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Which octane ?

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Trying to figure out which octane gas is most efficient in my 02' 650L. When I had the 08' L I always ran 87 octane and it seemed ok. The bike has Daves carb mod (55/160), uni filter, no snorkel, no smog, and stock exhaust. I run Sunoco gas that doesn't list ethanol in it. Right now my plan is to run through all the octanes (93,91,89,87) and which ever one I get the most miles out of a tank then go with that one. Anyone done this experiment or have any input? 

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Mine runs just fine on 87 octane, even though I have a 10.5:1 higher compression piston in there.

If one of your choices includes fuel without ethanol, you'll get better mileage than a fuel with ethanol.  Here in California, we don't have a choice, all fuel has 10% ethanol.  You'll also get higher mileage if you go down to a 158 or even 155 main jet.  Of you could just get a bigger tank like I have - a Clarke 4.7 gallon tank.  I get 40-45 mpg.

Edited by ScottRNelson

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I have an extra 155 main I could try. My concern isn't really the amount of mileage I get but what runs most efficiently. I figured whichever octane gets the most mileage must be burning the most efficiently. Daves mod suggested 55/158. I go to my friends shop where he kinda let's me dig around myself. All I could find was a 55 pilot with a 155/160 main. Should I put in the 155 or stick with the 160. I'm at sea level with stock exhaust but planning on going to powerbomb/powercore4 in a month or two. Like I said I'm more concerned with efficiency than mileage although I would imagine they go together.   

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Years ago I had a carbureted Ducati Monster that I was trying optimize the tuning on.  I went to an uphill section of frontage road and did a bunch of acceleration runs from a specific point to another where the bike was hitting about 65 mph.  I made multiple runs alternating blocking off the airbox or opening it up, observing the speed I was hitting at the end point.  The best run was with the airbox blocked off just a bit, which told me that I was slightly rich.  I went down one size on the main jet, then raised the needle one position to bring the midrange back richer a bit.  The bike ran excellent after that adjustment.

You can check yours by running as it is now, then take the left side cover off and see if it gets better.  If it does, that means it's rich and you would benifit from dropping a jet size.  If you go down to a 155 main jet and do the same runs, if it does better when you block off the airbox a bit, it will tell you that you're on the lean side.  If you find both rich and lean as I just explained it, you need a 158.

This isn't the recognized method of optimizing jetting, but it works well for me, and doesn't require any special tools or equipment.

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Remember, the definition of octane is "resistance to ignition". That said, it seems the lower the octane the easier to ignite the fuel and the higher the efficiency. So, if an engine runs fine with no pinging on low octane then use the low octane. I've never actually compared gas mileage with different octanes though.

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If you can run lower octane without detonation , the power and mileage will be better than with high octane. Higher octane allows higher compression , which gives more power than lower compression.

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1 minute ago, brianhare said:

Higher octane without a compression bump in these engines is like flushing money down the toilet..

 

B

Pretty much , BUT the local 93 here is ethanol free.

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True,,we have stations here that have none of that shit in all grades,,the Gas company only has to sell a "quota"  of ethanol crap,,so the high volume stations sell that,,leaving some smaller stations not having to sell any fuel with ethanol in it...

but all stations sell ethanol free in the high octane fuels...

I haven`t been able to use 87 in years in my bike,detonation would scare you to death at higher comp ratios,,lol

 

And all the lawn equipments gets no ethanol  fuel either,,someone else can use that crap,,I've seen the results when it sits unused for awhile..:lol:

 

In my car,,who cares,,

 

B

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Thanks guys. I've just been running 87 octane in it and it seems to be good. Any suggestions on what plug to use. I'm at 55/160 with a .025" needle shim. I'm not new to bikes just to trying to have everything as precise and efficient as possible. Didn't care so much when I was younger. Suggestions on a good plug would be appreciated. 

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1 minute ago, JoeRC51 said:

DPR8EIX-9  NGK Iridium plug. Iridium last much longer than a regular plug.

Just curious - how long does it take to wear out a regular plug?

I haven't even payed attention to the spark plug on my bike, which is the standard recommended NGK plug, and have never had a problem that I could blame on the spark plug.  When I used to ride two-stroke motocross bikes, I had to keep a couple of spare plugs with me just to finish a ride, but it has never been an issue with my XR.

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The edges erode round on the center electrode and the ground strap  over time and the gap opens due to this also. Sparks  jump better across sharp contacts. An Iridium plug will last for years.

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