View Poll Results: Which octane fuel do you use?
87 octane (regular)
89/90 octane (plus)
92/93 octane (super)
AV/ other or Allan 4 is a putz
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Which octane fuel do you use?

Old 04-12-2005, 06:34 PM
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Default Re: Which octane fuel do you use?

87 when on the trailer. But once on the lake, 93 is only option. So I always fill it up when ever it's off the lake for what ever reason. Too much hassle to pull trailer down just to pull it out and trailer a couple miles for cheaper gas just to fill it up.
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Old 04-12-2005, 07:30 PM
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Default Re: Which octane fuel do you use?

540s...9.8/1 cr=89 octane
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Old 04-12-2005, 10:28 PM
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Default Re: Which octane fuel do you use?

I . . . I feel so . . . used . . . .
Running PREMIUM all these years ( 502mags ) . . . . SH:T ! ! !

I'm gonna run 83 octane this season to make up for it . . .
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Old 04-12-2005, 11:49 PM
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Default Re: Which octane fuel do you use?

87 in the 496 ho's, thank God!
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Old 04-13-2005, 12:01 AM
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Default Re: Which octane fuel do you use?

Preignition occurs when cylinder temps, octane, and other factors combine to force the mixture to self-ignite, before the ignition system fires.

Detonation is a sudden rise and oscillation in cylinder pressure. The oscillation is within audible range and is very much like a hammer pounding on the pistons, rings, etc. Detonation is the pinging sound you hear.

Preignition is often followed by detonation, but not neccessary for detonation to occur.

Cars generally run lightly loaded compared to boats and can get away with some detonation. With the heavy load of a boat and consequent much higher average cylinder pressures, the effects of timing and octane become a lot more important.
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Old 04-13-2005, 11:26 AM
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Default Re: Which octane fuel do you use?

I heard this from a Merc rep. some years ago.

The timing is being adjusted constantly in milliseconds with the EFI and MPI engines. The engine knock sensor lets the computer know that the timing is advancing too far and stops advancing the timing. If you use higher octane fuel that does not allow the engine to knock and give the computer a baseline, then it is conceivable that the timing could be over-advanced in certain situations.

I never talked to anyone who could positively confirm or deny this, but it sounded plausible to me. Can anyone shed some light on this theory?
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Old 04-13-2005, 01:14 PM
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Default Re: Which octane fuel do you use?

93 octane
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Old 04-13-2005, 06:06 PM
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Default Re: Which octane fuel do you use?

Currently running a 454mag carb stock compression ratio and large rect. heads with 37 degrees total timing...89 octane. Runs great with no audible detonation. Has lived most of its life running like this and running pretty hard I might add. Have I been luck or is 89 still not necessary because of my low compression?

I am planning to raise my compression to 9.5 - 10 range with a rebuild soon. Planning on using my same cast iron heads but adding a cam. Is 93 octane necessary with this compression or is 89 or 87 sufficient?? Thanks for any imput and reccomendations that any of you may have.
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Old 04-14-2005, 01:20 AM
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Default Re: Which octane fuel do you use?

Originally Posted by Doug
93 octane
Doug, your carbs look like CandS carbs. Are they?
If so have you had good luck with them?
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Old 04-20-2005, 06:39 PM
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Default Re: Which octane fuel do you use?

FOUR FUEL PROPERTIES ----------------------------

Listed below are the four basic qualities of fuels. As in everything, there are trade-offs. You can't make a racing fuel that has the best of everything, but you can produce one that will give your engine the most power. This is why we produce different fuels for different applications. The key to getting the best racing gasoline is not necessarily buying the fuel with the highest octane, but getting one that is best suited for your engine.

1. OCTANE: The rating of fuels ability to resist detonation and/or preignition. Octane is rated in Research Octane Numbers, (RON), Motor Octane Numbers, (MON), and Pump Octane Numbers (R+M/2). Pump Octane Numbers are what you see on the yellow decal at the gas stations and represents an average of the two. VP uses MON because this test method is more prevalent in racing. Most other companies use RON because it is higher ad easier to come by. Don't be fooled by high RON numbers or an average. MON's are most important for a racing application, however, the ability of the fuel to resist preignition is more that just a function of octane.

2. BURNING SPEED: The speed at which fuel releases it's energy. In a high-speed internal combustion engine, there is very little time (real time - not crank rotation) for the fuel to release its energy. Peak cylinder pressure should occur around 20 ATDC. If the fuel is still burning after this, it is not contributing to peak cylinder pressure, which is what the rear wheels see.

3. ENERGY VALUE: An expression of the potential in the fuel. The energy value is measured in BTU's per pound, not per gallon. The difference is important. The air fuel ratio is in weight, not volume. Remember, this is the potential energy value of the fuel. This difference will show up any compression ratio or engine speed.

4. COOLING EFFECT: The cooling effect on fuel is related to the heat of vaporization. The higher the heat of vaporization, the better its effect on cooling the intake mixture. This is of some benefit in a four stroke engine, but can be a big gain in 2 stroke engines
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