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Sportster Motorcycle Air intake, Carburetor, EFI, Fuel, and Exhaust Problems, advice and/or how tos for Sporster and Buell motorcycle carburators, Electronic Fuel Injection (EFI), Air Intake, Fuel and Exhaust.

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View Poll Results: What grade gasoline do you use?
Premium Gas 91+ octane 3,709 81.95%
Mid-Grade Gas 89-90 octane 311 6.87%
Regular 87 octane 383 8.46%
I mix it up, don't care, etc. 104 2.30%
Change based on the weather 19 0.42%
Voters: 4526. You may not vote on this poll

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  #1291  
Old 3rd September 2013
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Originally Posted by Iceman62 View Post
One of the dealerships ~1-hr down the road sells high octane fuel...91, 95 & 100. I try to head that direction whenever possible. Usually get the 95, but might try the 100 in my Sporty-S to see if she'll fly...
That may be fine, but it will require a different tune to get the most out of it. Are you willing to retune the bike every time you try a different octane of fuel? If not, settle on one you like and stay with it.
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  #1292  
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Why would it require a different tune? There's no more energy in higher octane fuel, just more knock resistance.
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  #1293  
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Originally Posted by Folkie View Post
Why would it require a different tune? There's no more energy in higher octane fuel, just more knock resistance.
Basically, higher octane burns slower, so timing will not be optimum unless you advance it some.
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  #1294  
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Certainly no performance to be gained by using it on a motor that doesn't need it.
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  #1295  
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Originally Posted by Folkie View Post
Certainly no performance to be gained by using it on a motor that doesn't need it.

Right, so if it doesn't need it, it doesn't burn it fully and therefore it leaves behind more carbon deposits.
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  #1296  
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Originally Posted by Bone View Post
if it doesn't need it, it doesn't burn it fully
You've lost me there; could you explain that please?

Do you mean because it burns slower? It doesn't burn it all in the time it's got?

So, does this mean that higher Octane fuel actually produces less power? Poorer performance?

(Unless compression is high enough to need that slower burning, of course.)

Last edited by Folkie; 3rd September 2013 at 21:37..
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  #1297  
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Originally Posted by Folkie View Post
You've lost me there; could you explain that please?
OK, so you NEED higher octane to prevent pre-ignition/detonation on motors that run hot enough, or have high enough compression, or timing advance that is far enough ahead of time that lower octane would ignite at the wrong point in the compression stroke and finish burning too soon wasting some of the power before the power stroke is finished.

Higher octane is more stable and as such you do things like increase compression and/or advance the timing more to get more power out of the burn.

HOWEVER if you don't have sufficient compression or timing advance to make sure you get the combustion event over in time you can wind up with either incomplete burn and more carbon deposits and/or burn that is still taking place in the exhaust - neither of which are beneficial.

It's a long taught principle that using TOO high an octane will lead to carbon deposits, which increases compression over time and can actually make a motor that did not need higher octane require it at a later time.
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  #1298  
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Have I not posted this here before - good news report testing back-to-back regular vs. premium in a car not spec'd for premium and showing results - premium polluted more:

http://www.cbc.ca/marketplace/episod...mpfiction.html
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  #1299  
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Quote:
Originally Posted by Bone View Post
OK, so you NEED higher octane to prevent pre-ignition/detonation on motors that run hot enough, or have high enough compression, or timing advance that is far enough ahead of time that lower octane would ignite at the wrong point in the compression stroke and finish burning too soon wasting some of the power before the power stroke is finished.

Higher octane is more stable and as such you do things like increase compression and/or advance the timing more to get more power out of the burn.

HOWEVER if you don't have sufficient compression or timing advance to make sure you get the combustion event over in time you can wind up with either incomplete burn and more carbon deposits and/or burn that is still taking place in the exhaust - neither of which are beneficial.

It's a long taught principle that using TOO high an octane will lead to carbon deposits, which increases compression over time and can actually make a motor that did not need higher octane require it at a later time.
Thank you; that explains it very well.
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Still debating this..........
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