[35th_Anniversary...]

seems like stupid question, but Topeka sure as heck didn't have any 93.

[trackbird]

Right. We are actually discussing dynamic compression and not truly static compression ratios. I was trying to keep this light, so I over generalized it.

You've touched on one reason that cars with larger cams (longer duration) can use more compression on pump gas. The extra valve overlap allows the engine to push some air back out of the cylinders at lower RPM and not truly fill the cylinders. So, to correct for the loss of pressure (and therefore torque), we raise the compression ratio to help bring the cylinder pressure back up and restore some of that torque. Engines are less sensitive to detonation at high RPM. The mixture motion of the air and other factors contribute to reducing the tendency to detonate (as RPM increases) and at high RPM that longer duration cam can take advantage of the tuning effect of the intake runners and the high velocity air flow through those runners (which has considerable energy and is often just short of being super sonic at high RPM, air moving that fast has a lot of energy and it is used to pack air into the cylinder even though the piston is already coming up towards top dead center). But, we are now headed into a far more in depth discussion about engine dynamics (instead of just answering your question about altitude and octane).

If you have a cylinder in a vacuum. It has nothing inside. When we make that cylinder 1/10 its original size, we still have no pressure. It would have a mechanical compression ratio of 10:1, but the effective pressure is 0. Because 10 X 0 = 0.

If we super charge that cylinder and pack 20 psi in it (at bottom dead center, so it's truly full) and then compress it to 1/10th its original volume, we'll see 200 PSI. At 200 PSI, air can get hot (think of a 25 PSI turbo and the exit temps those can see, which is why we intercool). If it gets hot enough, BOOM. It detonates. Now, if we have sea level (roughly 15 PSI) and squeeze it 10:1, we are at 150 PSI (off the top of my head, I believe it's generically a linear relationship between starting and finish pressure...if not, our engineers will correct me, but this will illustrate the point). Again, at 150 PSI, things can get hot (which is how a diesel works and why they have no throttle plates, they need the airflow to make the compression to make the heat to detonate the fuel).

Now, go to 6,000 feet. Say we drop to 10 PSI atmospheric pressure (I don't think it's anywhere near that low, but I'm too lazy to look up the exact change). 10:1 only produces 100 PSI, far less than the 150 PSI we saw at sea level. So, the temperature rise is signifigantly less and our requirement for octane is also far less.

Again, if the pressure relationship is not truly linear, I'm sure someone will correct me. I'm trying to illustrate the big picture (as effectively as possible....without verifying my math 100%).

Better?