Quick Octain Help!

[RUFIO]

What Octain fuel should i run with this setup?

Sparks X-4
Uni Filter
155mj
Drilled Airbox Lid

[wheeltrax]

I would run what you were running before. since you didnt increase compression, there isnt a need to increase your octane.
I usually run a midgrade.

[RUFIO]

Wont it run better with higher though?

[KY Woods Rider]

Originally posted by RUFIO
Wont it run better with higher though?

That's a common misconception, but it doesn't work that way. A motor will run best on the least octane needed to prevent detonation (pre-ignition, pinging, spark knock, whatever you want to call it). The only advantage to running a higher octane fuel with a stock compression motor is that you might be able to get away with slightly leaner jetting. If you just put in high octane race fuel and leave everything else alone, you won't gain anything and might even lose a bit of power and throttle response.

Honda says the 400EX is designed to run on 92 octane or higher. The pumps in my area have 87, 89, and 93 octane, so I run 93.

[terko440]

Hey Rufio, Most people think of higher octace as turbo fuel or something. I used to sort of think the same thing. But the higher the octane rating the more is resists detotonation. Say you went with a higher compression pistion and used the same pump gas, before the piston reaches the end of it's stroke the gas will ignite by itself from the cylinder pressure before the spark plug ignites. So imagine the explosion happening while the piston is on it's way up. Not a good thing. So untiil you do some real engine work, save your time and money and stick with the pump! I believe the manuel of a 400ex dose prefer at least 92 octane though. Hope this helps. Good luck