Retarded timing: If the ignition timing on a car is retarded, then less power is made because the air/fuel mixture doesn't completely combust during the compression/combustion cycle. The flame front completely combusts after the piston has started its power stroke. This results in a loss of power.
Advanced timing: If the ignition timing on a car is advanced (to a certain degree), slightly more power is made because the air/fuel mixture is given time to fully combust and to allow the plug spark to travel. When the air/fuel mixture completes combustion, there is enough power created to slam the piston downward.
Over-advanced timing: If an improper grade of fuel is used combined with an over-advanced timing setting, the effects of detonation can damage the engine. Detonation is when the air/fuel mixture combusts too early or too quickly during the compression stroke. The flame front collides with the upward-stroking piston. Pressure in the cylinder becomes increasingly high. Temperature rises to extremely dangerous levels. This results in engine damage.
Is this mod worth it?
If and only if you're trying to squeeze every bit of power from your car and you're willing to use a higher grade of gas to do it. I can afford to use a higher grade of gas. I know some people can't, so it may not be worth it to them.
How much of a power gain should I expect?
Hopefully, you gain some mad horsepower so you can beat Civic's, but I doubt that'll be the case. I read that adding 1º of advance produces 2 horsepower to the wheels. Of course, this is only a guideline, and it varies from car to car.
