Have a 500 cc pilot. Looking at octane. Had mod done to increase size from 400 to 500. I live next to a small airport where they have 100 octane fuel.


Any suggestions



Rob

What’s your compression ratio? That’s what matters...anything over 11.0:1 needs a higher octane over the 91 pump gas. And don’t forget the 2-smoke oil drops the octane quite a bit, which is why their compression ratios are usually a lot lower. It is a 2-smoke right?

For reference a stock CR250 has a ratio of 8.5:1 and Honda recommends 95 octane.

Yes its a 2 stroke.

Since it's a mod (500) I'll know the compression later in the week.

Thanks.

It will really depend on what they do with the head, if they went to 500 without cutting the head, well then you better find something more like vp 110. If they cut they head, it depends on how much. I was always able to get abay with premium up to about 185-200 psi(a little better for telling compression in a twostroke) most companies tell you you need race fuel about 175-180(stoke is about 165-170 in most hondas), but my motor builder told me other wise, and well the bike ripped.

The compression is 160-175 psi.

I'm guessing 100 octane should be fine.


Rob

Just stay away from that airport gas. It was designed for hi altitudes and that's were it works best. If you want to run 100 octane then run VP or another race fuel.

Dune... the lower the altitude the higher the Octane rating. "AV GAS" is rated 100 octane at a certin elevation like 5000 feet or something like that. I have been running it for 11 years now and so have many other people I know with great success... Plus its only $2.35 a gallon

I think with that compression(test while motor is warm, and let it crank for a few revs) you would be fine running pump gas, especially if you can get it from sunoco ultra 94, good stuff.

As far as av gas, I have talked with plane mechanics, and a petrolium professor I have and I was told that it is a little dirtier than most race fuels, not as refined. Sometimes it also has a gas additive to prevent any water in it from freezing. for my motor I wouldn't use it, but what the hell, it is cheap. by the by, octane doesn't change with altitude, an engines need for octane changes, ie higher elevations need less octane. Here in colorado our pump gasses are 85-87-91, not because its cheaper, but because we only need about 2 points less than at sea level. Just fyi.