Dieselman said:
Most petrol engined vehicles will rely on the knock sensor to set the ignition timing. If a fuel that has a slower burn rate, (higher octane), is used then the timing will be more advanced.
The ignition retards back from the optimum setting to stop the onset of pre-ignition (pinking), when lower octane fuels are used.
This should then release more energy from the oxygen in the air, thus more power over a longer burn time.
If the said engine is in a low state of tune and has a low compression ratio then there will be little effect from raising the octane level.
The enhancing additives act to slow and stabalise the burn to release more power, as well as having detergent additives to clean the valves and piston rings.
I was just limbering-up my fingers up to say exactly this ^ ^ ^ !! Glad you beat me to it DieselMan as I have a feeling this will be a long post anyway!!
I don't think it's a coincidence that the two cars featured that showed the greater improvement in performance were both turbocharged (Golf and Imprezza), and that the one that showed the greatest improvement was likely to be running at higher boost (Imprezza). Careful selection by the producer and their advisors, and no doubt the fuel companies concerned - they want to show the fuel to it's best effect!!
The higher octane and greater burn control allowing more boost and therefore liberate even more power. It's this that 'magnifies' the gains on a forced induction engine, and therefore makes it more noticeable.
I drive an Astra Turbo Coupe and I can honestly say that there is noticeable difference in performance. I'm not talking different league with Optimax compared to without, but there is a difference in pick up. How much of this is real and perceived is difficult to know, but what I do know is that scientifically higher octane fuel does have greater potential for producing power, than lower octane fuel.
Forums for owners of turbo-charged cars are full of threads on Optimax and Ultimate, as they potentially have most to gain it's worth a read. It's probably the Subaru/Evo boys though that are keenest as the fierce rivalry encourages them to eek out every last bhp!!
Is it worth the extra money? I almost always use Optimax and have done for three years or so. I know the potential is there for the extra performance, but apart from the odd straight line prod I don't really use it. Fuel consumption is broadly the same. So you could say the benefit is limited for me, but I'd rather have it than not.
). The Sainsbury's Superunleaded didn't seem to make any difference at all over its standard 95 fuel. All of this is of course somewhat subjective. I didn't carry out any real scientific testing and maybe that's what is really needed.
