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compression ratio???

Discussion in 'General Tech and Maintenance' started by scottp11, May 24, 2004.

  1. scottp11

    scottp11 The Seniorest Member....

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    I currently go to school for automotive tech, and we learned about doing compression testing. We were taught that if you take the PSI minus 5, then minus 14.7, then divide by 14.7, you come up to your compression ratio, to one. And the same goes for the opposite if you have your compression ratio you can find what your PSI should be.

    Well... I did my PSI on my JDM B16A and came out with 210 PSI for each cylinder.
    If my math is correct, that means my CR is 12.9:1. I thought a JDM B16 CR was 10.4:1 and that would be about 175.5 PSI.

    Was I tought wrong? Help me out if you know... Thanks.
     
  2. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    That's a very inexact way to guess your compression ratio... it depends on your cam profile and a bunch of other stuff too. If you have an intake cam that leaves the valves open after the piston starts moving up, you'll lose compression- you can also gain more depending on how your engine is set up too.
     
  3. HondaCivicSiR93

    HondaCivicSiR93 Junior Member

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    Seems like a guess-timate type figure for a "perfectly" compressed car with absloutley no faults in the engine.
     
  4. scottp11

    scottp11 The Seniorest Member....

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    Whats the formula to find the REAL compression ratio? Thanx for your help!
     
  5. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    (volume of cylinder and combustion chamber @ BDC) / (volume of cylinder and combustion chamber @ TDC)

    There you go.
     
  6. MikeBergy

    MikeBergy Blah blah blah....

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    Mike, you need to invert that... other wise your cr comes out to less than one.
     
  7. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    Whoops- same thing... it's still the compression ratio- it's just not the same convention that everyone's used to. I guess I'm used to dealing with compression as fractions of a whole, since you're making things smaller after all.

    compression ratio = (volume of cylinder and combustion chamber @ BDC) / (volume of cylinder and combustion chamber @ TDC)
     
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