My dad works at a Chevron refinery and can get me about any level octane gas I want for my 1999 400 EX. What is a safe level? I know the manual says to use 92 or higher. Premium out of the pump only says 91. The few people I have talked to say around 100 would be perfect. I mostly trail ride but love to open it up and fly. What octane can I run in my quad all day safely?

 

You won't need higher octane gas unless you increase the compression of your EX. Higher octane gas is just slower burning for high comp engines so they won't overheat and preignite.

 

The 400EX is a low compression engine, stock is 9.0:1 No real need to run high octane fuel (especially considering the YZ400F runs fine on 92 with 12.5:1 compression). I have experienced preignition under high engine load at very low rpms, this is caused by a lean mixture.

Make sure it is jetted properly, and you should be more than safe on 92 octane premium...

 

On the YZ400F, I think the timing adjusts to the throttle position, allowing a higher compression ratio. When will the quads start using atleast 90's technology?

 

I have been told that higher octane does not make any difference, so I went and asked the Honda dealer that I bought my Y2K 400ex from and he said that if I ran Turbo Blue in the bike that it would NOT mess up my warranty. So I ran it on my second tank of gas and it was a INSTANT power boost. Considerable difference. Still can't go through the gears like a 250R, but was a big difference.

 

You cant get to much octane.. but when you break the 115 octane level your preformance will start to suffer. I would run 90-105 in your beast. 86250r is correct, light spark knock is ok, it happens when I dump the clutch to fast and lug it at half throttle, i get a few clings but nothing bad. Unless it is pinging like a old 80's hyundai then dont worry

 

I think some of you guys need to stop reading about octane and just go buy a tank full of high-octane gas. Then you'll see that it really DOES increase performance, just as I did.

Higher octane is a necessity in high compression engines to get rid of spark knock, true. But, that doesn't mean it can't help lower-compression engines, it's just that it won't help as much. You are making it sound like there is a sharp cutoff for X compression = Y octane level needed, and any more octane yeilds zero increase, and that's not true at all.

 

There is an excellent technical article dealing with fuel terminology at the hondaex.com web site. It's under the tech tip section -a must read.

 

I run 91 octane with an 11.5 : 1 piston here with no problems. It is simple less air equals less octane requirement period. Any more octane than is required to stop the pinging and predetination is a waste. The higher the octane the longer it takes to burn and on a 400ex which is cold blooded to start with excess octane makes this problem worse. I run a good octane booster when i venture to lower elevations but in Utah 87 octane should suffice. But 91 at this elevation is better than 92 at sea level at resisting detonation.

99 426ex , Web Cam, FCR carb, CT revlimiter, DMC exhaust, XCT's ,K&N, One kit,

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