Fuel Grade: Super or Unleaded?

[amoore658]

Hello!

I have a Yamaha-Mariner 40ELO (year c. 1990)
It has operated perfectly on Unleaded since I acquired it
However, I've been told I should change the fuel grade to Super Unleaded?

(Engine is 40HP, 2-stroke)

Thanks for your help

 

[henryf]

I would be surprised if you need to do that.

Given the number of outboards working away in countries where just finding fuel is a bonus let alone being able to choose brand and octane I think the proof is in the eating. Then you've got our cousins in the good old USA. They run their cars on low octane fuel. The big V8 in my Dodge Ram just needs green unleaded, no mention of super grade 98 octane.

The reason older engines needed (need) special love is because the valve seats use the metal of the iron cylinder head. Alloy heads need hardened valve seats due to the softness of the alloy. This resists wear. Cast iron heads needed lubrication from the lead in leaded fuel hence with unleaded they need additives.

Some engines require a higher octane to keep them cool (makes no sense but the higher octane burns cooler), or to stop pinking (valves getting slammed down on the valve seat by the combustion of the fuel).

Given a 2 stroke engine has oil mixed in with the fuel you get more than the lubrication of lead and any hope of ultra high octane goes out of the window so design the engine to run on lower octane.

All in my humble opinion of course.

Henry

 

[rafiki_]

Yep, Henry has given a very complete explaination. You might get a little more top end performance out of super unleaded, but it is not necessary unless you have a super performance 2 stroke, and you need the higher octane rating to prevent pinking as H says, where the carbon build up in the cylinder or piston creates a hot spot and pre-ignition.

 

[Alpha22]

+1
2-Stroke - Super unleaded?? No need. Unless particularly high performance racing machine and specifically specified in the manufacturers instructions..... even then I would bet it makes no difference......

 

[henryf]

It's worth pointing out that the octane rating of fuel isn't necessarily a measure of how much "power" or energy it has. It doesn't follow that a higher octane fuel will produce more power.

Higher octane fuels are more resistant to pinking or knocking. This means you can push the boundaries of engine design if you know you will only run on ultra high octane fuel, over 100 octane.

Modern engines have a thing called a knock sensor built into the electronic engine management system which listens out for knocking and retards the timing of the engine until it stops. So modern cars actually adjust to what you throw in them. Road engines are generally built with a view to longevity so even if you throw 105 octane fuel in the design of the engine means it is unlikely to be able to take advantage.

It does make for a good marketing advantage though and allows you to receive income from motorists desperate to buy the finest for their cars.

If you really want the very best fuel available then specialist companies sell what might be termed reference grade fuel. Precisely blended and handled to offer consistency. It comes at a price though. 98 or 99 octane fuel is over £2.50 a litre. If you think that's bad similar higher octane comes in at over £4 per litre and you do get through a drop or two in a 24 hour race weekend

Henry

 

[Spi D]

+1

As per page 4-1 in the manual your engine specifications call for Regular Unleaded Petrol, P.O.N. minimum 86 - equalling RON 90 Octane.

http://www.yamaha-motor.com/assets/service/manuals/1998/lit-18626-02-89_602.pdf


To add to it, octane is stated by more methods, RON, MON, RdON, AKI and PON. RON is the standard in UK, Europe, Australia and South Africa
Some of the major oil companies sell their fuel under marketing brands (eg. Shell Power xx), and actually tweak the composition between markets. So if you sense a difference when getting your usual fuel abroad, you may be right.



RON

Research Octane Number​

UK, Europe, South Africa & Australia​

MON

Motor Octane Number​

Motor sport applications​

Measured under more severe conditions than RON (higher revs etc). Usually 8-10 units lower than the RON.​

RdON

Road Octane Number​

Average of RON & MON​

Usually 4-5 units lower than the RON. USA, Canada​

AKI

Ant Knock Index​

Average of RON & MON​

Usually 4-5 units lower than the RON​

USA, Canada​

PON

Pump Octane Number​

Average of RON & MON​

Usually 4-5 units lower than the RON​

USA, Canada​

For further on the topic Google is your friend, eg.:

http://en.wikipedia.org/wiki/Octane_rating

 

[gordmac]

Only reason I can think of is that super won't have a lot of bio in it so may be more stable if it is lying around in a tank for a long time.