Compression question

Discussion in 'PSI Superchargers Tech Questions' started by Joe Percoco, Dec 18, 2003.

  1. Joe Percoco

    Joe Percoco Member

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    In figuring compression ratios, how do you determine the deck height volume on a Hemi? I usually plug in "0" into the equation, but if the piston is above the deck or below the deck at TDC then this needs to be taken into account. For a flat topped piston it's east to measure where the piston is in relation to the deck at TDC but on a Hemi it is pretty tough. Any ideas?
    Joe Percoco
     
    #1
    Joe Percoco, Dec 18, 2003
  2. mitch myers

    mitch myers Guest

    pour ir
     
    #2
    mitch myers, Dec 18, 2003
  3. Bob Holley

    Bob Holley New Member

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    Put the piston down one inch. Measure cc's to top of deck and subtract this volume from the calculated cc's of one inch down by your bore size and subtract measured cc's from calculated. This will give you the dome volume. You can figure the rest out.
    Hope this helps.
     
    #3
    Bob Holley, Dec 19, 2003
  4. Joe Percoco

    Joe Percoco Member

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    I finally realized the answer to this question is that there is no deck height volume. With a full hemispherical piston there is no flat above the ring lands like there is with, say, a rat motor. If the piston is up or down in relation to the deck it dosn't matter - it all becomes dome height and dome height volume. Plugging in "0" is correct in the calcs.
     
    #4
    Joe Percoco, Dec 20, 2003
  5. T.Howell

    T.Howell Member

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    >>"If the piston is up or down in relation to the deck it dosn't matter"<<

    You might want to rethink that Joe.
     
    #5
    T.Howell, Dec 21, 2003
  6. Bob Holley

    Bob Holley New Member

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    Joe,
    EAsy way!!!
    CC at bdc---cc at tdc
    divide top into bottom
    This is the easiest way with domed hemi piston.
     
    #6
    Bob Holley, Dec 21, 2003
  7. Joe Percoco

    Joe Percoco Member

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    Bob, the only problem with that is it makes it tough to compare ratios with different head gaskets. T. Howell, how does it effect it? If the piston is short of the deck by .010", there is no deck height volume. Isn't it all just dome volume?
     
    #7
    Joe Percoco, Dec 21, 2003
  8. Bob Holley

    Bob Holley New Member

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    Joe,
    Use any headgsk you want to. It is easy to figure the cc increase or decrease in combustion chamber with diff. thickness head gsk. The total cc do not change. As a rule of thumb. .010 will change compression ratio by .333. Use the formula in the back of the NHRA rule book to figure the cc's of gsk.
    I built an injected fuel motor from scratch. Had to figure compression height of piston, rod length, and head gsk thickness. It came out exactly what I figured. I also had a prob. for awhile with the deck height as the piston dome extends above the deck. The only way to figure the piston dome volume is the way I described earlier by moving the piston down one inch below tdc and ccing. Use the math formula to figure the cc of a cyl your bore size using inch length.
    The diff. is your piston dome volume in cc's.
     
    #8
    Bob Holley, Dec 21, 2003

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