In Reply to: Does octane affect mileage inversely? posted by Marketing hype? on December 10, 1999 at 16:49:19:
: I find that when I put a lower octane gasoline in my car,
: my mileage goes up. I get less miles with a higher octane.
: Am I missing something? Could all that marketing about
: higher octane being better be hype? I keep the octane above the minimum required for the car. I've tried the test several times, and it seems to be true every time. This is
: for an 1990 Acura Legend, not a BMW, but I wonder if the
: same would hold true for BMW's (which I'll own one soon). I
: notice NO difference in power with a higher octane level.
: The only reason I would use hitest would be because the
: engine would be kept cleaner (or is that a myth also)?????
Your findings are vexing in that there should be no discernable difference in milage as long as you avoid detonation regardless of the octane rating of the fuel. I would guess that small differences either in your use patterns or traffic conditions are accounting for the milage differences you are seeing. Is there a second driver of your car who might be fouling up your science?
As regards your octane requirements, it is recommended that you use a fuel rated to the minimum octane necessary to avoid "knock" or detonation. By the way, the conditions under which knock may present itself will change dependant on temperature, altitude, load, etc.
I'm not familiar with the ignition and fuel parameter controls on your Acura, but your new BMW will run well, and probably knock free, with any normally available gas. However, be advised that it may not make all of the power of which it is capable with 87 RON fuel, because there are knock sensors in the system that will limit ignition advance if knock is sensed. So, for most modern BMWs, assuming you wish to get maximum performance, 92 octane is recommended.
It is my understanding that some fuels are better than others insofar as keeping your injectors etc. clean. Many gearheads prefer Chevron because it is laced with Techron, which is an effective detergent in this regard. Though the amount of "scrubbers" in various rated fuels may differ, there is some truth to the fact that certain fuels burn cleaner than others. Finally, here in the west, high Octane (94 plus) fuels are unavailable at the pump, but I am told some companies still supply those on the east coast.