Originally Posted by BogMonster
Interesting, I always thought high octane fuel was more likely to go bang, didn't realise it was the reverse. I heard of somebody here a few years ago who was running 100LL aviation fuel in a trials bike, apparently won just about everything but didn't do the engine much good!
No, high octane fuel is LESS likely to go bang.
It is a common misconception.
In the quest to extract more performance from petrol engines, desginers raise the compression ratio (how much the piston squeezes the fuel/air mixture) as high as they can. Theoretically, the higher the compression ratio, the higher the engine efficiency. But as compression increases, the higher the chance that the petrol will 'pre-ignite' (burns before the spark plug fires). This causes an irregular burn, high pressure spots, and the 'pinking' noise.
Higher octane fuels were designed to allow these higher compression engines to function reliably. They are able to resist the higher pressures and temperatures of higher tuned engines before being ignited by the spark plug.
Super tuned race engines have very high compression ratios (either by head design or by turbos, etc.) and therefore NEED high octane fuel (98 up to 130+)
If you look in your car/outboard handbook, you can find your compression ratio. Most petrol engines are in the 8 to 10 / 1 range. In a 4 cylinder, 2 litre engine, a 10/1 ratio means that the piston/cylinder goes from a max.volume of 500cc to a minimum of 50cc. Using 98 RON in such an engine will gain you nothing, and will most likely reduce efficiency, albeit by a v. small amount.
I used to have an Audi S4, one of the few cars out there to officially 'prefer' 98RON, due to turbos, etc. Although I used 98 sometimes, it ran just as well on 95, and I never noticed any pinking.
I am not in the US, but I believe it is quite difficult to buy 95 RON petrol, let alone 98.