Ignition timing refers to the point at which the spark plugs ignite the fuel within an engine’s cylinder. Proper timing is all-important to the thermo-efficiency of the engine. The first thing to keep in mind is that fuel burns at a certain rate in an engine regardless of the rpm.
The Four-Stroke Engine
All automotive engines today have four strokes. Let's use a single cylinder in an engine as an illustration to demonstrate how all four strokes work. The crankshaft turns two revolutions, which moves the pistons up and down to one turn of the camshaft that opens and closes the valves.
The piston begins all the way up at top dead center. The burning fuel expanding forces the piston downward. At this time the valves are closed. This is the power stroke. As the crankshaft turns, the piston begins to go back up and the camshaft opens the exhaust valve. The upward moving piston forces the burnt gases out of the cylinder. This is the second or exhaust stroke.
Just before the piston rises completely in the exhaust stroke the intake valve opens, using the vacuum produced by the rapidly exiting exhaust gases to help draw in more fuel from the intake valve. As the piston reaches top dead center and continues downward once again it creates a vacuum sucking more fuel into the cylinder. This is the intake stroke. Just prior to reaching the bottom of this stroke the intake valve closes.
The crankshaft turns again and the piston begins to move upward, compressing the raw fuel and air in the process. This is the fourth or compression stroke. As the piston rises, the ignition spark plug ignites the fuel and the process begins again.
Consider that the fuel must be burnt as completely as possible before the piston reaches the top of the compression stroke in order to force the piston downward in the power stroke. It would not be efficient having a small percentage of the fuel consumed before the piston hits top dead center. This would mean that the fuel is still igniting while the piston is descending in the power stroke and would result in a massive loss of power.
Using an engine idling at 900 rpm further illustration, the piston is moving upward at a speed that the fuel, given the time it takes to burn completely, is ignited 1/16-inch from the top. This would equate to 10 degrees of crankshaft rotation before reaching top dead center to give the fuel sufficient time for a complete burn.
As the engine rpm increases to 3,000 rpm, the fuel, -- still requiring the same time to burn -- would never have sufficient time to burn if ignited at the same 10-degree timing. In order to have the same result, the fuel must be ignited much sooner in the compression stroke. As a result, the timing moves to 32 degrees before top dead center which allows the fuel to be ignited early enough so that it is burning while the piston is rising and completely burnt when the piston reaches top dead center.
Effects of Retarded Timing
In a situation where the ignition timing is retarded too far, the plugs ignite the fuel too late, allowing it insufficient time to burn completely. This causes a loss of power and poor fuel economy. Hesitation and backfiring will accompany the loss of power causing the plugs to foul leaving them with a black “sooty” appearance.
Computer and Environmental Effects
The unburned fuel enters the catalytic converter, causing it to overheat and turn cherry red. Over a short time this will cause the converter to fail. Additionally, the vehicle's computer, sensing a rich mixture in the exhaust, will set a code for the oxygen sensor that will often appear as a "Check Engine" light. You will smell raw fuel and see black smoke from the exhaust when this occurs.
Reasons a Late Model Vehicle will Retard the Spark
A late model computerized vehicle will automatically retard the spark if the knock sensor has failed. It is designed to sense spark knock associated with detonation and will automatically retard the timing to prevent the detonation.
The computer will also retard the spark if the engine is running too hot. Excess heat, coupled with lower octane-rated fuel, will cause the engine to experience spark knock. The spark timing is automatically retarded when knock is detected.