Octane requirements for HP500?
 

I recently purchased a Fountain 38EC with twin HP500's. Reading through the Mercury engine literature I was surprised to see Merc recommends 87 octane for these engines. Is this a misprint?

nope,,,,,87,,,its a low compression motor,,,thats all it needs

No misprint. They run fine on that. Personally I find 89 is the best for them. 91 and up is too high Imo. Better combustion with the lower octane stuff. They are only 500 hp 502s.

If I run better then 87 I have a transom full of black soot at end of day.

Me too . I tried 93 and all it did was create a make work project .

I run 87 in my 525efi, no problems.

What is the compression on a hp500 by the way???

Mi engine is 9.3 to 1. I been runnin 89........seems to do ok.

According to the Merc manual the compression ratio on the HP500 is 8.75:1.

87 octane.

agreed 87 is all you need.