What does it mean when a motor has 180 lbs of compression and another motor has 120? Does the bigger number mean a higher over all compression? Just curious.
On a compression test you are checking the cranking compression. It's different than static compression ratio and the cam play a big roll here , so its nice to know some info to better judge. The one producing 180 will be a lot more fun to drive on the street than the one putting out 120. If both of those cylinders are on the same engine , it needs work.
Also, battery voltage and actual cranking speed, as well as number of hits to record the pressure can make the numbers vary quite a bit.
Yes the bigger number means higher compression, more power, better gas mileage, but also higher octane gas required. There are other factors such as cam design, combustion chamber design etc but 120 would be considered a low compression engine, like a fifties six cylinder, while 180 would be very high.
In the dyno lab where I did business we had some dyno starters that spun the engine at 150 rpm and other dynos that were turned at [edit]210 rpm. Once I did a test comparing the two on the same engine. All other factors were rigorously controlled. There was a 10 psi difference - one was about 140 psi and the faster was 150 psi average. so yes, cranking speed makes a difference, as does cam timing, most notably the intake valve closing event.
Pumping ability versus leaks...leaks can be mechanical problems like bad valve seats or rings that allow air to escape rather than being compressed, or radical cam overlap.
Once in a while you will find very high compression numbers on an engine with a lot of carbon buildup stuck to the heads. They typically rattle badly when crowding the throttle.
My old BMW 2 cylinder motorcycle has 1 carb per cylinder on a short "manifold." A compression test done with the throttle closed returns scary low psi. The cars, with relatively large volume intake manifolds, I've tested did not make that much of a difference. Perhaps similar to the comparison Kevin Cameron used for another phenomenon - "you can take a deeper breath off a 55 gallon drum than a coke bottle."
I might add that when doing a static compression test the actual number may not be as important as having all cylinders test the same or within 10% of each other.
Compression testing is really just a test of the condition of the engine cylinders and has little to nothing to do with compression ratio.
A cam with very little overlap will produce higher static compression, has nothing to do with actual compression of the engine. Stock engines will produce the highest cranking compression, engine designers don''t like to let fresh fuel out the tail pipe, so overlap is tightly controlled. Big race cams with long duration have to have overlapping timing events, there just isn't enough degrees to get them all in with out it. Over lap is when both valves are open at once, and when doing a slow cranking test, a bunch of the compression leaks out creating the low static compression. Doesn't mean the engine is weak, its just a cam timing issue. It's said above, the most important part of the test is to be all the same number across each cylinder. Joe
Actually compression ( x:1) and combustion chamber pressure (PSI) are related but they are distant cousins, not identical twins. Compression is actually figured from swept volume, and it is the amount that the *volume is compressed or squeezed. If you have a volume of 100 and it is pushed or squeezed into a space of 10 then your compression would be 100:10 or simplified 10:1. Now in order for this to be true the compressed volume needs to be trapped in the space it is compressed into. In an engine that is functional, the space that the volume is compressed into is the space between the top of the piston and the dome (roof?) of the combustion chamber. Your compression number does not take into account leakage of any kind, like past the rings or past the valves or even like has been mentioned your valve opening and closing events. When you take a compression reading you are reading the pressure in the combustion chamber and the gauge reads the actual combustion chamber pressure. The gauge is dumb and does not know it but it is taking into account all the other factors already measured. I suppose that if one wanted a real compression reading across a given RPM range they could use something like a blower dyno that would spin the engine over a given RPM range and have a compression gauge that one could monitor. You could then take all the readings and give yourself an average compression reading for the range as well as a mean compression reading for the range, or any number of other ways to record and store the data. But none of that would change the compression ratio it would just give you cylinder pressure over a given range. Your compression ratio would still be the volume of the cylinder pressed into a specific size chamber. of course once the engine is spun there are other factors to consider like rod growth and piston movement and etc. but the differences would be so minute that there is really no reason to take them into account when considering compression ratio for our purposes. * volume of the cylinder for our purposes.
Yep, P&B is right on the money here. There certainly is a relationship between compression ratio and cranking compression. Try this experiment- take a belly button SBC with 76 cc combustion chamber heads and run a cranking compression test. Then swap only the heads to 58 cc and run the test again. The cylinder pressures will be greater. Ok now swap in some 1.6 rockers, run the test again, cylinder pressures go down because the intake closing event is a few degrees different at 0.050 and the overlap is longer delaying the exhaust closing. At 200 cranking Rpms, the cylinder pressure is down, but at upper lever higher Rpms the cylinders are filled better with more combustible material and less junk. Ok now swap back to 1.5 rockers and play with the cam. You can build cylinder pressure or loose cylinder pressure with cam choice. Another point here is that the cylinder pressure has a direct correlation to the rate and force of the expansion after combustion. A 7:1 cylinder needs to return to zero just as a 10:1 cylinder does. The 10:1 cylinder must do this faster and with more force keeping the times equal for both cylinders. However compressing a proper mix and more of the combustible mixture with less junk will greatly change the rate of expansion.
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