Octane, per se, is not about power, but rather ignition temperature. Premium grade fuel is used in high compression engines
to help prevent engine knocking (pre-detonation) since high compression engines heat the fuel/air mixture partially during the
compression phase of the piston stroke. FI adjusts the fuel mixture to accommodate the lesser atmospheric pressure at higher
altitudes for the proper fuel to air ratio. What you end up with is less power at a given throttle setting compared to what you
get at altitudes that are closer to sea level. Engines with carburetors on the other hand do not have automatic fuel metering
and would require adjustment of a pilot screw (if so equipped) or jet changes. Appropriate adjustments and/or changes that
would need to redone when returning to general altitudes. If not done, then the fuel/air mixture from a carburetor would be
rich and the engine would run poorly requiring more throttle to get enough power. Certainly using much more fuel per mile
than at lower altitudes. Returning to lower altitude with the carburetor still set for high altitude will have a too lean mixture
and the engine would run rough and/or poorly and would risk burning a hole in the piston.
Hard to say why you might be getting better mileage from a certain brand of gas, but the fuel that costs a little more just may
be fuel that does not contain ethanol. Gas with ethanol has less energy, so you might need to twist the throttle a little more to
get the same power as you would with non-ethanol laced fuel. In doing so you are using a larger volume of fuel; therefore lower
mpg. Premium fuel has about the same energy potential as mid-grade or regular grade fuel. (fuel excluding ethanol) There is
no advantage for an engine made to run on regular grade to be using a higher octane premium grade fuel... In fact, one would
only be spending more $$ for gas that works no better for that engine.
Southeast Michigan Darkside rider
Sometimes I'm so far outside of the box, the Hubble telescope can't find me
Last edited by bandito2; 11-17-2016 at 10:11 PM.