what octane?

[brantley moats]

What octane gas is recommended by Mercury for a 2004(maybe 05) , 225 h.p. efi motor?

 

[Bob Bourget2]

87. You can use any octane though, with no ill effect, as long as ethanol percentage does not exceed "10". There's a lot of talk about carbon build-up associated with use of higher octane IF the motor does not need it, but with use of the fuel additives most are running, it is NOT an issue. I am a firm believer that if your motor is propped correctly, timing is correct, 87 octane will provide trouble free performance. If you are lugging your motor at the bottom end of the recommended scale with too big a prop, the chances of engine damage from detonation / pre-ignition are increased. The absolute best performance is to be had with the motor on the "verge" of pre-ignition. No, don't ask, you can't tell without proper {read $$$$$} test equipment.

 

[brantley moats]

THANKS, Bob. B

Brantley

 

[Berry Lindley]

So Bob, why does the manual for 2005 my carbed 90 call for mid grade? If 87 is good I will start using it.

 

[Bob Bourget2]

Get me a serial and I will check when I get to work BUT.....that '05 90 2-stroke is a low compression, mildly tuned engine. Originating back in 1988, it was designed as a durable "work" motor capable of running on some "extremely" low-octane, 3rd world fuels. I don't know of any PRODUCTION Merc that requires better than 87, other than some early XS models, hi-perf engines and the 275 / 350 SCI Verados. :blink:

 

[Berry Lindley]

OT983830 2005 ELPTO

 

[Bob Bourget2]

The service manual states "87 octane minimum"...

 

[Berry Lindley]

Thank You Bob.

 

[Richard Hodge2]

Great info....I find it interesting that almost all lawn equipment companies want mid grade per their manuals and are extremely bothered by ethanol. It's good to know this because at first I was buying 89 octane out of habit from my other equipment.

I recently bought fuel at a boat dock and was surprised it was non-ethanol and not as outrageous in cost as it used to be. Back in the day I seem to remember them selling pre-mixed and it cost more then, than I paid recently for straight fuel. Every time I sit and read these posts I get the itch to get back out!

 

[Bob Bourget2]

Be careful Rich that you don't mix the non-ethanol fuel with the "E-10". It can and will cause you grief, by way of separation with methyl-tertiary-butyl-ether.

As far as the octane goes, there seems to be a trend towards higher octane....when it comes to length of time stored. Although there is not any performance to be had {with motors that do not need the octane}, there is a safety margin with octane degradation. I would prefer to see you buy fuel at a high-volume, high turn station, rather than be brand selective. Also, avoid filling at any station where you see a delivery truck filling them up. You don't want the "remnants" of the bottom of their tanks.

On a side note...we are seeing ethanol contents ranging from "0" to an astounding 22%! Talk about trying to keep these motors running consistently is a chore.....:blink: