Interesting discussion.
To get a lot more information about this, than you could ever wish for, search the technical section of
OSO.
From what I've gathered over the years, its always better to run the
cheapest gas possible! Premium gas will not make a normal (marine) engine run any better, but regular octane gas
will make a high performance engine run worse. In fact, Mercruiser recommends a
maximum octane rating of 89 for all their HP500 (500 horse power) engines. Back in the beginning of the Marine EFI's engines, the electronic control module (ECM) would set the spark timing based on how much "spark knock" the knock sensor would pick up. Well, a lot of people started filling these new fancy EFI engines up with 93+ octane gasoline.
What started to happen was that the ECM was not sensing any knock since the octane was so high. The ECM was not smart enough to realize the owner was running premium, so it thought there was a bad knock sensor. Thinking this was the case, the ECM pulled out
all the timing advance and ran a "failsafe" 0 degrees advance. You don't have to be a car nut to realize that: 0 degrees advance = significant power reduction. This was mostly the case on the first generation of 454 and 502 magnum motors. All of a sudden these 300 - 400 horsepower engines would barely be able to top 200 on the dyno with premium gasoline.
MerCruiser remedied this problem within a year or two, but is a good example of how running premium fuel in an engine will most likely yield zero results (and some times negative). In fact, a pound of 87 octane gas actually has more
energy than a pound of 93 octane. The higher octane has simply been more refined to sustain the increased temperatures found in high compression and supercharged engines.
Bottom line, running more than the recommended octane on a stock engine is a money waster. However, if you are really bent on throwing away money, please send a check to:
Bayley's Boat Fund
5200 Auto Club Drive
Dearborn, MI 48126