Calculating CR by Compression testing

01A79

01A79

Junior Member
I run a 59AB that I bought with low mileage. It has .060 pistons. I read 140# at cranking speed with WOT. How can I calculate the compression ratio?
 
You really can't. Compression ratio, as compared to compression pressure, is a purely mathematical/mechanical function. Cranking pressure can be affected lots of things such as ring and valve sealing, and cam timing.

Bob
 
I'll take a guess anyway. Roughly ~9/1 with no leaks, overlap, etc. and calculated max at BDC cylinder volume/TDC cylinder volume whatever you measure them to be. 8^)

Jack E/NJ
 
Isn't the compression ratio (CR) just the ratio of the measured pressure divided by the atmospheric pressure? Noting that the later is 14.7 psi, 140 psi yields a CR of 9.52.

But what do I know...I'm just an EE...
 
No, it's not measured pressure divided by atmospheric pressure! It is the volume of the cylinder with the piston at bottom dead center compared with the volume of the cylinder with the piston at top dead center. Volume is usually measured in cubic centimeters. For example, with the piston down, cylinder volume including the combustion chamber may be 480 cc's. With the piston up, cylinder volume (essentiually just the combustion chamber) may be 80 cc's. Thus it figures out to be 6 to 1 compression ratio. We have squeezed, or compressed, the mixture to one-sixth of it's original volume, in theory anyway. Since the intake valve stays open as the piston rises at the start of the compression stroke, compression at cranking speeds does not begin until the piston is well up from BDC. Thus 'effective compression ratio' may only be 5 to 1 or 4.5 to 1, depending on intake valve closing. This lowered effective compression is one reason a 'hot' or performance cam is doggy at low speeds.
 
If you read my book, Flathead Facts, the calculations and components of accurately determining CR from cranking pressure are explained.
 
I have a chart and formula for calculating CR in MY book, but it contains a 15cc fudge factor. This 20 year old play on numbers has proven to be quite accurate. HOWEVER this determines static CR. What really is important is dynamic CR. And this includes intake valve timing.
 
JWL,
140/1.3-14.7/14.7=6.32 x.05 =.31+6.32=6.63 which is less than the stock 6.75 CR
Have I done this right?
I highly recommend your book to all the Flathead people - lot'sa facts!!

Cliff
 
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40 Deluxe said:
No, it's not measured pressure divided by atmospheric pressure! It is the volume of the cylinder with the piston at bottom dead center compared with the volume of the cylinder with the piston at top dead center. Volume is usually measured in cubic centimeters. For example, with the piston down, cylinder volume including the combustion chamber may be 480 cc's. With the piston up, cylinder volume (essentiually just the combustion chamber) may be 80 cc's. Thus it figures out to be 6 to 1 compression ratio. We have squeezed, or compressed, the mixture to one-sixth of it's original volume, in theory anyway. Since the intake valve stays open as the piston rises at the start of the compression stroke, compression at cranking speeds does not begin until the piston is well up from BDC. Thus 'effective compression ratio' may only be 5 to 1 or 4.5 to 1, depending on intake valve closing. This lowered effective compression is one reason a 'hot' or performance cam is doggy at low speeds.
Click to expand...

Theoretically, recalling that pressure times volume is a constant (pv = Const), than the ratio of volumes is the same as the ratio of pressures. However, there is also all of this other practical stuff that you and others have noted.
 
Question: Does this formula (pv=Const) take into account the temperature rise that occurs during compression of a gas? The volume of a cylinder remains the same regardless of temperature, but pressure in a closed cylinder rises when it is heated. Rapid compression (as occurs during the compression stroke in an engine) causes a significant temperature increase, thus increasing cylinder pressure over and above that expected from the volume change of, say, a 6 to 1 mechanical compression ratio. So it seems to me that these two methods do not correlate.
 
If you could fill the cylinder with a liquid with the head in place at bottom dead center and then turn the crank so the piston is a top dead center and measure the amount of liquid that comes out of the spark plug hole you would have the compression ratio. It is a static measurement and not based on valve timing etc.

USING THE COMPRESSION TEST TO DETERMINE COMPRESSION RATIO HAS TOO MANY VARIABLES TO BE ACCURATE SUCH AS RINGS, VALVES ETC. THE STATIC COMPRESSION RATIO IS THE ONLY ONE THAT HAS MEANING.
 
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Yes, there are variables, yes, the normal expansion during compression is an element of the calculation, yes, it is important to be smart enough to use the information. After having done several tests of known static CR cylinders I can say the described procedure is reasonably accurate. It isn't often we need a 3 decimal accuracy level when discussing CR.
 
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40 Deluxe said:
Question: Does this formula (pv=Const) take into account the temperature rise that occurs during compression of a gas? The volume of a cylinder remains the same regardless of temperature, but pressure in a closed cylinder rises when it is heated. Rapid compression (as occurs during the compression stroke in an engine) causes a significant temperature increase, thus increasing cylinder pressure over and above that expected from the volume change of, say, a 6 to 1 mechanical compression ratio. So it seems to me that these two methods do not correlate.
Click to expand...

Good point. The actual base equation is pv = nRT, so it is pv/T which is a constant.
 
PV=nRT is the ideal gas equation, not to be used for a real world non-ideal fuel air calculations. If one takes into account such things as non-ideal gas fugacity, the earth's tectonic plate movements, global warming and the annual monarch butterfly migrations, you gotta conclude that 140 cranking psi with a WOT is an enviable measurement that even the most fussy flathead fanatic would lust after.

Jack E/NJ.
 
I'll take 140#, mine was at 60# (on the highest cylinder) before rebuilding. But that sucker was still running!
 
Maybe this should be in a new thread, but what is the most practical way of obtaining combustion chamber size? What is concidered an acceptable variation, cylinder to cylinder?
Thanks,Tom
 
Jack E/NJ said:
PV=nRT is the ideal gas equation, not to be used for a real world non-ideal fuel air calculations. If one takes into account such things as non-ideal gas fugacity, the earth's tectonic plate movements, global warming and the annual monarch butterfly migrations, you gotta conclude that 140 cranking psi with a WOT is an enviable measurement that even the most fussy flathead fanatic would lust after.

Jack E/NJ.
Click to expand...

Hey...what do you expect. I told you that I was an EE. Besides, I think that I got a C- in Hydrodynamics (or was it Thermodynamics....can't remember).

Thanks for the humor...really appreciated. Makes me want to go down to the garage tomorrow and take some cylinder pressure measurements (and maybe humidity and wind direction and ......) ;-)
 
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