Quote:
Originally Posted by ImDisaster
True...with a caveat. Higher octane doesn't have any more "power"...just a higher temp/pressure ignition point. That helps prevent knock. If you push it hard, with lower grade fuel you may experience knocking and the engine control system will then retard the spark which can reduce performance and fuel economy. The difference will only be if and when the control system has to kick in and retard spark. 99.99999% of the time you won't notice a difference.
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Higher octane permits much higher ignition timing advance without knocking most especially under WOT or heavy throttle condition, the end result being more power. But driving around with WOT and heavy throttle kills off the theoretical mileage gains of 92 Octane.
If you keep your foot out of it, you can get by with lower octane and your knock sensors never have to do any work [ignition timing retarding]. Which is to say, ignition timing advance on our cars runs pretty far out under light and medium loads, regardless of Octane. This is why you can't really get any better mileage with Premium fuel. The timing stays pretty far advanced with both 92 and 87 Octane under medium and light loads...exception being really hot weather, when just pulling off the line and A/C running. Then I always get a very brief ping with 87 until the knocksensors ramp up and start doing their business of retarding timing advance.
(I've often pondered what effect a more tropical climate would have on possible mileage improvement with 92 Octane???)
Generally speaking, in average ambient temps, you really have to stick your foot in it to get the knock sensors to wake up and only then will they activate to reduce timing advance,
if lower than 92 Octane is present.
Bit of a paradox, in that you have to drive the crap out of the car to really tell if Premium fuel is in the tank; which in reality defeats any increase in gas mileage.
A normally driven Avalon wont get better mileage with Premium fuel. Better performance yes! But not better mileage.
Reason I know all this stuff about timing is I have a better OBD-II analyzer which hooks up to my laptop and allows me to monitor (among other things) real time ignition timing advance - whilst driving. Soo Very cool.
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