87 Octane gas for ST1100

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finger lakes ny
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1999 ST1100
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The times I've been up to 14,000 ft, my ST1100's have not missed a beat (87). Never really noticed a change.

Sure noticed it when walking slightly uphill though!
 
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Here's what I found on using lower octane at higher altitudes:

"Air is thinner the higher above sea level you go. Less air going into the cylinders means less pressure at top dead center when things go bang. It's a lot like lowering the compression ratio in the engine, reducing the need for high octane. Cars will run just fine on lower-octane fuel when they're well above sea level-"

Ray
Correction for that assumption, pistons are drawing SAME amout of air in cyls., just not as much OXYGEN content in what's being drawn in:) My normal riding elev. is 5,000ft., and everything goes UP from here in three out of four directions to over 10,000ft. elev. when traveling those three directions some 40-80mi. Cyl. compression or pressures has to do with compression ratios, NEVER altitude changes;).
 
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Joined
May 30, 2007
Messages
1,387
Age
72
Location
Grand Junction, Colo.
Bike
92 ST1100
The times I've been up to 14,000 ft, my ST1100's have not missed a beat (87). Never really noticed a change.

Sure noticed it when walking slightly uphill though!
Only change that is obvious is the 3% horsepower loss for every rise of a 1,000ft. elev.:).
 
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soCal
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'97 ST1100
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687
Correction for that assumption, pistons are drawing SAME amout of air in cyls., just not as much OXYGEN content in what's being drawn in:) My normal riding elev. is 5,000ft., and everything goes UP from here in three out of four directions to over 10,000ft. elev. when traveling those three directions some 40-80mi. Cyl. compression or pressures has to do with compression ratios, NEVER altitude changes;).
STraymond's comment that at altitude there is less air going into the cylinders, creating lower TDC cylinder pressure, is correct, assuming a given throttle opening.

The ratio of oxygen in the air doesn't change with altitude, its simply a matter of the air being less dense. So per unit volume it does contain less Oxygen, but it also contains less Nitrogen, less water vapor, less CO2, etc.

Volumetric efficiency of an engine is a very complex topic, and the cylinders never really fill with a volume of air equal to their stated displacement, even at sea level. The throttle butterflies restrict the flow of air into the cylinder so that the volume of air which enters per cycle isn't something that's simple to calculate. Its some fraction of the available cylinder volume, and obviously varies with throttle opening, but its never constant.

That being said, if you have air that is less dense, being forced into the cylinder by atmospheric pressure that is lower than sea level, you are going to get less air molecules into that cylinder than you would with denser air at sea level pressure. Less air molecules being compressed in the cylinder means lower cylinder pressure at a given compression ratio. However, if you open the throttle more you can compensate for the lower air density, but only to a point. At full throttle you are limited by the air density/pressure relationship of the atmosphere.
 
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Here in the heartland it’s almost impossible to avoid 10%. Snip. Snip .....

John
That's an interesting comment, John. When I was in Iowa this year ( first time since 1974 ). I was surprised to see on the pump at two gas stations " All Grades Ethanol Free ". I have never seen that anywhere else in my travels. Usually the pump says something like " May contain up to 10% Ethanol".
 
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