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Thread: Compression test

  
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    StrnyGSI.T's Avatar
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    Compression test

    Hello guys and gals does anyone know what cylinder compression readings a healthy standard z20let should be any info will be appreciated.

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    Stock compression is 8.8:1...

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    Healthy compression per cylinder is between 160-180 psi i think.
    8.8: 1 is the compression ratio

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    Quote Originally Posted by GGX View Post
    Healthy compression per cylinder is between 160-180 psi i think.
    8.8: 1 is the compression ratio
    Definitely I gave the compression ratio.

    The difference between the compression ratio and the readings you get when checking compression are that if you know the compression ratio you can get a good guide for the expected readings you should have when testing.

    The compression ratio is simply the total volume of the cylinder and the combustion chamber above the piston at bottom dead center [BDC], divided by the combustion chamber volume above the piston at top dead center (TDC).

    If you know the compression ratio already in theory you can work out what the readings should be.

    Normal atmospheric pressure is 14.7psi or 1 bar.

    If at BDC the cylinder has 1bar pressure and then the engine is turned, you can gauge an estimated value for compression readings as 14.6psi x CR = X psi.

    For the Z20LET thats 14.7psi x 8.8 = 129.36psi.

    This is a very simple way to do it and doesn't take into account the "dynamic compression ratio" and all the variables it has such s allowances for valve overlap etc. It should be noted that the dynamic compression will always be lower than the nominal or calculated compression ratio (as above) due to the allowance of intake charge leakage through the open intake valve during overlap.

    Either way, the simple equation above can be used as a general guide for engine health and to see of one cylinder is significantly down on compression..

    Also, with a 8.8:1 compression, are you sure that it should be 160 - 180psi??

    Maybe 120 to 130psi would be closer, but I have never actually measured a Z20LET engine...

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    cheers for the reply guys

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    The z20 i checked yesterday was 145/146 psi across all 4 cylinders.

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    ZLETS are not as high as others from what i have been told..

    mine wer around 120ish and told thats ok for its age etc.

    160-180 imo is no right but im no mechanic
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    mine have always been 170 with standard pistons

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    as long as they are all the same i would guess the egine is pretty healthy.
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    @aussiegman: Thanx for the info. After a compression test my car showed 12,5/12 bar per cylinder. Somewhere here i read about 170 psi.

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