I'm assuming compression ratio is the ratio of the combustion chamber volume with the piston at bottom dead center to the combustion chamber volume with the piston at top dead center.
Anyways, what measurements are required to calculate compression ratio? Seems to me in addition to the bore and stroke you'd need the head gasket thickness need to know the volume between the top of the piston (@TDC) and dome of the head, which can change depending on the design of the top of the piston and headwork. Or is it just measured by air pressure difference at BDC and TDC? Eh but you'd need perfect compression for accurate results.
Say you've got 11:1 pistons and get some headwork, how do you calculate your new compression ratio?
Bottom line: how is compression ratio measured?

 

yes, thats exactly what compression ratio is, ratio of piston at bottom to when its at top taking into account, bore, stroke and head chamber volume. When you do headwork, some porters reshape the head combustion chambers which changes the volume ... if you now have more volume then your compression ratio goes down and vice versa. What they do to balance compression ratio when opening up head volume is they mill the head, by doing this you also reduce the amount of volume in the head's chambers tog et back at least the volume that you started from .... sometimes they even take off even more from the head to raise the compressiona bit. Obviously they would need ot know which pistons will be run with what cams so they can make a judgement on clearance (at least .0045" valve to piston for stock). The gasket thickess also is a variable to the amount of volume used to calc compr. ratio ... standard thickeness of the stock 3 layer headgasket (compressed or torqued as they say) is 0.7mm

HTH,
Greg

 

Cool, thanks.
Do you know how one measures the volume considering the irregular surfaces of the piston top and head dome? Just the same way we messured irregular volumes is chemistry class? Like filling it with liquid then measuring the volume of the liquid used? (which would have to be done with the head off and upsidedown of course)

 

here's a clue to do it while being practical as pulling the head off and measuring will not give the full story since the dome position cannot be recreated while head is off. There is an apparatus which is used by tech inspectors at the tracks for calc displacement.

Anyway, you know the stock manufacturer's compression ratio specs, the bore, the stroke and the head volumes are given:
b16/typeR/b17a = 42.7cc
P72 GSR = 41.6cc
and LS non vtec heads = 45.0cc

now just work the math and back into the new c/r. Use 0.7mm as headgasket thickness.


Greg