Some comments about theoretical and measured values of compression
pressure values with respect to the following:
Once you know the compression ration, all you need to do is to multipy the compression ration by 14.7 * (which is the mass at sea level in lbs of a column of air one square inch in cross-section). So about 136lbs.
This would be true if the
temperature of the gas didn't rise as it was compressed - which it does, because work is done on the gas to compress it. (Ever feel how a bicycle pump gets hotter as you inflate the tyre because of repeated compression of the air inside the pump?) Because the temperature of the gas rises, the
pressure of the gas rises by an additional factor to what would be expected solely due to its reduced volume. Hence it's not just the reduced volume (i.e. compression ratio) that determines the gas pressure. The pressure increase is more than a simple 10:1 (say) compression ratio would indicate; this is because the gas is not only compressed, but the work done to compress the gas has also heated the gas and the hotter gas will have a greater pressure (even if the volume had not changed). Those wishing to look further into the physics of this could look up "adiabatic" and "non-adiabatic" gas processes.
I thought many other things went into figuring out what compression numbers should be. For instance valve timing, or is everything already figured into the compression ratio figure?
The valve timing also effects compression readings. If the pressure calculation is done purely on the basis of change in cylinder volume, you are assuming that the valves are closed throughout the whole compression stroke. However, intake valve closure (sealing the cylinder) always takes place after BDC; hence only the percentage of the stroke after intake valve closure is compressed. This would lower the "real" compression ratio of the gas. But ... intake port tuning and scavenging may allow a greater mass of charge (at a higher than atmospheric pressure) to be trapped in the cylinder than the static volume would suggest. So the compression pressure also depends on the particular tuning characteristics of the engine.
Assuming that the valve timing and tuning characteristics can be ignored for simplicity, if the nominal compression ratio of an engine is given, the maximum theoretical cylinder pressure can be estimated using the following relationship:
pressure = cylinder pressure at BDC (e.g. 1 atmosphere, 14.7 psi) X (Compression ratio) raised to the power of gamma
where gamma is the specific heat ratio for the working fluid, which is about 1.4 for air.
Bottom line:
"You can expect the cranking pressure in psi for a road engine with a standard cam to be about 17 to 20 times the value of the compression ratio"
is quite reasonable. For example, my measured value of 165 psi (after I had the valves and rings done) is 19.4 times the quoted compression ratio for the 750 GT of 8.5:1.
dden's comment that:
I'm seeing 165-175. Gauge is good.
is consistent with the info above. And, of course, the condition of valves, valve seats, cylinder bore, lubrication and rings are also contributing factors.
Hope this helps.
Steve
).
