Octane ratings!!

[caspy]

OK, i know this is Thursday and this is a Friday sort of question.

What diff does an octane rating make to the power output of an engine? Would a rule of thumb be that 95 - 98 would means circa 3% or is it more complex as a result of look up tables in the ecu?

[uk hsv]

If you just put high octain fuel into a standard car it will make no difference, "IF" that car was origional tuned to use a lower octain fuel.

"BUT" if you re tune that car to use the high octain fuel it will be more powerfull.

The higher octain fuel is less prone to "knock" so you can advance the spark further and higher the compresion ratio.

a point of interest diesel fuel has more energy per kilogram than petrol!!!

[Dan_the_man]

Anything you can do to raise your octane level when on the juice is insurance against detonation and is a good idea. Octane is not a measure of the energy in the fuel so higher octane fuel does not give you more HP. Octane is a measure of stability - higher ocatane is more stable and less likely to detonate. It is possible for engines that are tuned for high octane to produce more power with those fuels due to the advanced timing the extra stability supports, but the fuel itself does not make more power on its own.

Many years of Jap import scooby ownership and octaine additives under my belt however I'm happy that these beasts run on supermarket unleaded quite happily

>> Edited by Dan_the_man on Thursday 14th April 09:52

[V8HSV]

Mine is currently set up for 100 octane rating & using 98.6 still induces some amount of knock, I still use the 95 stuff occasionaly when I'm not blasting about, say a nice run up north which is cheaper and produces no ill effects.

I think on standard cars no benefit will ge gained and possible could create adverse effects as the engine will run hotter, certainly does on my experience with smart cars.

[A57 HSV]

Worth mentioning that Optimax has a high detergent level, where as supermarket petrol is notorious for having low levels of detergent. Regular users of this "cheap" petrol have encountered various problems as a result. This is often mentioned in Honest John's motoring section in the Saturday Telegraph.
My car does slightly better MPG on Optimax. Fortunately I regulary have to pass a Shell station with low prices, as the price varies wildly from station to station.

[te51cle]

Thought I'd just add this info to the discussion: both vetteheadracer's and my Corvettes run better on supermarket petrol than on named brand stuff ! Nige likes Tescos for his and I have a favourite branch of ASDA for the LS1 in mine. The Z06 really hated running on Optimax apparently - at least when it was still at factory spec and hadn't been turbocharged !

[A57 HSV]

HSV reccomend Premium Unleaded, which I think in Aus. is at least 98 Ron. Do any Aussies know if this is right?

[V-Car]

Thats correct i think...our premium unleaded used to be 95, but with the introduction of Shell Optimax and BP Ultimate, that has risen to 98.
So what we have now is regular unleaded at about 92, premium at 95 (which HSV recommend as a minimum) and Optimax, Ultimate at 98.