octane on stock 454 Mags?
 

I've done some searches and couldn't find anywhere what Mercruiser recommends. I have 2 boats: one with a 1992 454 Mag. Carb engine 365HP. One with a 2000 MAG MPI 385HP. I've always run 89 or 93 in the Carb 454 is 87 OK? I'm pretty sure 87 is fine in the MPI engine but want to make sure.

Thanks

I have 1995s with EFI. I was running mid range like 89 for years. My mechanic said I was wasting my money. Been runnuing 87 for a long time and no problems and no difference in performance.

Jeff

they are set up to run on 87, in my area if you buy on the water thats your only choice in most places...

I can tell you first hand, if its says 87 thats what you should run 92 is a waste of $$. You will also notice alot less soot on your transom.

The only thing you might consider is when you store your boat for long periods of time, I read gasoline can loose up to a half a point of octane value a month. So I top off with 93 octane and fuel stablizer for winter storage. The marinas by me only sell 89 or 93 even though Merc says my 502 mags will run on 87.

The lower the octane, the easier and more completely the fuel burns. They will make the most power on 87 and have less soot on the transom. Both the carbed and efi 454mags are designed to run on 87 octane.
If you are always running 89 octane, then you can advance the timing 2* to 34* total.

mercruiser manual #16

mercruiser service manual #16