Ignition timing
In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke.
Early cars required the driver to adjust timing via controls according to driving conditions, but this is now automated.
These include the timing of the intake valve(s) or fuel injector(s), the type of ignition system used, the type and condition of the spark plugs, the contents and impurities of the fuel, fuel temperature and pressure, engine speed and load, air and engine temperature, turbo boost pressure or intake air pressure, the components used in the ignition system, and the settings of the ignition system components.
[1] The spark ignition system of mechanically controlled gasoline internal combustion engines consists of a mechanical device, known as a distributor, that triggers and distributes ignition spark to each cylinder relative to piston position—in crankshaft degrees relative to top dead centre (TDC).
The distributor's centrifugal timing advance mechanism makes the spark occur sooner as engine speed increases.
This typically applies to automotive use; marine gasoline engines generally use a similar system but without vacuum advance.
Engines so equipped carried special stickers on their valve covers reading “427-T.” AC Delco’s Delcotron Transistor Control Magnetic Pulse Ignition System became optional on a number of General Motors vehicles beginning in 1964.
By 1979 with the Bosch Motronic engine management system, technology had advanced to include simultaneous control of both the ignition timing and fuel delivery.
"Timing advance" refers to the number of degrees before top dead center (BTDC) that the sparkplug will fire to ignite the air-fuel mixture in the combustion chamber before the end of the compression stroke.
If the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly combusting mixture can actually push against the piston still moving up in its compression stroke, causing knocking (pinking or pinging) and possible engine damage, this usually occurs at low RPM and is known as pre-ignition or in severe cases detonation.
This results in lost power, overheating tendencies, high emissions, and unburned fuel.
Poor volumetric efficiency at higher engine speeds also requires increased advancement of ignition timing.
The speed with which the mixture burns depends on the type of fuel, the amount of turbulence in the airflow (which is tied to the design the cylinder head and valvetrain system) and on the air-fuel ratio.
With excessive advance, the engine will be prone to pinging and detonation when conditions change (fuel quality, temperature, sensor issues, etc).
Lighter weights or heavier springs can be used to reduce the timing advance at lower engine RPM.
Heavier weights or lighter springs can be used to advance the timing at lower engine RPM.
Vacuum advance works by using a manifold vacuum source to advance the timing at low to mid engine load conditions by rotating the position sensor (contact points, hall effect or optical sensor, reluctor stator, etc.)
This is a version of emissions control; the ported vacuum allowed carburetor adjustments for a leaner idle mixture.
At low temperature the advance allowed the enriched warm-up mixture to burn more completely, providing better cold-engine running.
Early emissions electronics would engage some in relation to oxygen sensor signals or activation of emissions-related equipment.
Most computers from original equipment manufacturers (OEM) cannot be modified so changing the timing advance curve is not possible.
Aftermarket engine control units allow the tuner to make changes to the timing map.
