400 stock compression ratio? need help asap

Discussion in 'Engine Topic' started by Baseballstud, Apr 30, 2005.

  1. Baseballstud

    Baseballstud Member

    99
    0
    Feb 12, 2004
    Jax, fl
    ok i have a 400 sbc standard bore with stock gm dished pistons, casting number is 3951511 and head casting number is 3973493 its a 76cc head.. what is my compression ratio?? im putting a 67 cc brodix race rite head.with a fel pro #1044 head gasket. it is .051 thickness. what would the compression be w/ those heads. thanks guys for all the help in advance.

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    71 hugger orange z28, 350, headers, holley street dominator intake, 1.94 heads, q-jet
     
  2. GetMore

    GetMore Veteran Member Lifetime Gold Member

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    Nov 8, 2004
    Patterson, NY
    I don't know, but my guess is that with those gaskets and the dished pistons you can still run 87 octane with the 67cc heads. My WAG is you will go from about 8:1 to 8.5:1
     
  3. night rider

    night rider Veteran Member

    6,401
    3
    Aug 6, 2002
    Bremen, Ga
    My info shows that as a 71 400 with 8.5:1 compression

    w/o gasket change.. The change in head cc will put you at 9.177.

    Can't give you know info about with gaskets cause i'm not sure what stock size was on that engine
     
  4. Damon

    Damon Veteran Member

    12,937
    20
    Nov 16, 2000
    Philly area
    That thick head gasket is going to give back some of the compression that the smaller chamber heads giving you. You're at about 8.3:1 with the original 76cc heads and a .039" head gasket. With the 67cc heads and a .051" gasket you're at about 8.7:1

    If you could use a slightly thinner .039" head gasket (Fel Pro standard 400 rebuilder gasket) you'd be up just shy of 9:1 with the 67cc heads. That's the way I'd go.
     
  5. Baseballstud

    Baseballstud Member

    99
    0
    Feb 12, 2004
    Jax, fl
    with the .039 head gasket would i be able to run 87 octane gas or would i have to increase the octane.
     
  6. Chuck78

    Chuck78 Veteran Member

    2,073
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    Oct 22, 2000
    Columbus, OH, USA
    If you got the block decked, you want a total of around .038"-.045" quench height (thicknes of compressed head gasket + distance piston is down in cylinder at tdc). That is what will enable you to run 87 octane safely.
    Factory head gaskets are the steel shim type, and are thinner. Say the piston is .020 down in hole at tdc, then you would use a .018" up to .025" shim type head gasket to achieve the best quench. When you run a zero deck height (have deck of block machined), then run the .039" gasket. if the piston is .010 out of hole, then run the .052.
     
  7. Damon

    Damon Veteran Member

    12,937
    20
    Nov 16, 2000
    Philly area
    If you're running stock, heavily dished pistons like the 400 came with (about -22cc dish volume) there's not much point in worrying about the quench distance- you're not getting quench with them regardless. And I doubt you're going to bother decking the block and all that since you said you were trying to reuse the stock bottom end as it sits right now.

    To directly answer your question, you can probably still run 87 octane in that motor even with the thinner head gasket (around 9:1 compression). You are putting aluminum heads on the motor which will help quite a bit with detonation resistance and plenty of factory motors are built with 9:1 compression and only still only require 87 octane. You'll be safe. Worst case, you pull the ignition timing back a couple of degrees on the street. When you go racing you put some 89 or better in the tank and turn it back up.
     

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