octane question?


8 replies to this topic
  • dabeyta3

Posted April 26, 2005 - 10:33 AM

#1

I was wondering what octane 650L owners run. My bike is stock except for exhaust, air filter and carb mods. Before I replaced my head gasket I had to run VP Red and 91 50/50 mix. But now I am wondering could I go back to 91. Does the 650L carb mods (drill slide and re jet) affect octane at all? Or maybe it was running lean before. Just interested to see what octane everyone runs.

  • Dual_Dog

Posted April 26, 2005 - 10:54 AM

#2

I was wondering what octane 650L owners run. Does the 650L carb mods (drill slide and re jet) affect octane at all?


The L doesn't have a high compression engine. The manual recommends a minimum octane rating of 87. In fact, some folks say higher octane isn't always a good choice if the engine was designed for a lower one. It doesn't matter that you've modded and rejetted the carb. You should be able to use whatever grade you want as long as the compression hasn't been altered.

I run mid-grade 89 octane with absolutely no problems.

  • XR Dude

Posted April 26, 2005 - 11:14 AM

#3

I run 87 most of the time, no problems. Plus its cheaper. I've never run any ethanol however, most bike guys don't recommend it and I don't know why.

  • qadsan

Posted April 26, 2005 - 11:35 AM

#4

Ethanol is an alchohol and alcohols have an affinity for water. It's also not the best for nitrile seals and its terrible on poylurethanes. Nitrile seals are used in the oil & gas industry and its used in the fuel systems of some motorcycles such as on fuel T's, o-rings on petcocks, etc. Unleaded gasoline on the other hand is very compatible with nitrile.

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  • dabeyta3

Posted April 26, 2005 - 12:21 PM

#5

The L doesn't have a high compression engine. The manual recommends a minimum octane rating of 87. In fact, some folks say higher octane isn't always a good choice if the engine was designed for a lower one. It doesn't matter that you've modded and rejetted the carb. You should be able to use whatever grade you want as long as the compression hasn't been altered.

I run mid-grade 89 octane with absolutely no problems.


Is the manual you are looking at the owner manual. I bought my bike used and it didn't come with one. I will have to get one. Also I just finished a whole bunch of projects on my bike and drained the old gas. I went and got 91 and it runs fine. I just remember that with 87 it used to run terrible. Has the quality of gas changed over the years? Next I'll try 89 like you. One last question - how does someone alter the compression?

  • truck6driver

Posted April 26, 2005 - 07:00 PM

#6

The myth about octane ratings is that higher octane fuels burn better. The truth is that octane is an additive that slows the combustion process to prevent detonation due to higher compression or advanced timing. Run what ever the engine will allow you to run. The XR650L engine is a low compression engine and will not get much above 9:1 ratio in stock form. The inside of my 94' air box cover states 91 octane recommended. I run 87 in the winter months with out any knocking or drivabality problems. When the weather gets above 85 degrees I change to 92 octane to prevent a knock due to the higher heat in the engine. Running any higher is just a waste of money, unless you have increased the compression ratio of the engine.

My 2 cents

Ray H.

  • qadsan

Posted April 26, 2005 - 07:17 PM

#7

The real myth about octane is that higher octane fuels burn slower. The fact is the octane number does not describe how quickly the fuel will burn or how much potential energy it contains, etc. It just so happens there are higher octane fuels that actually burn faster than lower octane fuels, but the flame speed itself is a function of chemistry and 'not' octane. The term octane describes the grade of gasoline and its resistance to engine knock (automatically ignite) and the only time your engine is aware of octane is when it doesn't have enough. The total energy in BTU's for a given fuel can also be a misleading. A fuel rated with more BTU's may have more energy potential, but that doesn't mean it's all usable or usable within a certain range for a given application. A fuel's distillation curve defines the temperature at which various amounts of fuel is vaporized. Generally, the race fuels for high performance motorcycles that offer more power with improved throttle response vaporize at lower temperatures and the distillation curve of a fuel can help clue you into good fuel choices for specific applications. Distillation curves are readily available from most fuel suppliers if you're looking to try various fuels.

  • Motosprtman

Posted April 27, 2005 - 06:42 AM

#8

after run out of gas (nearly I was on reserve) the nearest fuel was 87 octane, I took on 1 gallon of it and proceeded to the next station where 92 octane was availble (Shell) and filled it up. While running on that 1 gallon of 87 in a brand new bike (less than 200 mils at the time) - it ran - but popped back more than normal and seemed overall unhappy for the 10 miles I rode to the Shell station for 92 octane fill. It seems to me that it "prefers" 92 octane and the inside of the air cleaner side cover also recommends 91, octane whereas (it has already been stated) the owners manual says 87 or better.
I think it is a perosnal choice really - but for my new Baby NOTHING but 92 octane, Chevron or Shell. Avoid cheap gas like CITGO (had a bad experience with that in my Toyota Tacoma PreRunner truck)

  • dabeyta3

Posted April 27, 2005 - 10:09 AM

#9

Thanks everyone for the info, will run 91 from now on. (I can't find 92 anymore in Southern California).





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