I have a couple antique cars and have been reading some maintenance manuals from the forties. it’s mentioned that compression is less at higher speeds than lower. why is this so? is it because of the timing being advanced? I thought the spark starts early to catch up to cylinder speed and that the fuel mixture would be compressed the same. thanks in advance, so to speak.

This is just a theory, but this may have to do with the “L-head” or “F-head” design of those engines.

Those types of valve placements did not allow for the high compression of an overhead valve engine to begin with. Possibly this old type of engine design lost some of that already low compression at high speeds as a result of that valve placement. However, this is just a WAG, as I never worked on L-head or F-head engines.

Well, any engine that doesn’t “breathe” well isn’t going to completely fill the cylinders at high RPM–they will be under partial vacuum at BDC. Thus, compressing this mixture would result in a lower max compression PSI.

But I don’t see where this is anything other than academic, as I’ve never seen a means for measuring compression in operation.

The volumetric efficiency decreases as RPMs increase. Pistons draw in less air as the duration (time)of the intake stroke shortens. Turbo chargers and super chargers are used to compensate.

I would like to see how that statement is worded and the contex it is in, I bet if we knew this we could figure out what they are talking about.

Running compression, no matter what the cylinder design, is always lower. Try it. Remove a spark plug, install a compression gauge. Disable injectors/ ignition and crank over to get highest reading. Now reconnect everything and start engine. Compression’s lower.

Rod “nailed” the answer. Compression decreases with increased RPM. The volume in cubic inches of the piston pulling in the vapor stays the same, but the effect of the restrictions to the intake (and expulsion out the exhaust valve) become more pronounced as volume increases.

And you are correct that the spark is advanced at higher speeds to better time the combustion process relative to the position of the moving piston in the power stroke.

So the statement should have been put in the contex of “nothing you can do about it but compression will be lower at higher rpm”, not in the contex of something you should try and fix.

For anyone interested the physics of volumetric efficiency, cylinder head design, porting, camshaft profiles and supercharging may be quite enlightening. I attempted to patent a variable camshaft 40 years ago and after much investigation the old fashioned way in libraries I found that it was all for naught because patent law was not for amateurs.