knock , detonation and pinking.
These are three variations of the same theme.
In order of how much to worry.
1) Pinking , ignition is over advanced and the burn is too fast. The piston rattles off the cylinder wall and makes the sound. Less energy is used as kinetic energy (the piston is fighting it) and more heat is generated. Heat being particularly bad in an aircooled motor.
2) Knock , the spark is fired by the ignition but there is an uneven burn. There is a secondary ignition source and the two flame fronts meet creating the sound. Worse than pinking alone , more heat.
3)detonation/pre ignition. ignition without the spark, the charge self detonates and slaps down on the approaching piston. Very unhealthy, nasty forces on the whole bottom end and lots of heat.
Raising or lowering the compression ratio (subject to limits of induction and other components ) creates a denser charge. The more charge squeezed into a space the faster the burn . As compression ratio is raised the timing needs change, often less.
Fuel mixture.... This should be correct
. slightly over stoic. at wide open throttle. This allows for an imperfect swirl and distribution of fuel. At light throttle,lower revs this can be leaned out as there is insufficient charge to self detonate. If the mixture is "correct" it makes everything else easier.Timing. Too much advance and the burn is too soon and energy is wasted as heat, fighting the approaching piston. Too little and the burn continues as the piston is well down the cylinder, the energy is not converted into kinetic and becomes heat. So how much is just right? Well the best way is on a rolling road. The "correct" is the minimum advance to produce the maximum power at any speed (revs) and load point.
This advance angle changes with speed and load . To correctly time an engine you must measure both and adjust the advance based on both.
For example an Bosch 009, only measures speed so it advances to suit wide open throttle. even then its pretty poor at that. At anything other than full throttle (except where very over carbed) there is not enough advance. Energy is wasted and becomes heat.
Can you see the light bulb yet.
Finally. There are limits to compression ratio and its a very long story.
At some point Detonation will start and timing will have no effect.
However, in most cases the point of pre-ignition is still the spark plug!!
Put a cylinder heat (complete) on a bench and put a blow torch to the combustion chamber. Funnily enough the spark plug glows red within seconds, yes that pointy bit of metal sticking out of the head!
So, what. Well spark plugs are designed to run hot, too cold and the carbon builds up on the plug and it fouls. Too hot and it glows like a welder and lights up the fuel without asking.
If the timing is correct and the mixture is correct then you can "read" the plugs. The old method of checking the colour of the plugs works. If its shiney and glazed (insulator) its too hot, if its black with carbon then too cold. If its tan coloured from burnt off carbon its just right.
Once the rest is right you can adjust the plugs. This will maximise the amount of mixture the combustion chamber can correctly burn .
Basically thats it, and can you see why I believe mapped ignition is a must and fuel injection worth the extra hastle and cost.
All the Best............... Good luck!
