Ignition Timing Theory & Background

The torque of an engine is the turning force generated by the burning of an air/fuel mixture in the cylinders.  This burning creates high pressures in the cylinders, the burning mixture expands and fills the cylinder and continues to expand as it forces the piston down in its bore.  The force on the piston acts throughout most of the stroke of the piston as it travels down, subsequently spinning the crankshaft.

Maximum torque (and thus horsepower) is generated in the engine by making sure that peak cylinder pressures are generated at the right time.  Too soon, and the cylinder pressures want to force the piston down in its cylinder before it reaches the top of its stroke.  This is obviously bad for torque as the piston tries to make the crank spin in the wrong direction!  This causes engine knock which is destructive and can damage engine internals such as pistons, con rods, bearings, and the crank itself. 

If peak cylinder pressures are reached too late, then the cylinder pressure acts down on the piston as its already traveling down.  The pressure quickly drops because the volume of the combustion chamber is rapidly increasing, therefore the cylinder pressure has a reduced affect on torque and therefore much of the pressure is wasted.

To get the maximum cylinder pressures occurring at the right time, it is critical that the ignition of the air/fuel mixture by the spark plug occurs at the right time.  This is determined by the ignition timing.  Ignition timing is referenced from the top of piston # 1's stroke, which is called top dead center (TDC).  The crankshaft spins 360degrees to perform one revolution, and typical ignition timing will be set to operate in the range of 5 to 35 degrees before TDC.  Before TDC is abbreviated to BTDC.  The terms "advanced" and "retarded" ignition timing refer to making the spark happen sooner or later respectively.  If the ignition is altered from 10deg BTDC to 25deg BTDC, then it is said to be advanced 15deg because spark will occur 15deg sooner.  

An engine will produce optimum torque and power for a given engine speed and load at a specific ignition timing setting.  Advancing the ignition timing past this level usually leads to detonation and engine knock, and retarding the timing leads to a reduction in torque.  In general, assuming all other parameters are correct like air/fuel mixture, an engine will make maximum power by advancing the ignition timing to the point just before detonation.

The ignition timing required for maximum cylinder pressures to be delivered depends on several factors.  The main ones we can cater for are engine speed, engine load, air temperature, and water temperature.  

Only a computer controlled ignition system can offer some reasonable level of control of the ignition timing with respect to air and water temperature.  In general, an engine will perform optimally at a particular water temperature.  This may be 90deg C for example.  If the water temperature starts to climb for whatever reason (broken water line causing loss of water, etc) then you can alter timing accordingly.  Increased engine temps increase the likelihood of detonation, which can be countered by retarded ignition timing, so a computer can control this variable.  Retarded timing is also useful for increasing the amount of heat that is absorbed by the cooling system, so retarded timing can be used to reduce engine warm up times.

An increase in air temperature will also increase the likelihood of detonation, so you may want to retard timing at high air temps.  Low air temps may mean that you can advance the timing more to make more torque and power, because the detonation threshold has been increased due to the cooler air.

Engine management computers can also control ignition timing with respect to engine speed and load, but this can also be done successfully by mechanical means such as with a distributor.  The Gemini obviously uses a distributor to control the ignition timing and to distribute the spark to each spark plug from the ignition coil, hence the name.  

As the rate of the burn of the air/fuel mixture is fairly constant, it must occur sooner and sooner as the engine's speed increases (when the revs increase), so that maximum cylinder pressure happens at the right time.  If the spark didn't happen sooner, peak cylinder pressures would occur as the piston is well on its way down the cylinder, and as described above, this leads to a reduction in engine torque.  This is why we need to advance the ignition timing as engine speed increases.  As an example, the Gemini is usually set with a base timing figure of 6deg BTDC.  This base timing figure is called the static timing.  The Gemini distributor houses a mechanical advance mechanism that uses centrifugal weights to moves the ignition points so that it advances the timing as the revs increase.  By around 3500rpm, the timing has increased from 6deg BTDC to about 30deg BTDC.

Engine load is the last variable we can tackle, computers can deal with this by measuring the engine load by either reading the inlet manifold pressure, or by measuring the amount of air that is flowing into the engine.  Either method can give accurate data on the load of the engine.  Typically, as load is increased, the ignition timing needs to be retarded and vice versa.  This is because at low loads, the mass of air/fuel drawn into the cylinder is less than at high loads.  The lesser amount of air/fuel mix takes longer to burn as the molecules of the mixture are spread further apart in the volume of the combustion chamber, so the burning of the mixture takes longer.  Because it takes longer, the spark must occur sooner so that peak pressures happen at the right time.

The mechanical method to achieve this is with a diaphragm which references inlet manifold pressure.  Under light loads, a vacuum is present in the manifold which causes the diaphragm to advance the ignition timing.  As the load increases, the vacuum drops and the pressure increases to atmospheric pressure, where the diaphragm stops advancing the timing.  This can be taken one step further in turbocharged cars where the boost pressure in the inlet manifold acts on a diaphragm to further retard the ignition timing in an attempt to limit maximum cylinder pressures and to cater for the very densely packed (boosted) air/fuel mixture that will burn relatively fast.