87 Octane vs. 91 Octane in 250 Pro XS

[egammon]

Don:

I have a 2010 250 Pro XS. I have been fortunate enough (so far) to have always been able to find non-ethanol gasoline. And (so far) have always run 87 octane (per owner's manual). This engine has slightly less than 100 hours on it, and it is mounted on a fairly light hull (21' Bullet).

We have very few local stations that sell non-ethanol gasoline, and some have begun limiting their non-ethanol fuel to premium (91 octane) gas.

[/b]My question -- is one or the other (87 vs. 91 octane) better for my engine ---- assuming neither contains any ethanol and assuming I continue to add your "cocktail" to every gallon that I burn ??? [/b]

Cost isn't really a consideration -- by the time I drive to a station selling 87 octane, I have burned enough fuel in my truck to have bought the 91 octane from a closer dealer.

EDITED to add: I just did a better search and found your previous answers to this question -- "don't use 91 octane fuel."

So I have an amended question: Assuming I am filling a partially depleted fuel tank (for example adding 18 gallons to top off a 36 gallon tank) -- If the fuel already in the tank is 87 octane, can I top it off with 91 octane, and end up with ~ 89 octane fuel ??




Modified by egammon at 7:28 PM 6/20/2011

[esdbass]

I'm not Don but I don't think he'd recommend mixing ethanol gas and non ethanol gas together but if it was mixing like fuels either with ethanol or with out and you wound up with a tank of 89 I'd think he'd be fine with that?

[EuropeanAM]

<table width="90%" cellspacing=0 cellpadding=0 align=center><tr><td>Quote, originally posted by egammon &raquo;</td></tr><tr><td class="quote">Don:

So I have an amended question: Assuming I am filling a partially depleted fuel tank (for example adding 18 gallons to top off a 36 gallon tank) -- If the fuel already in the tank is 87 octane, can I top it off with 91 octane, and end up with ~ 89 octane fuel ??

</td></tr></table>

Theoretically, yes.

Actual octane may vary.... as pump postings are NOT always totally accurate, and we don't know how much (if any) your current fuel has degraded.

One CARDINAL rule to consider: If and WHEN the time comes to change from NON-ETHANOL to ETHANOL fuel, make absolutely certain that you do so with your fuel level AS LOW AS IS SAFELY POSSIBLE (and run the next tank out soon, do not let it "sit").