This is true. I have seen a 525 boat that used 93 octane and that transom was covered in soot!

If you always run premium fuel you can bump the timing a bit for some added power. The down side would be if you do that you must run premium fuel or risk detonation. If you dont know what your doing dont do it and run 87-89 octane with stock timing.

I'm not sure what years this applies to, but at one point in time, the merc engine management sytems would advance timing until the knock sensor detected ping, and then back the timing off a certain amount. Running 93 octane would actually keep the knock sensor from going off, and thus the box would think something is wrong, and dial timing back all the way - so you would actually loose performance by running higher than the prescribed octane. And, I think you'll also find that the idea of 93 octane "cleaning" things out is nothing but good marketing......people do it in their cars thinking they are treating it to something special.....total waste of money if the cars system is not set up for the higher octane....

I've seen 502 Magnum EFI dyno sheets. Lost 2 hp using 93 instead of 87. If you don't have the compression or timing to take advantage of the higher octane, you are losing power. Higher octane is harder to ignite, therefore more detonation resistant.