Question about octane

I've wondered the same thing (I also had a Probe GT). It probably has to do with the shape and volume of the combustion chamber. The room that the flame front has to work with has an effect on octane requirements besides basic compression ratio. The 2.5L DOHC was prone to carbon fouling (and thus carbon knock), and also had variable valve timing, which might have had an effect on octane requirement too.

I always ran 87 in my Probe. If the knock sensor ever dialed the timing back, I could never tell. It seemed to make the same power no matter what I put in it.

each engine has different quench and different thermal characteristics.


octane's only function is to increase the cumbustion temp of gas to prevent detonation (ignition of fuel before spark timing)

if your car doesn't knock with 87 it will gain no power with any higher octane rating.

octane is rarely used. variants of heptane are commonly used to increase combustion temps of gas instead of true octane

naptha or moth balls was commonly use by old timers to do the same thing but they didn't realize the naptha sublimes from a solid to a gas with no liquid state so mixing with liquid gasoline was minimal. it still helped but only to a small degree

diesel or kero can be added to gas in small amounts to do the same thing. or cetane but Cetane is hard to obtain

here is some home brew "octane" increasing recipies
http://www.gnttype.org/techarea/misc/octanebooster.html