octane and 2-strokes

[etpm]

Back in the 70s I rode 2-stroke bikes and used lawn mowers with 2-stroke engines. I really liked 2-strokes because of the high power to weight ratio. Interestingly, all the manuals for the bikes said not to use high octane fuels for the best performance. 89 was usually suggested. I did stuff to my bike engines to get more power. Things like porting and raising the compression and using expansion chambers. I had lots of success increasing HP and RPM. The reason for the low octane rated fuel, I was told, was because the gas would burn faster and this was necessary because as much power as possible must come before the exhaust ports are effectively open. I know, effective port timing can be changed by the use of expansion chambers and intake manifold changes. Exhaust and intake resonance can be used to control the fuel/air mixture flow into and out of the cylinder. RPM is part of the way resonances are controlled. Nevertheless the physical location of the ports cannot be changed like variable valve timing in a 4-stroke. The long preamble is just so you know why I am curious why so many folks these days talk about how higher octane is desirable in 2-stroke engines. I saw this once again in the recent E15 ethanol thread. Octane does not add heat energy, it just helps control burning. Higher octane being slower burning. And adding ethanol, while increasing octane, actually lowers the heat energy of the gasoline and ethanol mix. But by increasing octane the compression ratio can be raised and this will allow getting more energy from the fuel. Which in 4-stroke engines is usually a good thing.
Eric

 

[RustyA]

Owned a 1967 VW Beetle with an air cooled engine in high school. Right on the fuel door it said minimum octane needed was 89, which back then was non ethyl (premium) fuel. Now the only option for 89+ octane for most stations is E0 premium 91 octane.

 

[5030]

Always run my 2 stroke equipment on regular synthetic gas. I believe it's 89 Research Octane.

My understanding is, the higher the octane number is, the more resistant the fuel is to premature detonation (knock).

 

[Kyle_in_Tex]

todays 87 would be fine, but the ethanol is so so not so good. I'm a 2 stroker too.
RD400, RZ350, Got to ride an RZ500. These engines have low compression. No need for high octane.

 

[5030]

I'd love to own a Kenny Roberts Yamaha RD 350. Last screamer I owned was a Kawasaki H1 triple. Kind of surprised I'm actually still alive. Way too much motor in way too little frame with marginal brakes.

 

[orezok]

I'd love to own a Kenny Roberts Yamaha RD 350. Last screamer I owned was a Kawasaki H1 triple. Kind of surprised I'm actually still alive. Way too much motor in way too little frame with marginal brakes.

I still have a H1 750 Triple. Want another one? Me and a friend had the 6th Kawasaki dealership in the USA in the mid 60's. They would give a dealership to anyone who asked and would floor plan the bikes! We only got it to support our racing.

 

[Kyle_in_Tex]

I still have a H1 750 Triple. Want another one? Me and a friend had the 6th Kawasaki dealership in the USA in the mid 60's. They would give a dealership to anyone who asked and would floor plan the bikes! We only got it to support our racing.

I'll bet you have many a good story to tell. I test rode the Kawi 400 which was a 3 cylinder 2 stroke. But Cycle Guide did a report on the 76' RD400 and they were ecstatic about it, so I bought a Yamaha.

5030, I raced my RZ 350 at Texas World Speedway in College Station back in the mid eighties. You had to do 80mph on the banked curve just to hold neutral. My 350 with Toomey race pipes, mild port job, and a shaved head, would do close to 130mph. The pavement was so rough, it was like riding the pony express, bucking back and forth. It could outrun most Harleys, and I could start it by hand on the kick starter.

 

[ericm979]

todays 87 would be fine, but the ethanol is so so not so good. I'm a 2 stroker too.
RD400, RZ350, Got to ride an RZ500. These engines have low compression. No need for high octane.

I owned an RZ500 for a while. Spare parts being hard to find I never took it to the track. I used modified RZ350s for that.

It's not just compression that requires higher octane in two strokes. Heat needs it too. The evaporation of fuel absorbs a lot of heat. But the best power comes from leaning out the fuel/air mixture. Go too far and it will detonate at high speed. That can cause a seizure pretty quick.

Chainsaws are mildly tuned industrial engines compared to motorcycles. And the typical tuning where it four strokes when you lift in the cut would be considered rich by a motorcycle racer. Stock saws are petty safe on pump gas. Stihl calls for 89 octane for many of their saws so I run 89 or 91.

Speaking of leaning out for more power, Yamaha had Mikuni mis-mark the main jets in the RZ500- they were marked 195 but were really 165. The reason was so when people leaned them out for more power they would not seize them and then try to get it covered under warranty.

 

[ericm979]

I'll bet you have many a good story to tell. I test rode the Kawi 400 which was a 3 cylinder 2 stroke. But Cycle Guide did a report on the 76' RD400 and they were ecstatic about it, so I bought a Yamaha.

A Kawasaski S3 400 was my first bike. Unlike the 500 and 750, it handled ok. But I think the RD400 was much better all around. At the time it was beyond my budget but I could afford an S3 that needed work.

 

[etpm]

Always run my 2 stroke equipment on regular synthetic gas. I believe it's 89 Research Octane.

My understanding is, the higher the octane number is, the more resistant the fuel is to premature detonation (knock).

That's true, higher octane is more resistant to pre-ignition.
Eric