of2fsh wrote: Been running 98 in my new fourstroke 60 hp merc,do i need to or is 91 ok ?

what does the handbook for that motor say?
Like cars there are sometimes good reasons why 98 is used

of2fsh wrote: Good idea...ill go find it

Have often wondered the same thing with the 90 hp merc which states it should be run on 95 or higher.

The spec's on the merc web site state 87..

Recommended Fuel 87 octane/up to 10% ethanol

http://www.mercurymarine.com/engines/outboards/fourstrokes/40-60/

imho it can be a false economy as you go further in your car on 95 than 91 due to the more bang for your buck...

Also watch the brand of fuel you use, cheaper ones can be at a minimum on Octane rating, whereas big brands ie BP or Z will be on the money

of2fsh wrote: Thanks for the answers,next question ,engine does 6000 rpm,still running in so only been up there once or twice .while breaking in ive been running around low to mid throttle ( done 7 hours now so cruising at 3500 rpm up to max 4200 rpm) with varied throttle settings,when its run in what is a maximum rpm to cruise long distances ?

When I spoke to a dealer about fuel for my mercruiser I was going to use 91, because that is what the handbook says. But he said to use 95 to make sure the octane rating was high enough. If you use 91 and there is any determination in the fuel it will be too low. But using 95 you should be guaranteed at least 91. I suppose that depends how long the fuel sits in your tank, or the tank at the petrol station.

F1SH3N wrote: The spec's on the merc web site state 87.. Recommended Fuel 87 octane/up to 10% ethanol http://www.mercurymarine.com/engines/outboards/fourstrokes/40-60/ imho it can be a false economy as you go further in your car on 95 than 91 due to the more bang for your buck...

careful - thats a north american website and they measure anti-knock properties of fuel as PON - which is about 4-5 units lower than the rest of the world which quotes it as RON.

outboard manufacturers tune their motors to run on american gas - which is around 91RON - simply because thats the largest market.

so 91 fuel is fine.

modern efi outboards include knock-sensors which will retard the ignition if lower octane fuel is causing uncontrolled detonation.

improved engine output from 'hi-octane' fuel will only be realised if the ignition timing is advanced - so in essence you need to tune the motor to the better fuel

I dont know about the octane stuff, I just know you should avoid ethanol added fuels like the plague, as they chew the guts outa of your motors soft bits, seals etc, or so my mechanic tells.... and has shown me on a motor he was servicing. so....no Gull gas I think....

gull have 91 and 10% gas, i use 91 will go higher octane when i suss out a 4 stroke, as this stops the detanation or pinging you hear. as it's two stroke i'll run 91 for now..