do i need 2 use race gas

[roostking450F]

im gonna get a max rpms 523cc big bore kit.
the options i have are 11.5:1, 12.5:1, 13.5:1 compression pistons. would i have 2 use race gas for any of these?
oh its got 68mm stroker crank w/ knife rod and 99mm bore.

[Toadz400]

Originally posted by Quad_King
You wouldn't have to run race gas... but if you don't your YFZ will run like ***** and all that money you put into your YFZ will go down the drain because it will be running like *****, unless you run race gas...

:confused: :confused:

Anyway, 12.5:1 compression will be safe on premium but if you notice any pinging sounds (predetonation) try running a 50:50 mix of premium and race gas. Anything higher than 12.5:1 I would highly recommend running at least a 50:50 mix or 100% race gas. Running race gas in anything that doesn't need it will cause you poor performance and you will lose power.

[CHEVYZ]

I would also say that 12.5:1 is the max on pump, but even so I would run at least 50/50. You are better off just running race fuel, really. You will not lose any performance with that kind of compression... to be honest, I can only see gains. I ran a 50/50 mix of race fuel in my stock 450r (12:1) and noticed that it ran more efficiently and was more snappy than with regular pump.

[GPracer2500]

This may not be significant because the bore is only changed by 4mm. But, the larger the bore the greater the chance of detonation even if all other things are equal. Hypothetical example: 12.5:1 with a 95mm bore doesn't deto with such-and-such fuel. But 12.5:1 with a 99mm bore does even though the only difference is the bore size. I'm guessing a 4mm difference probably isn't big enough to make a noticable difference in this case.

Where this DOES matter is when the differences in bore are larger. For example, if you had an XR250R with a 11:1 piston it may not detonate. But an XR600R with the exact same dynamic compression could easily detonate.

Why? It takes the normal flame front longer to spread across a larger bore. The longer that takes the more time there is for the end gases to succumb to the pre-flame reactions that are responsible for detonation.

I suppose I bring this up only to highlight that there are many factors other than a piston's compression ratio that determine the octane rating required by an engine. That's why it's impossible to determine the octane requirement of an engine by only considering piston choices.

[CHEVYZ]

Thanks for that post GPracer2500, I was curious about that. Insightful.