VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 31 ---194ci compression readings

Hello to All,Can anyone tell to me what the compression minimums and maximums should be according to chev.specs? thanks

The published compression ratio is 5.0:1 for 1931 (ref. 1932 Engineering Features). So theoretically would be 73.5 psig but that ignores the heat generated during compression of the gasses. However with compression heating (assume adiabatic conditions i.e. no heat transfer) it becomes 135 psi @ ~ 540 deg F. Now we know that some heat transfer occurs and also there is cooling from evaporating gas droplets. So the real answer is between those two extremes.

Nuf of the science??? Okay then the actual expected pressure reading will be ~80 psig if everything is up to snuff. Of course that depends on the engine turning speed, leakage past rings, valves, etc. The key is that all cylinders are within 5 psig of each other and above 60-65 psig.

THANK YOU CHIPPER---You always amaze me with your incredable wealth of information---it is fantastic !!Did compression test yesterday---the results were as follows:
#1-85 #2-80 #3-80 #4-85 #5-78 #6-84 !! hope that is close enough for gov't work ---lol..I was worried because the acceptable or "green zone" on the insturment started at 95 lbs and went to somewhere about 120 lbs...must be for those cars that are still growin up!! THANK YOU AGAIN,jay

Ya'll would love my engine. average Comp. 45 at best for all cylinders. YET...starts and runs every time like nuthins wrong and doesnt burn an ounce of oil.

Is there a level at which compression my be too high and should cause one concern?

The only way that the compression would increase above what the factory engineers designed, would be by making modifications which forced more fuel and air mixture into the cylinders such as Supercharging, turbo charging , Nitrous oxide systems, and also the simple things like shaving the head and lumpy top pistons bigger cam lifts etc etc.

The compression test on a cold engine is the reading of the compession ratio of the engine, 1931 5 to 1 , multiplied by the air pressure ,14.7 at sea level, which gave the figure Chipper quoted at 73.5 PSIG. the lower figures we get are then due to leaking valves , leakage past the rings,head gasket and sparkplug .

The reason I asked is a compression test of my 1930 yielded results of 90-95 lbs for each cylinder. While it was consistent it seems too high from references in this thread. I checked the gauge and it was reading correctly. Any thought?

If you go to this website it discusses more on pressures and compression readings.
Particularly the part about compression ratio versus overall pressure ratio.(Chipper's science).
http://en.wikipedia.org/wiki/Compression_ratio

90-95 psig is not too high but definitely on the high side. I would suspect some carbon on the top of the piston which raises compression pressure.