cinque500
Member
Hi, how does one calculate the CR on an engine as there doesn’t seem to be a lookup table on the web? On a cold engine mine shows 127psi, it’s 740cc with a 64/105 camshaft. Thanks
It may be a bit messy, but if you measured the exact amount of engine oil poured into the plug hole at Bottom Dead Centre on the firing stroke, then carefully measured every drop coming out as you turn engine slowly to Top Dead Centre, you would then have the "swept volume" and the "un swept volume" ,Hi, how does one calculate the CR on an engine as there doesn’t seem to be a lookup table on the web? On a cold engine mine shows 127psi, it’s 740cc with a 64/105 camshaft. Thanks
Thanks. Your and bugsymike’s instructions both make sense, just put the engine back together and back in …… wish I’d asked earlier!!!Whilst there are rough guides online to calculate compression ratio from compression pressure numbers they rely on many assumptions (piston ring sealing, cam timing and duration, speed at which the engine is spun while testing) and can't really be trusted most of the time.
In theory, as you know it's a 740cc engine, you only need to measure the volume of the combustion chamber at TDC to do the same calculations?
But ultimately, yes, the only reliable way to measure the compression ratio is to measure the quantity of liquid required to fill the combustion chamber, and calculate from there. Normally you'd do it with the head off, but it could be done carefully, if messily, with the head on. You would then need to find a way to extract the majority of the liquid you've introduce before trying to turn the engine over.
I guess the next question needs to be, why would you like to know the compression ratio?
Thanks. Your and bugsymike’s instructions both make sense, just put the engine back together and back in …… wish I’d asked earlier!!!
Engine in, no mess. The swept volume you get from the bore and stroke, that's pi x (bore/2) squared x stroke.
The combustion chamber volume you get by filling it with oil (at comp.TDC) from a graduated seringe (a 60cc one is plenty enough), up to midway up
the plug hole. Note the quantity and immediately pump back out most of that oil with a thin tube attached to the seringe.
CR formula is (swept vol. + combust. chamb. vol.) divided by combust. chamb. vol.
The only difficulty is having the plug hole more or less vertical, but you can lean the whole car with a jack.
Thanks giardini, this doesn’t sound too bad at all. Might try it next week after tomorrow’s Brooklands event.Engine in, no mess. The swept volume you get from the bore and stroke, that's pi x (bore/2) squared x stroke.
The combustion chamber volume you get by filling it with oil (at comp.TDC) from a graduated seringe (a 60cc one is plenty enough), up to midway up
the plug hole. Note the quantity and immediately pump back out most of that oil with a thin tube attached to the seringe.
CR formula is (swept vol. + combust. chamb. vol.) divided by combust. chamb. vol.
The only difficulty is having the plug hole more or less vertical, but you can lean the whole car with a jack.
A very simple formula for measuring compression ratio--- 0.7854 x bore (in cm) x bore (in cm) x stroke (in cm) + full capacity of combustion chamber, divided by the combustion-chamber volume. If you have a standard crankshaft, the engine's stroke will be 70mm (7cm). To get to 370cc (1/2 of 740) the bore is going to be 82 mm (8.2cm) which gives a capacity of 369.7cc (times that by 2, and you have 739.3cc). Jack the n/s of the car up so that the spark plug is at the top of the combustion camber and measure the capacity (as advised earlier in this subject) with oil. Yes. it will be a bit messy, but with the head on, there is really no other way. If you have a combustion chamber volume of 36cc (have you had the head skimmed?---I hope not) you will have a C/R of about 11.3 :1---a mite on the high side for a 'road-use-only' car! I would suggest that you use the Esso "Super 99+" fuel!Thanks. Your and bugsymike’s instructions both make sense, just put the engine back together and back in …… wish I’d asked earlier!!!
Thanks hobbler, these are great instructions. I only use Super given the low mileage I do, the way I see it is it’s not going to cost me a fortune. The head has had a light skim but for flatness rather than performance gain and the safety exhaust channel is still there and deep too so it’s never had more than a few thou removed. I will eventually measure the combustion chamber as I’m fascinated to know. But presumably I’ll then have the theoretical CR? How does inefficiency with piston ring and valve sealing get taken into account and that PSI reading I have? Or are they exclusive? I wonder because the CR has potential to be pretty high and yet the psi reading according to my gauge is barely into the green zone.A very simple formula for measuring compression ratio--- 0.7854 x bore (in cm) x bore (in cm) x stroke (in cm) + full capacity of combustion chamber, divided by the combustion-chamber volume. If you have a standard crankshaft, the engine's stroke will be 70mm (7cm). To get to 370cc (1/2 of 740) the bore is going to be 82 mm (8.2cm) which gives a capacity of 369.7cc (times that by 2, and you have 739.3cc). Jack the n/s of the car up so that the spark plug is at the top of the combustion camber and measure the capacity (as advised earlier in this subject) with oil. Yes. it will be a bit messy, but with the head on, there is really no other way. If you have a combustion chamber volume of 36cc (have you had the head skimmed?---I hope not) you will have a C/R of about 11.3 :1---a mite on the high side for a 'road-use-only' car! I would suggest that you use the Esso "Super 99+" fuel!
Did you do the compression test with the throttle fully open? When I did my apprenticeship, I was always taught that the throttle should be fully open when the engine is being cranked. To be honest ANY compression-ratio check is "theoretical" because it does not take into account any "past the piston" leakage. If the valves have been 'lapped-in' correctly, there should be no loss past the valves.Thanks hobbler, these are great instructions. I only use Super given the low mileage I do, the way I see it is it’s not going to cost me a fortune. The head has had a light skim but for flatness rather than performance gain and the safety exhaust channel is still there and deep too so it’s never had more than a few thou removed. I will eventually measure the combustion chamber as I’m fascinated to know. But presumably I’ll then have the theoretical CR? How does inefficiency with piston ring and valve sealing get taken into account and that PSI reading I have? Or are they exclusive? I wonder because the CR has potential to be pretty high and yet the psi reading according to my gauge is barely into the green zone.