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post #1 of 29 (permalink) Old 05-13-2011, 06:47 AM Thread Starter
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What fuel octane?

What octane do you prefer? What differences have you been able to discern with your bike in the 87 to 93 spectrum? THANKS! (I have a 2011 Vision)
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post #2 of 29 (permalink) Old 05-13-2011, 09:51 AM
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Let me be the first to dispel some rumors on octane here.

Myth: Higher octane in a given motor will give more performance.
Truth: Engines are tuned from the factory to run a particular octane. Octane is simply (without going into the mechanics of it) the rate at which the fuel will burn and its resistance to detonation as pressure builds. An engine tuned for higher octane will have its ignition timing retarded to take advantage of the more stable fuel, and also have higher mechanical compression ratios built in. Cam timing is usually set to allow higher cylinder pressure, which in turn requires stronger rods, pistons, etc... Now, an engine tuned for 87 Octane will run worse generally with higher octane fuel. The reason is the fuel will not completely combust because of the slower burn time, and allow maximum cylinder pressure as a result, equalling less horsepower. The fuel is typically ignited BEFORE the piston reaches TDC and maximum cylinder pressure ideally reached just as the piston begins the downstroke. If you use fuel that burns slower than the engine is tuned for, maximum cylinder pressure will not happen until well after the piston is moving down the cylinder, meaning you will not get the "bang" for your buck so to speak.

Equally, by using a lower octane fuel than a motor is tuned for, the fuel could detonate or burn to quickly, causing cylinder pressure to max out before TDC, resulting in tremendous strain on the engine and possibly scattering it (this is why many people who use nitrous and dont tune for it end up with a pile of parts, but I digress...).

In short, unless you know what you are doing and retune for higher octane, use what the manufacturer recommends. Sorry for the long post.
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post #3 of 29 (permalink) Old 05-13-2011, 10:28 AM
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Preference is irrelevant. The manufacturer recommends premium octane ONLY!

As roughneck pointed out it is to prevent detonation. Detonation is very bad for your engine.
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post #4 of 29 (permalink) Old 05-13-2011, 12:51 PM
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with roughneck. Finding the same thing especially at my altitude..
One thing to keep in mind is that high octane gas actually burns slower than low octane gas. That was a particular advantage in older engines where poor flame propagation across an expansive combustion chamber made them susceptible to knocking. However, new head designs have such improved burn properties (better turbulence, squish area etc.) that they “tolerate” lower octane fuels much better even with much higher compression ratios. Typically, a modern engine will produce the best power and efficiency with the lowest octane (fastest burn) that does not induce knocking.



Now, most new high performance car engines have the ability to alter timing and other variables (adjust their “state of tune” as Route6T6 refers to) to achieve best performance/efficiency for a given octane, so they will run fine on low octane but often better on high octane. Most bike engines don’t (yet) have that level of sophistication (lacking knock sensors) and so are designed (tuned) to run without damage on lower octane gas even though they recommend higher octane for a safety margin (under harsh conditions such as high loads at low rpm and high temps.) So many bikes will run as good or better on relatively low octane gas. All the bikes I’ve had in the past 8 years (liquid cooled Ducatis, 110” Warrior, FJR, V-strom, Roadliner) ran as good or better (with as good or better gas mileage) on 89 octane as on 93 octane - my Ducati’s ran noticeably better on 89. On the other hand, if I had a sportbike with 13:1 CR I imagine I'd run 91 or higher whenever possible

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post #5 of 29 (permalink) Old 05-13-2011, 01:05 PM
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Cool vid. of combustion from inside. Burning not exploding.
up
Can't wait to see this with Direct injection.

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Last edited by busajack; 05-13-2011 at 01:08 PM.
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post #6 of 29 (permalink) Old 05-13-2011, 06:14 PM
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Quote:
Originally Posted by roughneck9725 View Post
Let me be the first to dispel some rumors on octane here.

Myth: Higher octane in a given motor will give more performance.
Truth: Engines are tuned from the factory to run a particular octane. Octane is simply (without going into the mechanics of it) the rate at which the fuel will burn and its resistance to detonation as pressure builds. An engine tuned for higher octane will have its ignition timing retarded to take advantage of the more stable fuel, and also have higher mechanical compression ratios built in. Cam timing is usually set to allow higher cylinder pressure, which in turn requires stronger rods, pistons, etc... Now, an engine tuned for 87 Octane will run worse generally with higher octane fuel. The reason is the fuel will not completely combust because of the slower burn time, and allow maximum cylinder pressure as a result, equalling less horsepower. The fuel is typically ignited BEFORE the piston reaches TDC and maximum cylinder pressure ideally reached just as the piston begins the downstroke. If you use fuel that burns slower than the engine is tuned for, maximum cylinder pressure will not happen until well after the piston is moving down the cylinder, meaning you will not get the "bang" for your buck so to speak.

Equally, by using a lower octane fuel than a motor is tuned for, the fuel could detonate or burn to quickly, causing cylinder pressure to max out before TDC, resulting in tremendous strain on the engine and possibly scattering it (this is why many people who use nitrous and dont tune for it end up with a pile of parts, but I digress...).

In short, unless you know what you are doing and retune for higher octane, use what the manufacturer recommends. Sorry for the long post.
Thank you, Thank You, Thank you! I've had similar discussions on three different forums (Mercedes, BMW & Acura) and nobody has yet summed this up so eloquently in such a brief period. Can I quote you outside this forum?

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post #7 of 29 (permalink) Old 05-13-2011, 06:50 PM
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Due to altitude gas in utah is different, our regular is 85 and premium is 91. Since I live at 5000' and ride mostly in the mountains up to around 8000' I run 85. I experiance preignition Roughneck is talking about every time I run 91. When I get over 6000' it happens with 87 so I ignore the manufactures recommendation. The loss of power is real; I feel like the bike is bogging anytime I give it gas -- I can do ok easing on the throttle.

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post #8 of 29 (permalink) Old 05-14-2011, 05:01 AM
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never heard of 89 in uk we have 95 or 98

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post #9 of 29 (permalink) Old 05-14-2011, 08:41 AM
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Quote:
Originally Posted by ndabunka View Post
Thank you, Thank You, Thank you! I've had similar discussions on three different forums (Mercedes, BMW & Acura) and nobody has yet summed this up so eloquently in such a brief period. Can I quote you outside this forum?

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Absolutely. I actually used to work as a Mercedes Benz technician. I have been a student of high performance since 16 years old and I still learn every day. This is one of those big misconceptions people have out there (there are many). Hope it helps some people out.
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post #10 of 29 (permalink) Old 05-14-2011, 08:42 AM
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Maybe that answers part of the question why gas in Europe costs so much more than in the U.S. There may be a few cities where you can find 98 Octane but I don't think it's standard in any state.

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