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Thread: Compression ratio help

  1. #1
    Rubths
    I just had a 454 rebuilt and bought some merlin heads that were cc'd to 123 and I went with a very slight dome piston, I dont have the engine specs to share with you but I was hoping that someone could give me a rough estimate of my compression ratio from the motor compression. Each cylender is at 185 what do you think?

  2. #2
    1BIGJIM
    YOU BETTER BE RUNNING PREMIUM FUEL!

  3. #3
    565edge
    to get the right comp ratio we need to know if the block was decked,or how far the piston is in the hole,what pistons and how thick your head gaskets were.I had a 565 inch chevy built that was supposed to be 14 to 1 comp,after i had it rebuilt i found the block not being decked,the heads were supposed to be 118 they were 123cc and the thickness of the head gasket put it at 12.9 to 1.the new motor after decking the block,cutting the heads to 114cc makes 14.8 to 1,my pistons were .035 in the hole until we zero decked it.it sound like you have big cc heads and with a small dome your probally around 9.5 to 10.5 comp ratio.

  4. #4
    Rubths
    1BIGJIM:
    YOU BETTER BE RUNNING PREMIUM FUEL!Is 185 considered high compression? When you say premium fuel do you mean 91-92 octain or 100+ octain?

  5. #5
    Rubths
    Where are all you gear heads when I need your input?

  6. #6
    Hotcrusader76
    I am sitting at 128psi all the way around + or - 1-2 psi. I know I am around 9.5. But I concurr with the later response. You need to know deck height, piston specs, head gasket thickness, amount of head cc's, etc. Otherwise its just a guessing game.
    Keep the Premium fuel in your boat. 91-92 octane with maybe a 10:1 mix of VP or Trick to be on the safe side. I run that in mine (probably overkill, but better than being killed) all the time.
    Good luck bro!
    Ty

  7. #7
    Infomaniac
    Rubths:
    Where are all you gear heads when I need your input?
    Your Avatar should answer that

  8. #8
    058
    Cranking compresson does not have anything to do with Compresson ratio. Camshaft profile is the biggest influence to cranking compresson. An 8.5 to 1 c/r engine can have 180 lbs. of cranking comp. and a 12.5 to 1 could have 125 lbs of crank. comp. It mostly depends on the cam profile.

  9. #9
    Jordy
    If you know what pistons were put in the motor, as far as dome cc and bore size, ie. 4.25 for a stock 454, 4.310" is .060 over, 4.35" would be .100 over, along with your heads that were cc'ed at 123, you should be able to find out from any speed shop, or even online at the piston builder's web site. They have it figured based on bore for just about every common cc chamber head from 103cc on up. This would at least put you in the ball park. If I had to guess based on the information you gave though, with a 123cc head and small domed piston, I wouldn't think much more than 10:1, if even that. Probably closer to 9:1.
    Jordan

  10. #10
    Liberator TJ1984
    I agree with Jordy..I put TRW .095 dome pistons in my 454 w/119cc heads, fel pro gaskets.039 compressed..comes out to 8 3/4:1 w/pistons .010 in hole...try to get some more info if ya can...

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