I know for sure now that my modded motor won't run on 87 octane because its crapped out twice. I did it once to save a buck and could barely break 2k rpms and sputtering like a SOB. I thought it might have just been only old 87 gas but I guess not cause it did the same thing with fresh 87. I know now I can only run 91 which I normally do but can't toss in 87 in a pinch Each time the spark plugs needed to be changed, fresh 91 octane and it fixed her right up.

Is it because of the head compression and modifications? I'm by no means knowledgeable on this subject. Polished and ported heads and some other minor mods is all it has. Anyone?

What mods do you have? It doesn't sound like octane rating is your problem. That (87 octane)wouldn't cause your engine to not turn up rpm's.

I can get the RPMS up but the motor will choke out and shoots out black smoke. I don't have a problem running with 91 octane now. The head is milled, ported and polished. Maybe I should ask what mods won't run 87 octane? There could be something else going on but right now, leaving it running 91 octane fixes it so I might just have to leave it at that.

Higher octane fuel combats predetonation with a higher compression motor.

I run 93 in my stock motor. It seems to run a little better and the motor dosent want to diesel when I turn it off like it did with 87.