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

 

[etpm]

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.

Yeah, I rode an RD350. It was fast. I made it even faster. It eventually burned a hole through a piston. I goofed somehow when modding it. It was so damn fast but was running too hot apparently. I sold it after I burned that hole in the piston. Good thing too, it was too fast for my skill.
Eric

 

[3Ts]

I've been told by a small engine mechanic that if you store gasoline it should be the higher octane because as the gasoline ages, the octane number goes down. Thus if you start with 87 octane and it goes down it becomes unusable. I've also been told that small engines need a minimum of 87 octane to run, so I've been using 92. Any one able to confirm this or correct this?

 

[Patches]

I've run chain saws, leaf blowers, snowmobiles, etc all being 2 strokes on everything from 87 to 94 octane fuel and never noticed any difference in performance or longevity

 

[rScotty]

I took 4 strokes to the track - Norton & Ducati.... , but loved the slower 2 strokes for weekend mountain riding and touring around the state. For longer trips to the East or West Coast I took other bikes of course,

But for just putting through the mountains I liked my old Jawa CZ 350 Californian. After all these years it's the bike I think about. Marvelous craftsmanship on that Jawa. Funny thing is I owned or worked on and rode most every British bike made....the Atlas, Goldies, REs, Matchless --- but it's the old black BMW and that Jawa I remember best. Both were simple & slow, both surprisingly well-balanced and comfortable. Actually sort of tractor-like.

The shop had a couple of Suzukis that were popular as loaners. The 250 X6 & 500 Titan were deceptively fast, nervous, flashy, quiet, indestructible and chrome/ugly.
rScotty

 

[Rockbadchild]

All octane dose is it makes the gas more stable to compression/heat so it initiate when the timing dictate it and not before. Any compression of any gas or object will generate heat so these two are link, it all depend the compression of that specific engine. I use 91 for my quad since its a 1000cc and it generate a lot of heat my 450WR he get 91 as well its a high performance engine so it need it, then my snow blower and chainsaw gets it as well only because there isn't any ethanol in 91 gas.

 

[MossRoad]

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

I had (still have in little pieces) a 77 RD400 that I used to drag race at the local strip. Higher Octane gas prevents pre-ignition, and that's about it. You can get pre-ignition in a 2 stroke from high compression, carbon deposits in the cylinder head, things like that. I had higher compression and free access to 100LL avgas at the time. It ran very nicely, especially on cold days when the air was denser. Otherwise, I just ran regular gas most of the time, when I wasn't beating on it.

It would turn very consistent 11:92 at 119mph. Not bad for 25 cubic inches.

 

[dodge man]

Im lucky I can get 91 octane ethanol free fuel. For a while I was using 87 octane ethanol free. I never knew what was worse, running to low an octane or using 10% ethanol. Stihl is saying 89 octane in their manual for their chainsaws.

 

[ruffdog]

The oil in 2-stoke fuel will lower the octane 1-2 points hence the need for slightly higher octane to start with.

Alcohol does act as a cooler but it is also an oxygenator which adds heat.

I tend not to mess with the ports in the cylinder. I have matched the bottom of the transfer ports to the bottom of the piston at BDC and get good results. This adds flow without changing port timing. Also, mill head to get squish area down to 0.04".

Past bikes: Two H1 500, Mach III 500, H2 750 (high comp,ported,carbs,chambers), and 2 Quadracers, a 250 and a 500. (short list)

 

[rScotty]

I never thought of octane in terms of more or less power or faster/slower burning.
To me, octane was simply a measure of the tendency of a fuel to ignite when compressed.

 

[ruffdog]

A lower octane fuel may ignite just from being compressed (pre-ignition) while a higher octane fuel will resist until the spark ignites it.