Unleaded Or Premium?

[majed1965]

Hi All.

I am using unleaded fuel 50:1 mixture. I have 2 different views from 2 marine mechanics. One says to use unleaded only as premium has high octane and the other says only use premium. What is best to use on a 1977 70HP Evinrude? Thanks.

[Guest rzep]

That is really only personal preference.

I would suggest running 1 tank of each and seeing which one is more suited to your Motor.

You will get arguments either way but you will find that one fuel will make YOUR motor run the smothest or gives it the more power and then I would be going with this fuel.

Neither fuel would be detrimental to the running of you motor.

[domza]

ive been using optimax 98 or vortex 98 in my motor for 4 years, no problems yet.

[wildfish]

i run only bp ulimate in my 1974 15hp, runs awsome.

[chrisg]

hi majed

Its quite simple in a way....how do you drive your boat? Are you a cruiser or a full throttle merchant? And do you know what the compression readings of your motor are?

That age motor probably had compression readings of 150 psi when new and if in good order may still have figures close to that. As such you should really be running 95 octane fuel particulary if you give the motor some stick. That said, if you are a cruiser and don't go over 4000rpm too often 92 octane will be fine..save yourself a few cents.

If you are using pre mixed fuel (not oil injection) which i think you would be with a '77 don't use Optimax 98 or vortex '98...it has agents that break down the oil over time stood in the tank. Not good.

Cheers

Chris

[Lasty]

Does anyone remember the old 'BP Zoom'? Two stroke from a pump, I think it was 25:1.

[wettingaline]

Premium is always gonna be cleaner and more octane hence making your engine combust more efiiciently and effectively. You just have to decide if its worth paying for, personally for an old outboard i wouldnt run it, but every now and then when you feel like spoiling the old fella give him a tank of premo, otherwise run standard and put the difference in your wallet...

cheers

Joe

[Sammy0884]

I run BP ultimate in my Merc Optimax. But....

Older outboards should be using High octane, 95RON minimum as that was the previous standard of leaded petrol, as two strokes have no valves, the non lead doesnt matter. Old outboards usually have massive compression ratios for two strokes and will benefit from high octane.. As will high revving four strokes.

Now i need to dispell a myth about high octane fuel. It will NOT increase the performance of your outboard, and the only time you must use it is when a manufacturer tunes an engine to run on high octane fuel and advises so.. the main advantage of high octane is the cleaning properties of the detergents in the fuel which will lessen the occourance of blocked carby jets and the like.

Outboard engines run much cooler than automotive engines, and are much less prone to pre-detonation, or pinging as some know it, as car engines are when run under extreme loads or during times of extreme air temp. The hi octane has a much higher 'flash point' meaning it is more likely to misfire or even not start at all in a cold engine that already struggles. Low octane is much easier to start and will burn slower at a lower temp..

So, what fuel do you use?

If you have an old two stoke (usually pre 1986) that is high compression, use premium..

If you have a newer outboard (post 1990) two stroke it will run best on regular unleadded.

If you have a NEW engine, use whatever the manufacturer advises, so if your new yammie 60 recommends 91RON unleaded, use that, anything else will be a waste of money.

[mick309]

Optimax is designed specifically for fuel injected motors and should be avoided with carby motors. Bp Ulimate and mobil synergy are the best fuels for carby engines