fuel octane

[bowman316]

Re: fuel octane

i think most people unnessecarily use high octane gas in their boats. Most put 93 in their tank. I think this just builds up more carbon on the piston rings. it would be interesting to start a poll asking which type of gas you use.
i don't know how to do a poll though.

 

[ezeke]

Re: fuel octane

Take a look at Bill Kelly's thoughts on the subject: http://maxrules.com/fixgas.html

 

[TN-25]

Re: fuel octane

The octane rating is a measure of anti-knock properties of the gas. If detonation is a problem then by all means up the octane. If there are no detonation issues then you may be wasting money on excessively high octane. Higher-octane gas is more resistant to detonation, and if it is not required you may have gas that is slightly less willing to fire. A lower octane gas may even make starting easier by it?s nature to ?light up? more willingly. I know that on my lawn mower they specifically caution against using high octane gas, probably for that reason. The caveat is that, at least locally here, that lower octane gas has more methanol in it. Shell around southern Ontario has up to 10% ethanol in it?s 87 octane gas, up to 5% in it?s 89 octane, and no ethanol in it?s 91 octane.

You can always experiment and use empirical observation.

 

[ondarvr]

Re: fuel octane

.Grem


Outboards and racing (motorcycle) 2 strokes differ in several ways.

Outboards run at high loads for long periods of time.
Racing 2 strokes, and I'll stick to off road because that's what most people compare them to, very rarely run WOT for any length of time, as in seconds, it's not even minutes, let alone an hour.

Jetting tends to be leaner on off road 2 strokes. When dessert racing, small engines often seize due to being held wide open for even a few minutes. This leaner jetting is for rapid acceleration and peak power as needed in MX or Supercross, not a sustained load.

The cylinder and head temperatures on motorcycles change rapidly, they do have water cooling, but there?s no thermostat and only a small reservoir of water, this can result very high temps after any sustained load. They rely on the throttle being off a significant amount of time giving the engine time to cool slightly before you exit the corner and accelerate.

Outboards run at 5,500 rpm or so at WOT, racing engines hit double that, they may have little power at 5,500.

Racing engines rarely get run long enough between teardowns and rebuilds to build up much, if any, carbon.

 

[Grem]

Re: fuel octane

.Grem


Outboards and racing (motorcycle) 2 strokes differ in several ways.

Outboards run at high loads for long periods of time.
Racing 2 strokes, and I'll stick to off road because that's what most people compare them to, very rarely run WOT for any length of time, as in seconds, it's not even minutes, let alone an hour.

Jetting tends to be leaner on off road 2 strokes. When dessert racing, small engines often seize due to being held wide open for even a few minutes. This leaner jetting is for rapid acceleration and peak power as needed in MX or Supercross, not a sustained load.

The cylinder and head temperatures on motorcycles change rapidly, they do have water cooling, but there?s no thermostat and only a small reservoir of water, this can result very high temps after any sustained load. They rely on the throttle being off a significant amount of time giving the engine time to cool slightly before you exit the corner and accelerate.

Outboards run at 5,500 rpm or so at WOT, racing engines hit double that, they may have little power at 5,500.

Racing engines rarely get run long enough between teardowns and rebuilds to build up much, if any, carbon.

Yes, my point wasn't that there are no differences between motorcycle and marine 2-strokes, merely that those differences in themselves do not have a direct bearing on the octane rating of the fuel required.
Over the years I've had everything from air cooled 50cc, through high compression racing 250cc upto my currently owned Suzuki GT750 triple, water-cooled (thermostatically controlled, large radiator-ed) one man environmental disaster, which will happily cruise all day WOT.

 

[ondarvr]

Re: fuel octane

Much of my response was to motorcycles being run flat out under a heavy load and DH saying don't confuse the requirements of outboards and motorcycles. In rereading your posts I may have been a little confused at what you were trying to ask and state.

Back to what DH said, your 750 triple does not run at WOT for more than a minute and normally much less time than that, its cruising, almost coasting once you get up to speed, even if that speed is fairly high.

If the 2 strokes were detuned (compared to racing) enduro or duel purpose type bikes (none of these are really made any more) then 87 would be fine.

The higher compression, heat and RPMs plus leaner jetting of off road motorcycles can require a higher octane fuel.

These are some big differences in the environment the fuel is being used in when compared to an outboard (discussed before) and it's critical to use high octane fuel in racing conditions or damage can easily be done.
Even the oil required in each type of use is different.

I'll only comment on what I know from 28 years of racing off road in the pro class and working in bike shops. DH can comment on his many years of racing and working on outboards as to the exact type of damage high octane fuel can do to a stock carb'd outboard (I don't want to get the details wrong) .

 

[Grem]

Re: fuel octane

Those old Suzuki triples were made in a fairly low state of tune for the sake of reliability and longevity. In Europe, I have run the thing WOT for long periods, two-up with luggage and no fairing. Believe me, the thing is under load in those circumstances. I had assumed (perhaps wrongly) that 2-stroke outboard engines designed to do a similar job would be sufficiently low tuned in order to cope with the load and endurance of the application.
In any event, manufacturer's recommendation is golden!