Thanks Sean, that all makes sense in it's self, and I had read similar before, but I still don't see how all the bits tie up. To quote what I think is your key paragraph,Now if the mixture at the far end of the combustion chamber is being spontaneously ignited by high pressure ahead of the flame front, I can see how having it at a higher pressure to begin with would cause a problem, either through a higher compression ratio, or forced induction, and I can see that lower octane fuel would be more prone to this scenario.

What I don't understand is how the timing at which the plug fires has any influence over this exact chain of events.

The only thought that occurs to me is that the exact position of the piston influences the initial pressure in the combustion chamber and also the shape of the combustion chamber. However my gut assumption would then be that having the piston higher in the bore, with higher pressures and a narrower flatter combustion chamber, would cause more problems. But that would rely on the timing being retarded, which is obviously at odds with all anecdotal evidence.

Oh and Ken, the quote that you were so uncomplimentary about;Came from here http://www.superchargersonline.com/content.asp?id=15 , which in turn seems to have been cribbed from here http://www.powerdyne.com/tech.htm , the FAQ page of a major supercharger manufacturer. So was it written by a salesman rather than an engineer, or is the situation different when talking about forced induction?

And was it the bit that I've underlined that you had a particular problem with?

Dare I put my post in?
It is quite simple. Wrong timing is the main problem here. The compressed fuel/air mixture is ignited to early by the spark plug, not by itself. If it did, you could drive without sparkplugs, right. The piston is still compressing the mixture when the spark ignites the fuel. It doesn’t not explode but burns by spreading from the sparkplug. A huge amount of heat and released energy is pushing against the piston crown while the piston is still heading toward TDC (top dead center) This is causing the famous pinging you can hear. If the timing is correct, the piston is ‘’allowed’’ to pass TDC and the burning mixture is actually helping by pushing the piston down. Imaging the piston, conrods crankshaft etc been given a huge knock on the way up. Bad timing must be avoided at all times. I may be wrong here but high octane fuel (98) is slower in burning the fuel than low octane fuels. Please grill me if I’m wrong. I know someone more than willing to do that.

The way I had previously 'understood' it was that the mixture was igniting itself, through being compressed. In the same way that an engine sometimes 'runs on' after the ignition is turned off. Although obviously not reliably enough to drive like that.

Normally caused by low octane fuel in conjuction with sod igniting the fuel, not igniting itself

Isn't it to do with the heat of the engine as well though, Viking? Hence it can be caused by an over-lean mixture, insufficient cooling (muff left on!) or, although I've never encountered it in an A-series engine, over-retarded ignition.

Ok so I'm trying to understand all this from you all, I'm guessing mine isn't a pinking problem, what I feel when I sit at 40 is the car slightly judder, almost misfire but I'm not convinced it is, I just put it down to it pinking. Trouble is the 2 into 1 exhaust and open air filter etc make it hard to hear any noise. I know ignition is good unless my 123 is wrong. But I can sit with my fit to the floor running full chap and not feel a thing.

There's another thing, lean mixture, how does that contribute? directly or indirectly? does a lean mixture burn faster or slower, is it more susceptible to the process that Sean described earlier. Or is it an indirect link, i.e. a lean mixture burns hotter, and the extra heat in the piston crown and cylinder head then heat up the following charge which makes that more likely to suffer detonation.

And Viking I've never heard of fuel being ignited by sod. Sorry to sound rude, but if that's a typo, it doesn't help an already very confusing topic.

Maybe wrong word. What do you call the black deposits inside the combustion chamber?

Think of a gas burner, yes more oxygen and the temp goes up.

Sigh
there is Pre ignition where by a hot part of the engine such as plug electrode, valve edge or big flaky bits of carbon ignite the charge often a cause of "dieseling" or running on

thats not whats going on at full chat

pinking/knock is something else
for the curious who want to educate themselves( altho ill get slapped by V and reminded that "wikipedia is NOT a valid achademic litrature source")

lean charge will cause the engine to tend towards pinking due to the heat as well as being more condusive to burn as petrol engines tend to run better at just over its
Stoichiometric ideal...im not 100% on that last bit and am trawling to try to confirm it

Sean
Ps "soot" hydrocarbon particulate pollution, un burned carbon