Really? I ran 87 Oct in my 525efi, no issues. That's what the manual calls for.

I have a labbed LH 26P Maximus for sale if you are interested.

Yes the book says that, he switched to 93 octane and his runs much better, planes out faster, he is amazed at the big difference,, like a different boat.

Are you sure it's not the beer? Just playin... Typically when an engine requires a particular octane ordinarily it won't affect the engine performance wise rather can actually build a little more heat from higher octane. More so with two cycle small engines. I'm not trying to say your BSing cause hey anything is possible. Did he buy his boat new? Just wondering if it's possible if someone owned it before him maybe had the heads decked etc? I know I tried 104 In my lawn mower and all I got was a woody and smelt like red neck cologne. Ha

Ya he got it new, and always used the cheap stuff 87 octane, we boated together for 15 years now, last July he filled up with 93 octane , runs much better, one Merc. mechanic he asked said the knock sensor was probably keeping the timing cut back with the cheap 87 detecting spark knock on the 525 efi engine

Is the 93 non-ethanol and the 87 ethanol ?

That I don't know, all I can find for my boat in our area is 10% ethanol

on that engine should not make that kind of a difference...smitty been dynoing a EFI engine, tried 93 and lost 10hp so ?? unless was some really junk fuel he was getting

I know he always bought the cheapest 87 octane he could find , at off brand, cut rate gas stations . so he most likely was getting junk 83-84 octane in reality.
Last summer he filled up with Shell 93 Octane, same thing I used for 15 years,, it runs tons better.