Please tell me about ignition timing

[NoHachi]

First read this: http://auto.howstuffworks.com/ignition-system.htm

Try to get a good idea what it is that that an ignition system (or advance) is actually trying to achieve. To get the pressure from the explosion at exactly that moment where the piston is ready to perform work (see the vid on howstufworks) The reason why you need to do this earlier and earlier (in terms of crank degrees) is because the time it takes to ignite the mixture stays the same regardles of rpm. But at higher rpm the crank is moving through more degrees in the same XX milliseconds it takes to ignite the mixture.
So what ignition systems do is fire the spark at exactly the right time to provide maximum work. At idle this is called the base timing. As rpms increase, the crank moves through more degrees per second and firing needs to be advanced a few degrees. This is called the advance. The advance you are running plotted against the RPMs is called the advance curve.
There are two ways of getting an advance curve. The 4A-GE uses a system where the ECU fires the coil according to an advance curve stored in the ECU. It starts at basetiming at idle and sensing the rpms starts advancing the stored amount of advance.
Older cars used mechanical or vacuum advanced distrubutors. These had inbuilt curves that depended on mechanical or vacuum gadgetary to achieve advance given a certain rpm. If you wanted another ignition curve, you had to have these distributors "recurved".