390 Rebuild
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390 Rebuild
I am planning on rebuilding my 390 in the near future, and one of my main concerns is having a compression ratio that I can handle. I want to be able to run pump gas and not have it ping. A compression ratio in the 9.6:1 range should allow me to run 93octane and not ping, right? Anyway, one question is how do I get to that ratio? I have a C4AE heads, so I believe those have 73cc combustion chambers...so I can plug in the numbers and figure out how thick my head gasket would have to be in order to get to my desired compression ratio...but how did Ford manage to lower their compression ratio for the truck 390? After all C7AE or C8AE heads have smaller combustion chambers, so they should have even higher compression than a 64 390, but of course they don't. So my ultimate question is, what's the difference between the car 390 and the truck 390 that yields a different compression ratio?
Dan
Dan
Cargo Master
Joined: Aug 2000
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From: SW Michigan
390 Rebuild
Dan, to lower compression in the truck 390s, Ford used leftover 410 mercury pistons. The 410 mercury piston has a pin hieght of about 1.65 instead of 1.76 like a regular 390. This lowered the compression to about 7.8 to 1. If you build your engine with the car pistons (silvolite 1130s) and felpro gaskets (.041 thick ), you should have about 9 to 1 with the C4s. DF
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Joined: Aug 2001
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From: Collierville, TN
390 Rebuild
Actually, Silvolite 1130 pistons would yield only 8.4:1 compression in a .030 over 390 with 73 cc heads and a .041" gasket. That's because they have a dish of 19 cc. Silvolite 1131 pistons aren't dished, though they have valve reliefs totalling 8 cc. With 1131 pistons in a .030 over 390 with those chambers and gasket, you'd have a compression ratio of 9.3:1.
I'd go with the 1131's if you don't mind using hi-test or if you think you may switch to aluminum heads someday.
I'd go with the 1131's if you don't mind using hi-test or if you think you may switch to aluminum heads someday.
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390 Rebuild
Yeah after running the numbers I was getting a compression ratio of around 9:1 with the pistons I have. I actually wouldn't mind being a little higher and running high test. Felpro head gaskets are .041, but weren't stock ford steel head gaskets like .018? If my pistons didn't have valve reliefs quite as big that would help, however I roughly cc'd them and they look to be about 14cc. I just can't justify getting new pistons though...so maybe I should just stick with 9:1.
Dan
Dan
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