# Compression?

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

1. ### Tshane1/2 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
/forums/images/graemlins/truck.gif

2. ### HarryH31 ton statusAuthor

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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. ### HarryH31 ton statusAuthor

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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. ### TopOff1/2 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!

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