Ignition timing, in a spark ignition internal combustion engine, is the process of setting the time that a spark will occur in the combustion chamber (during the power stroke) relative to piston position and crankshaft angular velocity.

Setting the correct ignition timing is crucial in the performance of an engine. The ignition timing affects many variables including engine longevity, fuel economy, and engine power. Modern engines that are controlled by an engine control unit use a computer to control the timing throughout the engine's RPM range.













Setting the ignition timing


Timing light"Timing advance" refers to the number of degrees below top dead center (BTDC) that the spark will ignite the air-fuel mixture in the combustion chamber during the compression stroke. Retarded timing can be defined as; changing the timing so that fuel ignition happens later than the manufacturer's specified time. If the ignition timing, specified by the manufacturer, was to be set at 12 degrees BTDC and it was adjusted to a number lower than 12 degrees BTDC, it would be retarded. In a classic ignition system with breaker points, the basic timing can be set statically using a test light or dynamically using a timing light.

Timing advance is required because it takes time to burn the air-fuel mixture. Igniting the mixture before the piston reaches top dead center (TDC) will allow the mixture to fully burn soon after the piston reaches TDC. If the air-fuel mixture is ignited at the correct time, maximum pressure in the cylinder will occur sometime after the piston reaches TDC allowing the ignited mixture to push the piston down the cylinder with the greatest force. Ideally, the time at which the mixture should be fully burnt is about 20 degrees ATDC. This will utilize the engine's power producing potential. If the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly expanding air-fuel mixture can actually push against the piston still moving up, causing detonation and lost power. If the spark occurs too retarded relative to the piston position, maximum cylinder pressure will occur after the piston is already traveling too far down the cylinder. This results in lost power, high emissions, and unburned fuel.

The ignition timing will need to become increasingly advanced (relative to TDC) as the engine speed increases so that the air-fuel mixture has the correct amount of time to fully burn. As the engine speed increases, the time available to burn the mixture decreases but the burning itself proceeds at the same speed, it needs to be started increasingly earlier to complete in time. Poor volumetric efficiency at lower engine speeds also requires increased advancement of ignition timing. The correct timing advance for a given engine speed will allow for maximum cylinder pressure to be achieved at the correct crankshaft angular position. When setting the timing for an automobile engine, the factory timing setting can usually be found on a sticker in the engine bay.

The ignition timing is also dependent on the load of the engine with more load (larger throttle opening) requiring less advance (the mixture burns faster). Also it is dependent on the temperature of the engine with lower temperature allowing for more advance. The speed with which the mixture burns depends also on the octane rating of the fuel and on the air-fuel ratio.

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1. Disconnect vacuum hose(s) on distributor and plug them.

2. Remove the #1 plug wire (again, check your manuals to determine the #1 cylinder) either from plug or distributor and attach correct lead of timing light here (see directions with timing light).

3. Connect electrical leads of timing light per directions (to battery or AC outlet).

4. Set engine to idle according to manufacturers recommendations with the engine at operating temperature.

5. Turn off the engine and loosen the distributor hold down clamp.

6. Start engine again, aim timing light at flywheel or harmonic balancer, whichever has the timing marks on it " note: marking with a white crayon on the timing mark will help to locate it easier when using the timing light", . Rotate the distributor each way until the mark on your flywheel or balancer lines up with the correct mark on your timing pointer.

7. Shut off the engine and re-tighten the distributor.

8. Recheck dwell angle and timing; in tightening the distributor you might have jarred it.

9. Remove dwell tach, timing light, and plug from vacuum hoses; reinstall vacuum hoses on distributor.

10. Start engine and set the idle to manufacturers specifications.
Check your owner's or shop manual for the correct timing settings for your vehicle.
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Ignition timing
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Older engines that use mechanical spark distributors rely on inertia (by using rotating weights and springs) and manifold vacuum in order to set the ignition timing throughout the engine's RPM range. There are many factors that influence ignition timing. These include which type of ignition system is used, engine speed and load, which components are used in the ignition system, and the settings of the ignition system components. Usually, any major engine changes or upgrades will require a change to the ignition timing settings of the engine.