Gas options

AJ/FormS said
All I'm suggesting is that an elevation change of 4000 feet represents a cylinder pressure change of about 20 psi. And a jet change of two sizes at WOT.
All that change will drive the Part-Throttle tune crazy, and when it goes lean, detonation is sure to appear with 87 gas.......... especially if the engine left the factory already tuned lean;
and, IMHO, the recommended 91 gas is pointing exactly to that.

But I totally agree that any 318 should run on 87, less even, when tuned for it, and operating within a narrow elevation range. The Effective compression ratio, could be down to 4/1 at times, and that will support skunk-pee quite nicely..

Totally depends on what elevation your tune is for. I drive my sea level tuned cars over the 8,000 foot elevation passes here without issues. They run a little rich, no big deal. The difference from 0 to 4,000 feet is pretty negligible, maybe a tenth or two on my AF/R gauge. Idle speed slows down a smidge.

Now, if you tuned for 8k feet and then drove to sea level you might have an issue going into lean detonation. But not the other way around. And a low compression 318 tuned for 4k feet works just fine at sea level. You lose maybe a tenth or two on the AF/R, which shouldn't hurt anything unless you're already way leaned out.