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Thread: Determining compression ratio

  1. #1
    78Eliminator
    So I have been asked on several occasions what my compression ratio is. As a matter of fact, I am damn curious myself. Since the previous owner had no information to offer me, how can I determine this? I was told by one member to get a casting number off a piston. If I can't find one, what is an alternative method? I need specifics here....
    Thanks
    MC J

  2. #2
    Dennis Moore
    The compression ratio is determined by cylinder combustion chamber size (they vary with manufacturing tolerances between heads even with the same casting numbers) head gasket thickness and diameter, piston dome/valve relief size and the height of the cylinder block (again this varies with manufacturing tolerances, some are taller than others even with the same casting numbers). The best way to figure ratio is to measure all of the different dimensions of the parts involved.
    More importantly is the question "what is my cranking compression with a compression tester?"
    A low compression ratio engine with a small duration camshaft will have the same compression as a high compression ratio engine will have with a long duration camshaft. If the camshaft closes the intake valve early (small duration cam) the compression cycle will start sooner and build more compression. If the intake valve closes later (long duration cam) the compression cycle starts later and will not build as much compression. Matching the camshaft and compression ratio so you have around 180 PSI of cranking compression will give you an efficient engine for use with premium pump fuel. Keep it down to 160 PSI for regular pump fuel.
    Hope this helps someone!
    Sincerely
    Dennis Moore

  3. #3
    78Eliminator
    Dennis Moore:
    The compression ratio is determined by cylinder combustion chamber size (they vary with manufacturing tolerances between heads even with the same casting numbers) head gasket thickness and diameter, piston dome/valve relief size and the height of the cylinder block (again this varies with manufacturing tolerances, some are taller than others even with the same casting numbers). The best way to figure ratio is to measure all of the different dimensions of the parts involved.
    More importantly is the question "what is my cranking compression with a compression tester?"
    A low compression ratio engine with a small duration camshaft will have the same compression as a high compression ratio engine will have with a long duration camshaft. If the camshaft closes the intake valve early (small duration cam) the compression cycle will start sooner and build more compression. If the intake valve closes later (long duration cam) the compression cycle starts later and will not build as much compression. Matching the camshaft and compression ratio so you have around 180 PSI of cranking compression will give you an efficient engine for use with premium pump fuel. Keep it down to 160 PSI for regular pump fuel.
    Hope this helps someone!
    Sincerely
    Dennis MooreMy cans are all at 205-10 except one that is like 179...

  4. #4
    gnarley
    [QUOTE]Originally posted by Dennis Moore:
    Matching the camshaft and compression ratio so you have around 180 PSI of cranking compression will give you an efficient engine for use with premium pump fuel. Keep it down to 160 PSI for regular pump fuel.
    Hope this helps someone!
    Sincerely
    Dennis Moore

    OK so how does 180 at cranking with a blower compare to 160 cranking & a blower??? Then throw in the % percent of the drive or additional boost that could excced the cranking compression, what are your thoughts Dennis?

  5. #5
    Dave F
    To answer your direct question, the proper way to find out your compression ratio is to measure it with water.
    You must remove the head and measure the amount of mililiters of water the combustion chamber of the head will hold with the valves installed of course. That will give you the amount of cc's. 1cc holds 1ml of water.
    Do the same with the cylinder with piston at TDC and the head gasket should have it stamped on the package. Add them up and look it up. I do have a formula laying around here somewhere, that I (of course) cant remember right now. When I find it I'll let ya know.
    Anyway thats how to determine the comp ratio of your engine.
    DAVE

  6. #6
    Senior Member
    Join Date
    Jun 2009
    Posts
    10,871
    Heres a link that both tells you exactly what it is (simple statement) and how to calculate it...
    http://www.turbofast.com.au/TFcomp.html

  7. #7
    Dennis Moore
    Dear Gnarly,
    Tough question, that borders on a question that only an engineer could answer. I hope that we can get someone to respond to it!
    Dennis moore

  8. #8
    058
    Dennis Moore:
    Dear Gnarly,
    Tough question, that borders on a question that only an engineer could answer. I hope that we can get someone to respond to it!
    Dennis mooreI'm not an engineer but I think I can answer this, Static C/R + Boost [lb/sq/in]= Dynamic C/R. wink

  9. #9
    Hotcrusader76
    058:
    Dennis Moore:
    Dear Gnarly,
    Tough question, that borders on a question that only an engineer could answer. I hope that we can get someone to respond to it!
    Dennis mooreI'm not an engineer but I think I can answer this, Static C/R + Boost [lb/sq/in]= Dynamic C/R. wink Nice!....

  10. #10
    jslimjeff
    OK yes you can do it with water but the way you can do it without removing the heads.
    Titl engine on side so spark plug hole it directley up.
    remove that cyl. lifter. goto TDC adn measure the amount of oil it takes to fill the cavity.
    write this number down. Then move engine to BDC and continue filling with oil. ANd this amount to the first amount and write that number down.
    take the 2nd number ( Bigger ( and divide it bvy the 1st ( Smaller ) the answer will then be the first number in the compression @@ to 1.

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