10.25:1 pistons- what octane rating would you suggest?

FWIW, I did a rebuild on my 900, bumping it to 1,024 and approx 10:1 then tried running it on 92-93. It didn't like it. Now back to straight 87 and everything's running great.

trippivot wrote:

dyna coils are the root of so many problems
avgas makes you slower

Nothing like some sweeping generalisations in the mix! I've had some good results with both

aviation fuel uses a differant rating system than pump gas as far as octane is concerned.

...hmmm, I've run every bike I've had on the highest octane you could get at the pumps. I'm running an 1170 at 10.25:1, maybe I will try the next lower grade and see how it runs. So far, I thought it was running pretty happy on the 93 octane.

Fossil wrote:

...hmmm, I've run every bike I've had on the highest octane you could get at the pumps. I'm running an 1170 at 10.25:1, maybe I will try the next lower grade and see how it runs. So far, I thought it was running pretty happy on the 93 octane.

Wouldn't you be a happy camper if it ran even better on less expensive gas!

If you have a 10.25 air-cooled engine tuned at max ignition timing it should not run without pinging upon acceleration on 87,89,or 93. Keeping the timing retarded some at full advance lets a lot of these bikes keep runing on the cheap stuff. I have my z1r running on 93 and am down about 20 horses because of backing off the timing. Its good enough but you gotta roll the throttle on,not whack it. 108 is another story. But 8 bucks a gal. at 15mpg = Ka-ching!! All these newer fuelie bikes will alter the spark and injection pulses explaining why they can run on 87 at 11to1. Be careful guys. I would hate to see y'all hurt an engine.

Here's some interesting reading:

en.wikipedia.org/wiki/Gasoline
en.wikipedia.org/wiki/Octane
en.wikipedia.org/wiki/Avgas

Effects of octane rating

Higher octane ratings correlate to higher activation energies. Activation energy is the amount of energy necessary to start a chemical reaction. Since higher octane fuels have higher activation energies, it is less likely that a given compression will cause knocking. (Note that it is the absolute pressure (compression) in the combustion chamber which is important - not the compression ratio. The compression ratio only governs the maximum compression that can be achieved).

It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation (pre-ignition = engine knock) as opposed to the ability of the fuel to burn (combustion). However, premium grades of petrol often contain more energy per litre due to the composition of the fuel as well as increased octane.

en.wikipedia.org/wiki/Octane_rating#Effects_of_octane_rating

You probably should have quoted this part also.

The power output of an engine depends on the energy content of its fuel, and this bears no simple relationship to the octane rating. A common myth amongst petrol consumers is that adding a higher octane fuel to a vehicle's engine will increase its performance and/or lessen its fuel consumption; this is mostly false—engines perform best when using fuel with the octane rating they were designed for and any increase in performance by using a fuel with a different octane rating is minimal.

Same link as above.

avgas has a different specific gravity than street fuels- you have to rejet to run it for good effect.

timing and camshaft duration, not nessesarily compression ratio would be cause for higher octane fuel, the higher the octane the slower the fuel burns,if you had advanced the cams, timing or lenthend the ignition advance more octane would be needed, bikes with short runner intakes deliver a good a/f mixture so uping the octane isn't always nessesary, if you went to 12.5 then maybe

the 10.25's in my bike (1105cc) just gets by with 94 and 104+ booster till I go to the track, then I need to run race gas (110) or after about a lap and a half it starts to sound like a tin can 1/2 full of rocks at anything over 1/3 throttle. In fact on a ride through the badlands of South Dakota, (108 air temp) could only buy 89 oct. gas, and it could only hold 70 mph without detonation. Stock cams and carbs, head ported.