octane and 2-strokes

[Rockbadchild]

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.

That's the biggest misconception out there...

 

[Snobdds]

Elevation also plays a role in octane. The higher the elevation, the easier it is to pre-ignite a lower octane fuel.

For my twin turbo truck, premium is a must at high elevation to not have the timing so retarted that it robs all the power turbos were designed to produce.

 

[dodge man]

I actually think it’s the other way around. You see a lot of 85 octane fuel sold at higher elevations and it’s my understanding that’s fine if you stay at higher elevations.

 

[ruffdog]

I actually think it’s the other way around. You see a lot of 85 octane fuel sold at higher elevations and it’s my understanding that’s fine if you stay at higher elevations.

You are correct. Fuel sold in Denver has 2 points less octane.

Things To Know About Buying Gas At Altitude | Mike Shaw Subaru.

 

[Rockbadchild]

you guys are correct...Also elevation will reduce your HP, mountain sleds will run on 91+ octane or even airplane fuel since they turbo charge them swap the heads to increase compression and play with their fuel/air delivery via fuel controller in order to produce the same HP at elevation then at see level...

 

[Oaktree]

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

Mixed results here. I've had a couple of cheapie string trimmers as well as an old 2 stroke Polaris ATV that were definitely happier with higher octane fuel. Other stuff not as much.

 

[Kyle_in_Tex]

All these old 2 stroke street bike riders coming out of the woodwork. Next, y'all are going to tell me you like The Who too!

 

[MossRoad]

All these old 2 stroke street bike riders coming out of the woodwork. Next, y'all are going to tell me you like The Who too!

Meet the new boss.
Same as the old boss.

 

[Grumpycat]

A lower AKI (not "octane") is acceptable at higher elevations because the issue is not compression ratio but the compression chamber pressure.

A lowly Prius has a 13:1 mechanical compression ratio but with variable valve timing and throttle-by-wire happily runs on 87 AKI. The ECU manages load keeping combustion chamber pressures down. Will give an CEL if fed 93 AKI.

 

[Grumpycat]

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.

That was then. Look up "power valve". The exhaust port timing is controlled in every modern 2-stroke dirtbike the past 25 years. Lots of discussion on KTM forums as to the most desirable spring to control the opening for different conditions. Beta adjusts theirs with a threaded shaft making changes in the woods very easy.

The last RD400 was the first I remember having a power valve in the exhaust.