I have always used high test fuel, but with rising prices I may have to reconsider. With a pair of factory 502 mag/ 415's what is the recommended octane? I assume 89/90 will be fine, just looking for feedback.
Russ C.

Russ,
I use what Mercruiser recommends for the 502 MPIs, 87 Octane. Anything else, I'm told, is a waste of your money.
Dean

I'm not sure what you have to use, but I must use 93 this yr.
Or I'll blow a piston.

When I had 502 magMPI's I used 87.

Now in blown mills 93

If I could use 87 I would....but I have to run 93 in mine.

Use 87 octane in mine. Benefit of factory 454 mag.

600hp/540 ci Innovations - 89 octane, but knock sensors will allow me to use 87 in a pinch.

You have to remember the higher the octane rating, the "slower" the combustion burn. Normally large bore/stroke engines require a slower burning fuel/air mixture for the longer stroke of the pistons during combustion. A faster burn or ignition for the shorter strokes. A common myth is that the higher the octane you put in, the better/stronger your motor will run when in fact the opposite is true. You should only use what the manufacturer tells you, which in most cases is 87/89. In automobiles you could void your warranty if you don't use the recommended octane rating. IN the "old" days it was thought using the higher octane was best because the the hot cars of the day required it with their big blocks. So the urban myth of well if it's good for them then it's got to be good for the ole family car. Not true.

04 496 H.O.,, 87 only.. 01 5.4 f-150 87 only,,,, 01 nissan maxima 93 or it runs like Chit...

Depends on your compression ratio (naturally aspirated) or boost level (blown).

If you're running a CR of 12 or higher, I'd definitely run premium. You can get away with 87 under low load and cold air temps, but once you put the engine under load and or hot air temps, detonation is likely.

Same is true for even moderately blown engines. 90+ degree heat and even 4-6 psi means you'll need 93 or better.

Just remember, you can always limp an engine home if you can't find premium fuel, but be wary of hard acceleration (high loads) on the engine and temperatures. Detonation is hell on pistons.

This all assumes your timing is in tune, advanced timing will make the problem worse.

But like said above, a properly tuned engine, at 11.5 or lower CR, you're basically wasting money if you go with premium fuel (unless it's like 110 degrees outside and no humidity)