Ignition Timing

So why is ignition timing so important?

Firstly I would like to clarify what is mean't by "ignition timing". Timing is the point, in time, at which the spark plug "sparks" in relation to the piston position on it's compression stroke.
If the spark is created at the wrong time, power is lost together with efficiency and engine damage can occur.


Let me explain why these events happen with incorrect timing.
Firstly lets take a look at what happens if the ignition sparks to late. This is also known as the ignition being retarded. As the piston approaches TDC on it's compression stroke the timing at which the spark will occur is calculated, whether that be by mechanical means or electronic the fact remains it it gets defined by some means. If the spark is created too late the burn of the air fuel mixture, creating expanded gases, will tend to chase the piston down the bore rather than push the piston down the bore, hence the loss of power.


If now we look at the spark being created to early, i.e. Too advanced the expanded gases will start to push the against the piston on it's way up to TDC. Again losing power. So between these two points there is a sweet point where a spark could be created just at the precise moment to enable the gases to expand and push the piston down the bore at the optimal time. To complicate things further this precise timing point changes with speed and load of the engine.


A conventional ignition system uses mechanical mechanism to control this timing point over the rpm range. The mechanical device responsible for ignition timing is known as the distributor also known as the dizzy. Inside the distributor there is a cam some weights and springs. The cam pushes open a switch the switch is called points. At the time the points open a high voltage is sent to a spark plug to create the spark. The cam inside the distributor rotates at half the speed of the engine. As the engine speeds up the weights inside the distributor start to throw out due to centrafugal force. The weights are connected to the cam in such a way to rotate the cam as they move outwards. This opens the points slightly earlier causing a spark at the spark plug slightly earlier i.e advancing the timing. The springs hold the weight back. The speed of the rotating cam, the weight of the weights and spring strength define the advance curve i.e. The timing value at any given rpm in the engine rpm range.


Because of the inherent crudeness of this design the advance curve compared with the engine's optimum advance curve is difficult to achieve, as optimal advance curves are non linear. To try and solve this problem in a mechanical ignition timing system two stages of advance are built into the distributor. This uses two weights and two springs each of the weights and springs are rated differently. This allows the two linear timing curves to be produced. One for the lower rpm range and one for the upper rpm range. The main problem with this is due to the mechanical constraints of the system the curve can never produce an optimal timing curve.


Load an engine also demands a change in timing to obtain optimal efficiency. The distributor addresses this by having a diaphragm that is connected to the internal workings of the distributor. The diaphragm is operated by a vacuum taken from the inlet manifold. When the engine is not under load high vacuum exists in the inlet manifold. This in turn creates a vacuum on the diaphragm, creating a pulling force. This moves a lever inside the distributor, rotating the internal workings, advancing the ignition. As engine load increases the vacuum in the inlet falls away, the diaphragm moves towards it's natural resting position, the lever is moved back, reducing the advance. So as we can see there are two main factors that change the engines timing needs, rpm and load, and also see how these demands are catered for by a mechanical ignition system.


Before I move on I just want to mention a few other problems associated with mechanical ignition.


  • 1) points bounce. When building a high rpm engine this a fundamental problem when points are involved. Imagine the cam in the distributor rotating, opening and closing the points. The cam opens the points and as the cam falls away the spring on the points pushes the contacts closed. If the cam starts to rotate fast enough the spring isn't strong enough to follow the cam profile. As the cam pushes the points open and the cam falls away faster than the spring can close the points. They are in effect left open when the points should be closed. By the time they close the next cam lobe is pushing the points open again. Because the points haven't closed for the correct amount of time the charging of the coil would have failed, subsequently a spark would not occur at the plug, resulting in a missfire.
  • 2) distributor cam wear / distributor cam bearing wear. After the engine has done a number of miles cam wear and bearing wear occurs. Cam wear can cause asymetrical cam lobes this results in none optimal coil charging times resulting in weak sparks causing misfire's. Cam bearing wear causes spark scatter due to the cam moving around in it's bearing. This is direct result of the points opening at slightly different time due to the placement of the cam in the bearing. There may be only a small amount of play in the bearing but this is significant to cause a number of degrees timing fluctuations, resulting in power loss.
  • 3) Rotor arm. This has to be replaced at set intervals to keep ignition system in optimal working order. The rotor arm is responsible for taking the high voltage from the coil and delivering it to the correct terminal on the distributor cap.
  • 4) The distributor cap. Again has to be replaced at set intervals to maintain an optimal ignition system. Responsible for delivering the high voltage from the rotor arm to the spark plug, via the ignition lead.

The ignition spark lasts around .002 seconds(2ms).

At 6000 RPM an engine just takes .010 seconds(10ms) to do 1 revolution.

After ignition, combustion pressure can easily reach 400psi