View Poll Results: Which octane fuel do you use?
87 octane (regular)
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44
23.78%
89/90 octane (plus)
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27
14.59%
92/93 octane (super)
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97
52.43%
AV/ other or Allan 4 is a putz
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17
9.19%
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Which octane fuel do you use?
#1
21 and 42 footers
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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 C.
#7
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600hp/540 ci Innovations - 89 octane, but knock sensors will allow me to use 87 in a pinch.
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Retired! Boating full-time now.
Retired! Boating full-time now.
#8
Rough Seas Lie Ahead
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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.
#10
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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)
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)