Basically, when the engine is at low rpm's like out of the hole, you want the ignition timing to be advanced more. When the engine climbs to higher rpm's, you want the timing to retard. The pcm controls all of this. It can be adjusted through the use of a hand tuner, but should not be attempted by anyone but an experienced tech. If the timing is too far advanced for the fuel you are using or if you have too much air flow compared to the fuel flow, this will cause detonation or pinging. I'm sure you have heard an engine ping. The timing of the ignition spark in relation to where the piston and valves are in their cycle is very important. If it's off, you will either lose power or cause detonation which will burn the pistons. When you purchase a hand tuner, it should be set up with your current mods taken into consideration with the program it comes with. The dealer that sells you the tuner should be able to handle this. Your best bet is to have the car tuned on a dyno after you have completed mods that affect fuel/air flow. You can buy kits that have a variety of bolt on parts and a specific tune ready for the bolt on mods and a tuner that is set up for the kit. Like for example, the kit would come with a CAI, UD pullies, axle back exhaust, 4.10 gears or whatever. The tuner would be set up for all that's in the kit including correcting the speedo do to the gear change.
