Originally posted by DemonSVT:
Originally posted by warmonger:
the burn rate of the fuel doesn't mean it has more power or in your terms energy.
Relatively speaking and definitely in low efficiency car engines, the small difference in structural changes affects available energy very little.
I doubt it affects the fuel economy at all!


For a car programmed to run on low octane (87) fuel running high octane fuel will lower efficiency.

The slower burn rate & later point of ignition coupled with the smaller amount of ignition timing (later to initiate the flame front) will give less time to adequately burn the fuel. (in effect it's like retarding timing) Therefore the engine has lower efficiency which means less power & mileage.

I'm adding that in case you meant it both ways instead of the SVT, for example, retarding the timing curve to run lower octane fuel. Mileage will drop a hair but you are right it will be very minor at best. (once the PCM adjusts) However the torque (and subsequent HP) drop off will be notable. A good tuned chip is proof of that. Timing curve is EVERYTHING.



This is very sketchy. In principal you are correct for significantly different grades of fuel, however, in terms of available pump grade gasolines it isn't applicable. A car tuned for 87 octane will run the same on 87 through 93 and probably even upwards towards 100 octane. It definitely won't run less efficient, it just won't see any benefit from it.

Now when you talk about running gasoline and then later methanol or propane or a fuel that is VERY different in its octane and power capacity, then yes, I think you'd be correct.

Former owner of '99 CSVT - Silver #222/2760 356/334 wHP/TQ at 10psi on pump gas! See My Mods '05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red '06 Mazda5 Touring, 5spd,MTX, Black