I agree...run the 91. The EFI system compensates to a degree for lower octane but for a couple gallons who notices the price really? With no scientific data to back me I say it runs better at low speeds and gets better mileage. Also, with absolutely no science to back me up I say buy gas at a nationally known supplier. I prefer Sunoco, since they have the highest octane around here, 93. I think it's higher quality..
Octane 101 Octane is a measure of how slow or fast the gas burns. High octane = slow burn. Low octane = fast burn. It's help in performance is rooted to being matched to the engine's needs. Use the right octane for the engine. . Gas does not instantly explode, it burns at a rate predicated by the octane. Spark does not happen at top dead center. Spark ignites the gas at distance before top dead center that ensures a complete burn and maximum power at TDC. Because of higher piston speeds, high compression and high revving engines require more spark advance. This requires high octane for a controlled burn. Low octane would cause pre-ignition/detonation in a high performance motor as it would burn out before TDC. High octane in a low performance engine would not burn completely; the unburned gas will form carbon deposits that will affect running over time, damage valve seats, and cause sticking rings. Modern engine management systems are much more forgiving of different grades of octane. But even if the system manages it, it's not really needed or cost effective to run premium fuel in an Enfield. The octane # is an average of two different testing methods here in the US. Other countries use only one method, so the rating may be higher or lower and you can't draw a direct correlation to our numbers. Other misunderstandings: You don't need lead. Modern valves and seats are hardened and do not need the cushioning effect lead gave to 1930 era engines. Don't sweat the ethanol. Modern formulations are pretty good. It does tend to bond with water from condensation during periods of non-use. So ride more or use a drying agent such as Heet during storage.
Strangely, all the damage seems to be to bikes.....
When I make my trip up to Flagstaff where I'll be climbing 7% grades I do fill up with the 91 octane fuel just to be on the safe side. (Running with a wide open throttle to maintain 50 mph for over 5 miles without stopping does heat up the engine a bit and I think the higher anti-knock fuel gives better protection under these conditions.)
The only problems I've had were some bubbling of the paint inside the tank and around the filler neck and the cracking of the exterior layer of the fuel line early on. To be fair, some bikes come out of the crate with the fuel line cracked.
About the only fuel we can get at a filling station here in Arizona has 10% ethanol in it.All of the local filling stations have 87, 89 and 91 octane.Being a low tech engine I've had no problem using the 87 octane around Phoenix. No pings. Runs smoothly and gives me a little over 70 mpg (US gal).When I make my trip up to Flagstaff where I'll be climbing 7% grades I do fill up with the 91 octane fuel just to be on the safe side. (Running with a wide open throttle to maintain 50 mph for over 5 miles without stopping does heat up the engine a bit and I think the higher anti-knock fuel gives better protection under these conditions.)
Bubbling paint around the filler necks is another problem seen out of the crate. I saw it on a batch of '11 - '12 vintage RE's at a dealership...