Ignition timing is the relationship between the spark and piston position in an engine. It affects power output, fuel consumption, and engine longevity. Timing is established using a timing light and reading the engine’s harmonic balancer. Pre-80s engines are tuned before TDC, post-80s engines are tuned after TDC.
Ignition timing is the relationship of the spark to the position of the pistons in an internal combustion engine. Measured in degrees of the piston’s stroke within the cylinder wall, it is relative to the leading or leading relationship of the piston to the top dead center of its stroke, or trailing or trailing position. The position of the piston in relation to the spark plug that ignites the fuel-air mixture, which is the ignition timing, dictates an engine’s power output, as well as its effectiveness in burning fuel. A vehicle’s fuel consumption, peak power and engine longevity depend on ignition timing in relation to the 360 degrees of rotating crankshaft.
In a four-stroke engine with an intake, compression, ignition, and exhaust stroke, the crankshaft drives the piston to its absolute highest position within the cylinder twice in one cycle. As the piston approaches the top of the compression stroke, the ignition timing determines when the spark plug will fire. If the mixture is fired too soon, the piston will fight going all the way up to the top of the stroke. If the ignition timing sparks too late, power is lost, as the piston is already on its stroke.
The effectiveness of the on time can best be compared to pushing a person on a swing. If you grab the swing early and then follow up and push down violently, the swing will go up with great force. If the pusher only comes in contact with the swing itself when walking away, the person on the swing will hardly notice the power of the push. The same is true for a piston inside an engine; ignition timing must occur at precisely the correct point in the piston travels to provide maximum power.
With the engine running, timing is established 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 before and after TDC of the piston in its stroke. Typically, 1970s and earlier engines are tuned before TDC and post-80s vehicles are tuned after TDC. This is in response to attempts by manufacturers to tune vehicles to save gas. These so-called “smog engines” were tuned conservatively in an attempt to provide better fuel economy, with some manufacturers going so far as to relocate the balancer position on the crankshaft several degrees to the retarded position to improve readability.
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