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Does anyone know how to figure out your compression off your bore/stroke and a PSI reading off a compression tester?
 

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Not too simple

Compression ratio = total volume : compression chamber volume

or

Compression ratio = (swept volume + cylinder head volume) : (cylinder head volume - piston crown volume)

With a flat topped piston the swept volume is (bore/2) squared X pi X stroke. If the piston has a pop up or crown that volume must be subtracted to get the actual swept volume.

The combustion chamber volume is the cylinder head volume - the volume of the piston pop up or crown (if there is one).

On something like the SV you need the volume of the piston crown, the volume of the cylinder head, the bore and the stroke.

The reading you get on the compression gauge is affected by how well the rings are sealing and by the intake efficiency.
 

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We realize the reading from the compression test isnt exactly the way to calculate the compression ratio...but I figure there has to be some rough estimate. I mean, if you pull 160 psi on an engine that has a bore of 81mm and a stroke of 63mm there should be some way to estimate compression ratio. I mean, if you have a stock bike that is 11.5:1 compression and you threw a guage on it, what would you get?
ac
 

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theoreticly, you multiply your compression ratio by atmosperic pressure 14psi± and you should come up with the pressure you get doing a compression test regardless of bore & stroke
 

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How accurate?

Not too terribly accurate. Things that will affect the reading on the gauge includes humidity, ambient temperature, barometric pressure, how fresh your battery is (cranking speed, faster = less ring leakby), number of compression cycles in the test. It's a rule of thumb.

There is little practical value in calculating the theoretical compression ratio unless you are making modifications to one of the key parameters (bore, stroke, cylinder head volume, piston crown volume, intake and exhaust tract efficiency). If you are making one or more of those changes, what is your target? Theoretical compression ratio can fool you. For example, how many threads have you seen on this site claiming that there is no way regular gas will run in our engines because of the high compression ratio?
 

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avc8130 said:
Thanks Randy. That was basically what we were looking for. How accurate is that? Do you have a proof for that?
ac
not terribly, but consider it if you doing a compession test on a mountain top, or higher elevation

example (numbers are only a guessed example

at sea level, you do a compession test, you ratio is 11.5:1
11.5x14.7psi= 169 psi
in denver @ 5000 feet, the pressure might only be 14.0psi
11.5x 14.0psi= 161psi
 
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I think it is more like 10psi less for every 1000 feet.
I am at 4500 feet and reading is low,I get around 170 here.
 

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TWF said:
I think it is more like 10psi less for every 1000 feet.
I am at 4500 feet and reading is low,I get around 170 here.
could be, I was only trying to illistrate the principle, I know ambient atmopheric pressure at sea level is 14.7± and less the higher elevation you are,

if a repair manual sez you should have a certain compression, you shouldn't be alarmed if you get less at a higher elevation because of the lower ambient atmospheric pressure
 

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What would the compression be on Busa pistons with one layer of the head gasket removed? I'm now curious just in case I need to measure this. I'm about 600 above sea.

Thanks in advance,

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