Active Mike

I'm running a 500Hp what octane fuel is recommended?
I have heard that these engines like the 87 octane and run their best.

mcollinstn

heard right.

cheaper is better

Active Mike

mcollinstn,
What is your experience with this octane? Do you run a 500 hp? I'm wondering if anyone has done test on this engine on a dyno with different fuels?

mcollinstn

AM,

"Additional octane" over and above the design spec of an engine NEVER gives additional performance, if the engine is in a proper state of tune.

The higher the octane rating is, the slower the fuel burn is, and the less btu per pound the fuel has. This is true for all "pump gases".

(We're not talking about race fuel or horsepower cocktails employing oxidizers and/or high volatiles like Nutec for example).

Now if you choose to increase your compression ratio or advance your ignition timing beyond the factory specs then you *may* be able to increase your engine's octane needs beyond the design spec. In that case, you will indeed obtain additional performance because lower octane fuels will be handicapped by the knock sensor on the motor retarding ignition timing away from optimum.

If it makes you happy, though, run 93 in it. Some HP500EFI's will soot up the transom with 93 fuel, but it can always be wiped off...

Active Mike

If I don't have to run the higher octane that is fine with me specially with the price of fuel this summer.
What octane is recommended the lowest on the pump or stay around the 87?

mcollinstn

HP500 EFI - 500 HP
Specifications
HP: 500
Full Throttle RPM Range: 4800-5200
Compression Ratio: 8.75:1
Fuel System: MPI
Fuel Requirements: 87 Posted Octane (R+M)/2 (92 RON) International

mcollinstn

As long as you are in the good ole U.S. of A., the yellow 87 sticker on the pump will be just fine.

If you travel outside of the US, you will need to know what method they use to figure their octane...

Stormrider

How about the same question for a 454mag 365hp carb'd engine? What octane?

Tricky919

Iv'e heard the same is true for the 496's, is this also right?

mcollinstn

Guys, it is clearly spelled out in your manual.

And I believe the rule of thumb is that all black Mercruiser motors are setup for 87 octane.