m1super90

I have the 7.4L MFI motor in my 1999 212 Islander. This is one of those questions nobody seems to be able to answer. Especially the dealer. Is this motor able to take advantage of 92 octane fuel? Meaning, does it have an adaptive fuel system that will attempt to advance spark to predetonation, or is the curve fixed? The manual says the minimum octane is 87 but doesn't discuss the issue at all.

99Hammer

I have the same engine in a hammer and all I run is 87. Put 92 in it twice (in a row) and noticed no difference.

m1super90

Thanks a bunch! I'm going to go top her off with 87 now! I'll try to get my 20 cents/gal back from the state on it

Luv-N-It

I agree. I have tried 92 octane as well as 94 Ultra octane from Sunoco and I did not notice any performance difference with my boat. I have the 502 EFI.

I have had some dealers tell my friends, when buying new boats and taking possession, run 87 and you'll be fine. That is for the black series motors. Don't know about the blue racing series motors though.

I would think you would want to run a higher octane in those babies. Any one with those type motors, or even a high performance motor with the works, l would be interested in reading your post in regards to this subject.

I have found that Shell 87 even burns better with my boat. What I mean is, there is not much black exhaust fumes under the transom when I have finished boating for the day or for several days for that matter.

Shooter

92 octane has no more BTUs in it than 87. The octane rating is the speed of the burn / flashpoint. I run 92 becasue I have a blower but I can run lower octane as long as I'm mellow. Preignition is the consideration when looking at octane. Reports have stated that you can actually marginally hurt perfromance because of the slower burn characteristics when appled to a motor specifically designed for 89. At the price, stick with cheaper gas ass long as you can!

Team V

I've always been told that there is a point at which your compression ratio (I don't know what it is off hand) will take advantage of the higher octane fuel. For example, if you only have 8:1 compression, you are wasting your money buying higher octane gas. Has something to do with the burn/flashpoint like shooter said.
If you have a high compression engine, then you are required to run a higher octane fuel, or your motor will knock and ping and possibly damage valves.

dloftis

Shooter is the one I'm going to agree with here.

Octane deals specifically with the volativity of the gasoline. Higher compression engines require higher octane fuel because the compression levels will cause detonation (knocking) with low octane gas. (Essentially, compressing the gas causes it to ignite before the spark plug ignites it.)

To obtain higher octane the fuel has to burn slower... which actually hurts performance.

In the end, the lower the octane, the better performance... but if your compression is too high for low octane, you have to trade that fast burn for stability (low for high octane.)

Now some EFI/MPI engines are capable of advancing ignition timing to take advantage of higher octane fuel. These programs will get better performance out of your engine just by using higher octane gas. To my knowledge these don't exist (at least alone) for boat motors, and if they do I doubt they can "sense" your higher octane... it is a fuel mixture map in your ECU.