cinque500
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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
Technical Compression ratio
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Technical Compression ratio
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" ,
So if you poured in 110cc and collected 100cc I would call that 10:1 Compression Ratio.
I am sure there are cleaner ways of doing this and someone will tell us.
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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?
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?
If someone decides to go the "messy route", after drawing off most of the oil then a spin over engine with the spark plugs still out and a cloth over the plug holes should collect the majority, so only a little smoke on first start up. Much the same as when doing a "wet " compression test to check piston rings.
The advantage to me is often pistons don't actually go to the top of the bore, plus the shape of piston crown dome may affect results also so result may be arguably more precise?
The one time I should have done this check and didn't was as a young apprentice I fitted the crankshaft and conrods from a 109E Ford Classic engine into a 105E Ford Anglia engine (a straight swap) using original Anglia cylinder head, the boost in capacity from 997cc to 1340cc meant, even using the then 5 Star petrol available, I couldn't stop the engine "pinking" which coupled with a teenage heavy right foot shortened the engine life even more.
The advantage to me is often pistons don't actually go to the top of the bore, plus the shape of piston crown dome may affect results also so result may be arguably more precise?
The one time I should have done this check and didn't was as a young apprentice I fitted the crankshaft and conrods from a 109E Ford Classic engine into a 105E Ford Anglia engine (a straight swap) using original Anglia cylinder head, the boost in capacity from 997cc to 1340cc meant, even using the then 5 Star petrol available, I couldn't stop the engine "pinking" which coupled with a teenage heavy right foot shortened the engine life even more.
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cinque500
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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.
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cinque500
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Thanks giardini, this doesn’t sound too bad at all. Might try it next week after tomorrow’s Brooklands event.
the hobbler
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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!
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cinque500
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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.
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the hobbler
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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.
Assuming the gauge is accurate, to me that reading is not that high @the hobbler can better advise on these vehicles, however I would have thought a reading somewhere nearer to 155/160psi, maybe the piston rings need bedding in a bit.
I suppose a lot is defined by the head design as to how high a compression you can run without pinking etc. Older car designs had a poor burn rate and left pockets away from the spark plug of unburnt fuel resulting in pre ignition etc. so high compressions were a problem with the fuel available at the time. Cars like Jaguar in those days had a semi hemispherical head design which gave good efficient burning and so ran higher compressions.
In the 1970s working at a Mazda Dealership I recall their 818 models had compression reading of 180psi and didn't need higher Octane rated fuel than was then currently available (pre unleaded etc) due to an efficient design.
Some time ago I rebuilt the engine of a Ford Tickford turbo Capri that previous garages and owners couldn't stop pistons burning holes in when using the full boost power, even after using Octane booster additives etc. Compressions on the non holed pistons were around 180psi when I checked before rebuild, so with full boost it was a problem. I had the engine rebored and I assembled it with a special set of Mahle pistons which brought the compression to around 150psi.
Customer was well pleased as once run in, he was able to reliably give it full power.
I suppose a lot is defined by the head design as to how high a compression you can run without pinking etc. Older car designs had a poor burn rate and left pockets away from the spark plug of unburnt fuel resulting in pre ignition etc. so high compressions were a problem with the fuel available at the time. Cars like Jaguar in those days had a semi hemispherical head design which gave good efficient burning and so ran higher compressions.
In the 1970s working at a Mazda Dealership I recall their 818 models had compression reading of 180psi and didn't need higher Octane rated fuel than was then currently available (pre unleaded etc) due to an efficient design.
Some time ago I rebuilt the engine of a Ford Tickford turbo Capri that previous garages and owners couldn't stop pistons burning holes in when using the full boost power, even after using Octane booster additives etc. Compressions on the non holed pistons were around 180psi when I checked before rebuild, so with full boost it was a problem. I had the engine rebored and I assembled it with a special set of Mahle pistons which brought the compression to around 150psi.
Customer was well pleased as once run in, he was able to reliably give it full power.
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