This is a new one on me?

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Deseret Rider
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This is a new one on me?

Post by Deseret Rider »

I have a friend---a design engineer--and a very smart guy. We sometimes ride the high mountain country of Southern Colorado together. On these rides I usually ride my fuel injected WR-450 4 stroke which calls for Premium fuel---91 octane. He rides an older 650 XR Honda, He claims that there is no reason to run hi octane fuel anywhere in the Rocky Mountains--that regular unleaded 86 octane is all that is needed. ???? I'd never heard this theory before but had to buy a tank full of regular gas recently at a hi mountain gas station that did not offer anything other than 86 octane. Whoa? My bike ran fine on it at 12,000 to 13,000 feet elevation----I could not detect any 'knock' nor anything different from the way it runs on 91 octane?

I offer this subject for conjecture-----anybody have any comment as to why his statement (that there is no need to run premium gas at altitude) is---or is not----true?
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This is a new one on me?

Post by SS109 »

Depends on the bike. My 200 hates anything under 91.
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Re: This is a new one on me?

Post by newbbewb »

interesting. I haven't played much with fuel injected bikes. Question for you...does is have a knock sensor? If it's like most cars, the ECU will see knock and retard timing in order to get rid of detonation. so it will run fine, just reduced power. higher elevation= less oxygen, the ECU knows what the barometric pressure is, it will/should adjust injector pulsewidth to compensate for both elevation and temperature. Fuel injection is a complicated story really, yet so simple at the same time.

Again, I am NOT familiar with dirt bike FI. I would THINK that unless you have No2 or positive displacement apparatus, you'll be fine.

*edit* My guess is that you'll get to lower elevation and that bike will have a considerably less power on 89 octane. And please don't let your buddy into thinking that his engine and your engine are anywhere CLOSE to the same design. you can cut your fuel with piss in any xr tank and it will still run that same.
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Re: This is a new one on me?

Post by chkdx »

In general, octane requirement on naturally aspirated 4-stroke engines goes down as elevation increases. Atmospheric pressure is lower, air density is lower, so cylinder pressure is lower. In fact, some testing has shown a slight power increase using lower octane fuels compared to higher octane fuels, at higher elevations. The burn rate of the lower octane fuel is a bit faster, and helps offset the slower burn rate caused by lower cylinder pressure at high elevations. This slower burn rate is also why increasing the ignition timing advance can help performance at higher elevations. If you don't change the timing from sea level, you can usually get away with using lower octane fuel. I emphasize all this applies to normally aspirated 4-stroke engines, and not every engine responds to the same degree.

Air density is MUCH lower at 12,000-13,000 feet elevation; I'm not at all surprised that your WR450 ran fine on 86 octane -- or that the station didn't offer higher octane fuel; engines at that elevation don't need it.

Our family cars, a Toyota and an Acura, both call for 91 octane, but run fine, no issues and no power or mileage loss using 87 octane, since we live at 5000' elevation. I'm careful to use 91 octane when we travel to sea level.

Forced induction 4-strokes, and 2-strokes with an expansion chamber exhaust ( which causes forced induction ) should stick with higher octane fuels. Their cylinder pressures don't go down as much with increased elevation, compared to a naturally aspirated 4-stroke.
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Re: This is a new one on me?

Post by Deseret Rider »

I think the post by CHKDX pretty well sums it up---and explains logically why the 86 octane worked well in my bike at altitude. Thanks ChKDX! the insight you have provided plugs a lot of holes in my knowledge. Good job.
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Re: This is a new one on me?

Post by chkdx »

Glad I could help!
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