Quote:
Originally Posted by oab_au
There is no different in the burn speed of 92 octane to 98 octane. The only difference is in the 'flash point' of the fuel. This is the temp/pressure that the fuel can stand before it will self-ignite. A higher octane can stand a higher combustion chamber temp/pressure, before it will self-ignite/detonate.
It won't make it run rich or hurt the cats.
Harvey.
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I agree and at the same time disagree, nothing personal. You are correct on the octane rating and the flash point of the fuel. However, the slower burn prevents knock in a number of ways beyond raising the flash point. Knock can be caused by lots of different things as you're well aware. Hot carbon chunks, hot spark plug bases, bad plug wires, excessive engine heat, and many others can all lead to detonation. However, even on an engine running at normal temperatures with no carbon build up or any other flaw or defect, knock WILL still occur, albeit minor. This is a less serious example of the fuel in the combustion chamber igniting at both the piston and the plug at the same time. Picture a combustion chamber from the side, with the plug being top center. In lower octane fuel, due to the high rate of burn speed, it is possible for the center to not get burnt as fast as the sides. As the sides burn together, the now separate flames are able burn into each other, colliding and causing knock. This will produce enough of a signal for the knock sensor to respond, but not with enough voltage that the computer will care to do anything about it. A slower burn reduces the frequency of this occurring.
As for the effect on the cats, the slower burn means that by the end of the exhaust cycle, some of the fuel is still burning or is not burnt as it leaves the engine. Without tuning adjustments for the octane change, this will cause a rich condition. Running consistently rich is one of the most frequent causes of catalyst failure. Think of a muscle car running a short, wide open exhaust running on race fuel. I know I've seen flames come out the rear. There would be no flames if the fuel was fast-burning.