why does my bike get better range on 91 gas?

91 from which gas station? Shell has 0 ethanol in their V-Power gas which could explain this.

Then again, this topic has been beaten to death here. Many people (including myself) feel that if your bike isn't rated for anything more than 87 putting in premium gas is just a waste of money.

My bike, on the other hand, requires 91 gas... /shrug
 
I don't know but it just does. Don't question, just go with it before the oil companies catch on.
 
Because 87 is so lean that it often burns before the gas is fully sprayed when hits the chamber/cylinder. Therefore not efficient enough to push the piston at full power rate. 91 octane does not burn as fast, sprays and hits the chamber fully, compress and ignites properly pushing each piston at full force therefore engine requires lesser feed because every spray and burn of gas in the cylinder is effective. Google it, they can explain better than my words. =0)
 
Higher octane = higher flash point. If your bike is not high compression and set up for high octane then your waisting your money. Will 91 give you further range then 87? YES. Will it increase hp in a low compression engine? NO
 
menacingone said:
Replace "fast" with "easily" and I think the statement is a little more accurate
Click to expand...

Except it won't have any meaning. What would "does not burn as easily" mean?

In an engine using 87, neither 87 nor 91 will combust before the spark ignites. The air/fuel ratio will be the same and the mixture pressure will be the same. Octane rating does not correlate with flame speed in general, so we could assume that it's the same in both cases. If the fuel economy is indeed different (in a repeatable way, using fuels from different sources, etc.), then the cause must lie somewhere else.
 
Does your bike have a knock sensor? If your bike is ment to have 91 and you put 87 in your motor could be detonating. If it is the knock sensor will pick this up and pull timing to try and save the motor. If u pull timing you will lose mpg and horsepower


Now I swear if I use ultramar 91 over esso/shell 91 I get less horsepower in my turboed car
 
Here's a simple way of thinking about a few things...gasoline is a mixture of hydrocarbons from mainly C5-C8 chains, that means that for every "one" of those carbon chains you get 5-8 times as much CO2 (and about the same H2O vapour) produced on combustion. 1 (mole) of the hydrocarbon liquid (gasoline) occupies about 100ml (ish) and on combustion this 100ml turns into 5-8 X 22.4 L CO2 and 6-9 times 22.4 L of H2O vapour, the gasses that expand and push the pistons around.

Ethanol when it burns turns into 2 lots of CO2 and about 3 lots of H2O vapour...ie less gas volume. Produces less work but because the combustion products are the same it can be used in fuel. A bit like cutting heroin if you like.

These chained hydrocarbons aren't just found in straight chains, sometimes they have carbon branches on the chains, these are generally better in terms of igniting properly in the engine. One of them is 2,2,4-trimethylpentane which means that it is a C5 hydrocarbon with 2 single C groups on the second carbon of the main chain and one on the 4th carbon of the main chain. The octane rating you see basically refers to the characterisitics of a fuel containing a certain percentage of this compound. So 94 octane gas has the same characteristics as a mixture of 94% 2,2,4-trimethylpentane (it doesn't mean that it actually contains this much, just that it has the same properties as a mixture of that much of the pentane derivative and 6% heptane, a C7 straight chain hydrocarbon).

When you branch a hydrocarbon you increase the boiling point and combustion point (generally more carbons = higher boiling point/combustion point and higher surface area, such as with branched hydrocarbons, = higher boiling point and combustion point). More branched hydrocarbon in the mixture = higher combustion point (higher temp at which fuel ignites). Higher combustion point = less tendancy to knock or ignite at the wrong point in the combustion cycle. Higher octane fuels are thus igniting at higher temps and if your engine isn't designed for this then over time things may obviously wear a lot quicker.

Edit: Apologies for this..I was bored and I finished marking a class today. Really need to switch off.
 
a few have said it, but i think it boils down to the ethanol.. really has nothing to do with the octane, but different grades of gas(rated by octane) have different amounts of ethanol.. if we where able to get that crap out of all levels, you would experience the same performances from all. in low compression engines that is.
 
Why does no one like chemistry? :(

"Cutting" gas with ethanol is a smart business move and there's most likely a massive lobby group from the ethanol people (agriculture types) with all sorts of propaganda about local product vs foreign oil etc etc. In a sense they have a point, but if you're paying through the nose for a finite resource (oil/gas) I want gas in my tank, not ethanol. I'll happily pay less for ethanol laced gas.
 
just_luc said:
a few have said it, but i think it boils down to the ethanol.. really has nothing to do with the octane, but different grades of gas(rated by octane) have different amounts of ethanol.. if we where able to get that crap out of all levels, you would experience the same performances from all. in low compression engines that is.
Click to expand...

The octane rating doesn't have anything to do with ethanol amounts, a gas of 87 octane rating could have no ethanol, or a small percentage of ethanol, but whatever it has as a mixture will be equivalent to the ignition properties (NOT the energy) of an 87% mixture of 2,2,4-trimethylpentane and 13% heptane (this is a refernce mixture used to determine octane ratings, it is not necessarily what you put/get in your tank).
 
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