Contented Peugeot Driver
I thought Roger's comments deserved better than being buried in a post that many might gloss over on their search for meaning:
Nothing to add myself other than opinion. :wink2:lhs2.1 said:think some clarification is needed.
Firstly, as far as I'm aware there is no combustion phenomena known as "pre-detonation".
For a gasoline fuelled IC engine there are two separate abnormal combustion processes, which are separate but can be interlinked. These are "pre-ignition" and "detonation". The latter is also known as "knocking" or "pinging" or "pinking".
Pre-ignition is when ignition of the fuel-air mixture in the cylinder occurs before the spark plug event. This ignition can be initiated by a incorrect heat-range spark plug, or incandescant hot-spots in the combustion chamber.
Detonation is when there is spontaneous combustion of the the fuel-air mixture in the combustion chamber ahead of the flame front progressing from the spark plug ignition. When this happens there is a rapid rise in local pressure causing a shock wave in the combustion chamber which may be heard as a knocking sound.
Detonation can result from too much ignition advance. too low an octane rating of the fuel, too high a compression ratio, and from pre-ignition. It is influenced by manifold pressure ie engine load, and if detonation is allowed to persist, the shock waves can damage piston crowns and ring lands.
For the engines we're discussing here, pre-ignition shouldn't be a problem assuming that correct spark plugs are used, and that there haven't been running problems in the past which may have resulted in excessive deposits in the combustion chambers.
Octane ratings of gasolines are really indicators of the detonation resistance of the fuel. Even the un-leaded 91 octane crap that we are served up today is far superior (in detonation resistance) to the fuels available when the Dees were launched. Consequently, there is scope to advance the ignition timing beyond the original OEM recommendation.
However, setting the optimum ignition setting is a trial and error process and relies on the ability of the operator to hear when detonation is happening ie when to back off the advance. Some people are good at hearing "pinking", others aren't.
My advice is to set the static ignition timing to the manufacturer's recommendation, find a nice clear road, roll at 60/70 km/hr and floor the accelerator in a high gear and listen for a tinkeling sound. If no sound, tweak the ignition advance and repeat until you hear it. Then back off the static timing a degree or so.
In a previous life I spent some years testing fuel octane requirements of cars, including verification of this for pre-production for most of the major Oz manufacturers.