Ignition timing is the relation of spark to the position of an internal combustion engine's pistons. Measured in degrees of the piston's stroke within the cylinder wall, it is relative to the before or advanced relation of the piston to the top dead center of its stroke, or the after or retarded position. The position of the piston in relation to the spark plug igniting the air fuel mixture, which is ignition timing, dictates an engine's power output as well as its effectiveness in burning the fuel. A vehicle's fuel mileage, peak power and engine longevity are all dependent upon the ignition timing in relation to the 360 degrees of the spinning crankshaft.
In a four-stroke engine with an intake, compression, ignition and exhaust stroke, the crankshaft brings the piston to its absolute highest position within the cylinder two times in one cycle. As the piston approaches the top of the compression stroke, ignition timing dictates when the spark plug will fire. If the mixture is fired too early, the piston will fight rising completely to the top of the stroke. If the ignition timing fires the spark too late, the power is lost, as the piston is already on its way down the stroke.
The effectiveness of ignition timing can best be compared to pushing a person on a swing. If the swing is grasped early and then followed up and pushed down violently, the swing will go up with great force. If the same swing is only contacted by the pusher as it is heading away, the person on the swing will hardly notice the power of the push. The same holds true for a piston within an engine; the ignition timing must occur at precisely the right point in the piston's travels in order to provide the maximum power.
With the engine running, timing is set by using a timing light and taking a reading from the engine's harmonic balancer. The balancer is attached to the crankshaft and is marked with lines and numbered in degrees both before and after top dead center of the piston in its stroke. Typically, 1970s and earlier engines are tuned before top dead center and post 1980s vehicles are timed after top dead center. This is in response to the manufacturers' attempts at tuning vehicles down to conserve gasoline. These so-called "smog engines" were conservatively tuned in an attempt at providing better fuel mileage, with some manufacturers going as far as relocating the position of the balancer on the crankshaft several degrees in the retarded position to enhance the reading.