Pud 237, isn't the speed at which fuel burns what causes knocking anyway?????
Knocking is the fuel/air mix igniting prior to the spark going off. Octane is a unit that measures the fuels resistance to oxidisation in conditions of high temperature/pressure. Ideally the fuel/air mix doesn't ignite until the spark goes off, this is timed to ensure the most force is generated on the pistons during the powerstroke. Its no good if the fuel takes so long to burn that there is still some left at the end of the powerstroke..
Maybe we notice a difference with Vpower because once an engine has built up deposits, is worn.... pre-ignition is more likely and so the ecu cannot keep to the (95) map because the anti-knock sensors are telling it the pre-ignition is taking place. It would then retard the ignition and so performance is down.
That's actually quite a clever theory, I think you're probably correct there - and that is why some people feel a difference on V-Power and others don't, some are already on the 95 map with 95 fuel, for some it takes V-Power to prevent knocking when running on the standard 95 map, due to deposits or wear.
The supposed virtues of using Vpower are that it has detergents to clean the engine (less hot residue to cause pre-ignition) and a more controlled flame ( less likely to cause violent uncontrolled burning or pre-ignition). This suggests that the anti-knock sensors are cutting in less often and so the ecu map can be followed more closely.
The Ron of the petrol was originally about the proportion of octane to control the burning, so that higher compression ratios could be run. The lower the Ron the more quickly and violent the explosion when ignited.
What do you think?
I certainly think V-Power is good for the fuel system, in comparison to as-cheap-as-you-can-find 95.. But I'm pretty sure that the octane rating isn't about how quickly the fuel burns, but just it's resistance to self-ignition. One's the spark plug has ignited the fuel, you want it to burn very quickly, so as much energy as possible is transferred onto the crank.
The way I see it, its similar to riding a pushbike, when you are in the correct gear for the speed you are travelling, you can exert a lot of energy downwards onto the peddle, accelerating you along. But when you are in a very low gear (normally for uphill and setting off), the peddles are moving very fast, and the 'explosion' in your leg muscles doesn't happen quick enough to do much pushing on the peddle during the power-stroke. So when it comes to fuel, you want a fuel with a high octane (resistance to pre-ignition), so it goes off when the spark activates but not sooner, and also you want a fuel that burns fast (not sure what unit measures this, I think it is seperate to octane), so that the fuel can burn fully and give as much energy as possible to the crankshaft through the pistons. I've always thought that octane doesn't have any relevance to how fast or violently the fuel burns once it is ignited by the spark plug, it is purely a measure of resistance to self-ignition prior to the spark.