Octane ?
Road Warrior
Joined: Jun 2008
Posts: 1,437
Likes: 80
From: currently an Okie
FI
Actually, with todays fuel injected engines, that's not correct as the FI system will compensate for the increased altitude and keep the fuel mixture pretty much were it's supposed to be. For that matter, the CV carburators they used to put on the bikes will also self compensate a little for changes in altitude. They're not as good as FI but they're way better than the older non-CV style carbs that were around when I first learned how to ride.
The reason an engine doesn't need as much octane at altitude is because with altitude, there is less "air pressure!" Because of the lower pressure, the engine can't pull in as much air at a given throttle opening as it could at sea level. For example, an engine at full throttle and 10,000 feet "above" sea level might only pull as much air in as it would at 2/3's throttle at sea level because of the difference in air pressure. That example is only given as a reference and I have no idea what the difference would really be. Anyway, with a lower effective throttle setting comes less load on the engine and a lower need for octane in the fuel.
That would be true if we were all stock or runniong wide band O2 sensors. Most of us aren't though. Most of us have no feedback from the O2 sensors because we have increased the A/F ratio to higher than the narrow band O2 sensors can register.
The reason an engine doesn't need as much octane at altitude is because with altitude, there is less "air pressure!" Because of the lower pressure, the engine can't pull in as much air at a given throttle opening as it could at sea level. For example, an engine at full throttle and 10,000 feet "above" sea level might only pull as much air in as it would at 2/3's throttle at sea level because of the difference in air pressure. That example is only given as a reference and I have no idea what the difference would really be. Anyway, with a lower effective throttle setting comes less load on the engine and a lower need for octane in the fuel.
That would be true if we were all stock or runniong wide band O2 sensors. Most of us aren't though. Most of us have no feedback from the O2 sensors because we have increased the A/F ratio to higher than the narrow band O2 sensors can register.
Road Captain
Joined: Jan 2010
Posts: 620
Likes: 61
From: Castle Rock, CO
High altitude
I use the 91 because when I tried the regular or mid grade it felt like my bike was running hotter. Don't know why, just felt like it.
Elite HDF Member
Joined: Jun 2007
Posts: 3,873
Likes: 13
From: NW Arkansas
You may find that you don't have to make that decision...the decision is already made. I live in northern Colorado and all the stations around here have 91 octane as the max.
I use the 91 because when I tried the regular or mid grade it felt like my bike was running hotter. Don't know why, just felt like it.
I use the 91 because when I tried the regular or mid grade it felt like my bike was running hotter. Don't know why, just felt like it.
Road Warrior
Joined: Nov 2008
Posts: 1,296
Likes: 3
From: Cheltenham, UK
As stated by others, I too am no expert and going back to my thermo studies at uni 20 years ago, my understanding of octane ratings for petrol (does not apply to Diesel) was that it also quantifies flame propogation rate on ignition. Too fast and too slow are not good for the engine. Knocking occurs as a result of flame propogation being to high.
I may well be wrong but that is how I have always understood it
I may well be wrong but that is how I have always understood it
Road Warrior
Joined: Mar 2007
Posts: 1,383
Likes: 3
From: Texas City, TX
Oooo, BT, DT, GTTS! Don't envy you on that on. Be sure and stay hydrated and..........
Ride Safe,
Steve R.
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