2 stroke motor have a lot less compression than 4 strokes, for this reason they don’t make use of the extra benefits of higher octane fuel. Increasing compression in a 2 stroke robs the engine of hp because there is only 2 cycles and the extra compression competes with the engine ability to reach TDC on the compression/ignition stroke. So there is no benefit in using 98 in a 2 stroke.

As for timing, ignition timing can be changed on a 2 stroke, but dynamic or engine timing is set by height of the ports in relationship to the combustion chamber. No cam or valves.

The problem with ethanol is all the plastic and rubber parts in the fuel system that ethanol eats.

Kandrew wrote: 2 stroke motor have a lot less compression than 4 strokes, for this reason they don’t make use of the extra benefits of higher octane fuel. Increasing compression in a 2 stroke robs the engine of hp because there is only 2 cycles and the extra compression competes with the engine ability to reach TDC on the compression/ignition stroke. So there is no benefit in using 98 in a 2 stroke. As for timing, ignition timing can be changed on a 2 stroke, but dynamic or engine timing is set by height of the ports in relationship to the combustion chamber. No cam or valves. The problem with ethanol is all the plastic and rubber parts in the fuel system that ethanol eats.

Older 2s was told to run 95 or betterless smoke and more economical.running 95 I am saving roughly 2 ltd against 91. Advised by mechanic and South Auckland marine

The ports take care of the intake and exhaust timing on a 2 stroke, effectively the same function as a cam in a four stroke engine. Two strokes do indeed have lower compression ratios as Kandrew says and that is why they are happy using lower octane fuels. The ignition timing is very critical and a pre-ignition scenario (detonation) is not what you want in a 2 stroke engine but you wont need to change it if you put in a higher octane petrol. Outboards are relatively low state of tune  beasts. A 90hp Etec is 1300 cc approx 60hp/l and CBT's little Kawasaki is 45hp from 350cc -approx 130hp /l. Bear in mind that bike is from 1972 or thereabouts and two stroke technology improved in leaps and bounds in the 80's with electronic ignitions, variable exhaust port timing, reed valves and carburettors that went directly into the crankcases. Things improved massively again with direct fuel injection and the corresponding refinement of all the other stuff. A modernish 250cc motocross bike engine puts out more hp than CBT's 350 from the 70's, close to 50hp. Of course you don't want to be rebuilding your outboard every 100 hours though. Be nice if the motor had lots of low end power too, which is easy to get when the hp per litre is lower.

Pcj wrote: Kandrew wrote: 2 stroke motor have a lot less compression than 4 strokes, for this reason they don’t make use of the extra benefits of higher octane fuel. Increasing compression in a 2 stroke robs the engine of hp because there is only 2 cycles and the extra compression competes with the engine ability to reach TDC on the compression/ignition stroke. So there is no benefit in using 98 in a 2 stroke. As for timing, ignition timing can be changed on a 2 stroke, but dynamic or engine timing is set by height of the ports in relationship to the combustion chamber. No cam or valves. The problem with ethanol is all the plastic and rubber parts in the fuel system that ethanol eats. Older 2s was told to run 95 or betterless smoke and more economical.running 95 I am saving roughly 2 ltd against 91. Advised by mechanic and South Auckland marine

ok fair call but 91 to 98 I wouldn’t except to see a saving with the price of 98. Remember not all 91s are the same and the ratings in the US which is what most of the manufacturers recommendations are based can be way over stated. No need to alter timing as you say.

But at the end of the day my 115 opti runs fine on 91 and it’s heaps cheaper because petrol prices are just ridiculous at the moment.

Cheers everyone. I'm staying with Z91. Same price as the (marina discounted) Gull 98.

Pierre'o wrote: Hi guys, where can I get the best deal on two stroke oil? B and MD has 4l Valvolene for $59.99. That’s $15 per litre - seems v expensive? Can anyone beat this? Pierre

found valvoline my motor didnt like it and smokes more,using mercs or quicksilver? less smoke and runs better,sure $24 lt 

Wow, I’ve only used Valvolene since new. Now on 85 hours.

e.m.p! wrote: mind if i barge in? I have some spare Motul DiJet 2 Jetski oil. Is that ok to use in my 30hp 2 stroke Tohatsu? Can't imagine to be too much of a difference