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Compression?

Discussion in 'The Garage' started by Tshane, Feb 11, 2004.

  1. Tshane

    Tshane 1/2 ton status

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    Today I check the compression of my engine and the tool I was using measured it in lbs per sq. in. I was wondering if there is a formula to change that to a proportion (ex. 10:1).

    Thanks
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  2. HarryH3

    HarryH3 1 ton status Author

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    At sea level, the atmospheric pressure is 14.7 PSI. So let's do some math:

    If your compression tester showed 147 PSI, then 147/14.7 = 10 would give you roughly a 10:1 compression ratio. 135 PSI would work out to 135/14.7 = 9.18.
     
  3. HarryH3

    HarryH3 1 ton status Author

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    BTW, there's a fudge factor that will vary from engine to engine, so don't expect an exact figure. Due to various cam timing, valve overlap, etc. the pressure from the compression test is not a perfect repesentation of the mechanical compression ratio. A really radical cam will show low compression pressures simply because it has so much valve overlap. This causes some of the air to be expelled from the combustion chamber before the valves are fully closed. /forums/images/graemlins/eek.gif
     
  4. TopOff

    TopOff 1/2 ton status

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    Of course, if you spun the engine faster, I am pretty sure you can make the PSI come up too!
     
  5. HarryH3

    HarryH3 1 ton status Author

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    [ QUOTE ]
    Of course, if you spun the engine faster, I am pretty sure you can make the PSI come up too!

    [/ QUOTE ]
    Not by much. Spinning faster won't result in much of an increase in PSI because the compressed air is allowed to escape through the exhaust valve during stroke 4 of each complete cycle. You could probably see some increase with a radical cam if you could get the engine to spin at say 5,500 RPM. At that point you would be in the sweet spot of the cam, where the combustion chamber is able to pack in more air due to a mini "ram air" effect of the long valve overlap.
     

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