He made a good point as to compression ratio and cam profiles. The 67 Stang coupe I bought a few years ago had a warmed up 302 in it, with 289 heads, cast flat top pistons, .015 deck clearance, and an RV type cam. It wouldn't run on 87 at all, pinged like crazy. It would run on 93, never had the chance to try 89. My 68 Merc's 390 did likewise with the stock cam and 10.5 to 1 compression, wouldn't run on 87 (or even 89), but with the Crane Energizer cam it's got now, is happy with 89. And my 331 stroker, just didn't ping with 87 ( this was when I first fired it up, and had a tank full of 87) the noise it made sounded like a rock crusher. Never heard one do that before, scared me enough to drain the 87 out and replace it with 93. Comp ratio with it is 10.4 to 1, cam is a Ford Racing Z303 roller.