Quote:I'm thinking it will go down, but it depends on which centered dish piston you use. USA offset-dish and non-USA offset-dish differs.

Ross will make you a set of flattops for about $400/set. Give you 10.9:1 with a standard head gasket. Requires Premium or E85 fuel, but will return better fuel economy so it ends up fuel costs are the same.

A15 Piston
* Three different factory A15 centered-dish pistons
* 2 or 3 different factory A15 offset-dish pistons

Ok guys. I got a more accurate syringe at a feed store and its in ml which is equivalent to cc. Measured it several times. GX head is 28 ml/cc and the piston dish is 13 ml/cc. Can someone run the numbers for me? I really prefer not to use premium gas if I dont have to. But DD makes an interesting point that if I have higher compression it will get better fuel economy which makes it a wash.

You guys may notice I am a novice at all this but slowly picking up alot along the way and will be better prepared for my next build. I have worked on a variety of automobiles for many years but not with the technical components (cam lift/duration, compression etc) of an internal combustion engine. You all must be engineers.......

Use the link on the first page that Thaifrog provided; I checked and using the standard felpro head gasket you are looking at 9-1 compression ratio. If you go to the .6 thick head gasket you are looking at 9.5-1

Tom

Thanks Tom. I can still use regular unleaded fuel with that ratio?

Yes, 9:1 was the standard USA compression ratio for 1971, and spec'd for 87 R+M/2 Octane.

If you go with an open combustion (type C) it might ping or run-on, but with the closed chamber (type A/B), no worries

Block is being decked 5 thousandths and GX head was shaved 5 thousandths. Any idea what my compression ratio will be? Piston dish is 13ml/cc and GX chamber was 28ml/cc prior to shaving.

This CR calculator will tell you: http://www.csgnetwork.com/compcalc.html , you'll need to measure the head CCs again. For a rough estimate, subtract about 1/2 of 5/1000" from the gasket thickness

I can't remember what motor you have in the car but somewhere between 8.6 and 9.1 dependent on A14 or a15.

Tom

Previous estimate for his engine was 9:1 to 9.5? ... CR will be higher after shaving the head and block