What octane gas are you using?

[Karl]

I was reading in the general forum that those guys have to use high octane gas or their engines run like dogs.

You hear more about 4-strokes having high compression engines, but do 2-strokes? I'd read a long time ago that they typically don't.

What grade gas do you run? Currently, I'm running high octane only because it was left over from my dirt bikes. I need to review my manual to see what's recommended.

[90GTVert]

I try to run premium most of the time, no real difference either way for me. The issues they claim with buildup shouldn't have a great effect on a 2 stroke anyway. Technically, you're supposed to de-carbonise a 2 stroke every so often anyway. If you actually do that, I wouldn't think any additional gunk would do much.

There are high compression 2 strokes, but most stock 2 strokes prob won't need premium. You can get race kits with compression ratios up around 17:1, maybe higher.

I did run C12 (112 octane r+m/2 or 108 mon i think it was) in a 2 stroke pocketbike with a race kit that had 15:1 compression. It didn't run right with pump gas.

[Karl]

What is considered high compression for 2strokes then? 13:1 and up?

[kaintuckdave]

I have heard all over the map with the octane opinions and I'm still confused. Maybe 2 stroke brings a whole nother aspect into the argument??? Big Guy claims he's gone back and forth and gets better mileage with 87. So far, as I've just gone from 89 to 87, looks like my needle isn't going down nearly as fast, and also seems like I'm stronger riding so gonna wait and see. There are really good points with high and low octane with scoots, but maybe there needs to be a discussion among each cc group. Something needs to be broken down cause I'm totally confused on this topic.

[90GTVert]

I honestly don't know what would be condisdered a starting point for high compression for 2 strokes.

When building a hot rod, most people like to stop at 10.5:1 CR if they have plans to run on pump gas, and of course that's premium pump gas. There are other factors besides just the static compression ratio that come into play for determining what CR is safe, so that number will vary and so will opinions on a pump gas CR.

You can take an engine with 9:1 static compression that's safe on 87 octane and make it need premium. Advance the timing and it will spark knock wihout the additional octane. That's just one example of how octane rating isn't everything, but a good baseline.

[rallyrrr]

Kargo. When you get in the neighborhood of 13:1 coventional or 8:1 effective c.r. in two strokes alcohol fuels become necessary. The determining factor is the amount of time spent with the throttle pinned at maximum load. Two strokes can't stand near as much as four strokes since they have half the time to cool off between power strokes. Detonation takes hold and pistons start to melt. There is almost no end to the blends of exotic fuels used to avoid detonation and/or for oxygenating (power adders) in both petrol or alcohol applications. There is a fairly good treatise of this as well as of all aspects of two stroke tuning in "Two Stroke Performance Tuning" by A. Graham Bell, an Australian, published by Haynes Publishing. Get your bookstore to order you a copy. I was absolutely ignorant of two stroke performance enhancements until I got this book.

[90GTVert]

Thanks for the info and book reference. I'll have to check that out. +1 Bone