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Join Date: Oct 2017
Location: Traverse City, MI
Re: HP/Torque ?
Flattop and open chamber heads get you 7.8:1 - the original with a dish would get you 7.3:1 even though it was advertised at 7.8:1 - bad habit GM has used over the years, either rate the compression with a smaller dish (or flattop) piston than what is actually used, or rate with a smaller chamber than what is used.
The standard 350 used in the 1970's and early 1980's was rated at 8.5:1 when the actual compression was 7.8:1. My 1982 Corvette was rated at 9:1 (actual 8.5:1), 1969 Grand Prix 400 rated at 10.5:1 (actual 10:1), 10.25:1 Olds 455 is actual 9.5:1..... you get the idea.
Your open chamber heads depending on where you get your information are listed as either 113cc chambers or 118cc chambers - most actually measure around 121cc chambers. If you use a .025" stock deck clearance using a flattop piston and about a .025" shim gasket you come up with a whopping 7.8:1 compression (real world). If you want 9:1 you need to go with about 100cc chamber head.