you want but you really need to know your own chamber volume for good accuracy. Easy enough to measure with a piece of flat acrylic or glass drilled to accept a syringe/pippet, some grease to seal. Valve cutouts should also be accounted for but I didn't.