I know that this subject has been debated over and over again but rather than more debate i would like some input on what fuel octane levels you have had success with and how they have performed for you.I would like to run as low an octane as i can in my 12.5:1 690 but i would feel better hearing from some of you who have done this for an extended period of time without damage.With my first 686 at 11:1 i found that too much octane had a negative impact on performance so figuring out the best for this higher compression motor should be worthwhile.Another question i have is about wether or not aftermarket cdi units have any effect on what octane levels you can run since they can change timing or at least have the same effect or am i wrong?

 

12.5 :1 your going to have to mix 50/50 with race gas and premium.

 

I should make it clear that the cost of the fuel is not an issue with me but what i am after is the very best performance with reliability.After what it cost me to have this build done the fuel cost is pretty insignificant.

 

50/50 is right mix. 13:1 and up is all race gas. 100% race gas wont hurt, but it is more expensive.

 

You don't need to run any race gas at 12.5:1 compression. How do I know? My last 2 motorcycles (2002 YZ 426F & my 6 month old YZ 450F) have that same exact compression ratio & ran perfect on 91 octane pump gas. That's what the manual call for & that's what they like..never experienced any pinging or detonation even at Glamis wide open up Olds. That new VP 4-stroke gas will give you more hp, at the tune of $12 per gallon, because it is oxygenated. It's octane is only rated at 92. Too much octane(in the 102-108 range) will cause the fuel to burn too slowly.

 

Anything over 11.5:1 will require some type of mix or race fuel period, take your chances all you want.

 

There aren't rules carved in stone here, you have some leeway on the general thoughts on how to mix. Heck the YFZ is factory compression of well over 11-1 if I remember right. The swirl in the combustion chamber and how the burn occurs has an effect on your needed octane at given C/R, as well as the ignition timing. Some careful trial and error is in order, beginning with a known safe mix and working downward. JMO.

 

When you hear that awful ping, you will know you need a better fuel, I've heard it running lean on alky sometimes, it's not a good sound. Just EGT the motor in is the only way.

 

I have the 470 in my YFZ and 12.5:1 and run premium unleaded 98.
It would be slower if I put a higher octane in it.

Geoff.

 

Premium around here is 93, won't do