What do you run 87 or higher and why.

 

I have never found any reason to use anything other than 87 octane. The higher octane burns slower and produces less heat, but not enough that you will ever notice.

 

2007 400ex owners manual says 91 octane

 

The benefit of higher octane is reduced detonation/pinging. Generally, the higher compression the more likely an engine is to require higher octane. Combustion chamber temperature and ignition timing also play a role.

Anyhow, I'd run what the owner's manual suggests, and not pay to run higher than what the machine calls for, unless the compression ratio has been increased.

 

<div class="FTQUOTE"><begin quote>Originally posted by: rvessar

What do you run 87 or higher and why.</end quote></div>


Well....what quad???? What does your owner's manual spec???

Fuel degrades quickly in storage or sitting in your tank, so I always run a couple octane higher than what the owner's manual specs, just to be sure I never get detonation. But going way higher is just a waste of money, and may even result porer performance.

 

Yes, like the fellers have touched off on... What quad ?
I have guidlines I follow when building an engine for what fuel it should be running per application. Cam and timing specs go hand in hand with head cc, swept deck volume, exhaust, carb, head flow etc... in order to figure the correct octane.
In a nut shell... Detonation occors after TDC into the power stroke (downward movement of the piston after compression) and pre detonation occors early during the compression stroke.
Example of pre-detonation would be an engine demanding 91 octane when 87 is used. The 87 will ignite quicker so with the engine timing set for 91 (slower burning fuel) which is advanced timing, it will cause harmfull pre-detonation.
Hence an engine demanding 87 octane, running with 91 octane the flash is late and causes detonation.
Both can cause engine component failure. both make a pinging sound under throttle up situations. Pre detonation is often noted through blown head gaskets, bent rods, burned intake valves. Detonation is more noted on broken ring lands burnt exhaust valves etc...
If you are really good tuner, looking at the piston you can identify fuel wash and see it's affects from both pre-detonation and detonation affects are having on the piston.
The detonation difference when running 91 in an engine designed to run 87 is mainly as described above, a waste of money, think it out. Some detonation relief is absorbed through the exhaust valve opening. carbon deposits are accelerated.
Pre detonation from running 87 octane fuel in a 91 octane engine will do harm as the timimg is advanced too far for one. The 87 in this situation flashes it's burn quicker and will build heat.
Why higher octane in some engines ? Number one is they will run cooler at higher RPMs. Normally these engines are designed to generate their power curve higher in the RPM range where heat is more a factor to be concidered than in a low RPM engine.

Confused yet ? LOL

Lower octane fuel, burn flash is quicker = hotter= less compression needed
Higher octane fuel, burn flash is slower = cooler= higher compression needed

----- Gimpster -----

edited for spelling

 

I was asking in general but I have. 1996 300ex high comp piston sons
2004 300 ex stock comp wifes
2006 400ex stock comp mine
2001 90 arctic cat daughters

I keep a fuel barrel around and just run 87 but maybe I should get one just for the atvs and put 91 in it.

 

Have you ever even read your owner's manual? You may have been toasting you engine all this time. It's pretty simple....heat favors detonation, so air cooled engines (even low compression ones) need higher octane because they can overheat more easily than water cooled.

My 400EX manual specs 92 octane....not 91 For me that means 91 octane pump gas (highest available around here) and some Klotz Octane Booster, or a mix of pump gas and race gas.

 

No I dont have any of the owners manuals,like I said I was just asking in general.

 

<div class="FTQUOTE"><begin quote>Originally posted by: rvessar

No I dont have any of the owners manuals,like I said I was just asking in general.</end quote></div>

Why do we see this "I don't have an owner's manual" excuse over and over again???

Well, obviously there can't be any "general" answer to this question, becasue every individual engine design is different.

All I can say is don't keep running 87 in your 400! Willful ignorance has its price!