Ignition Timing

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.  To get a more detailed view on why this is critical, you can read this ignition timing background information.

I will talk about four different things; 

- static timing

- mechanical advance

- vacuum advance

- total timing

The Gemini is usually set with a base timing figure of 6deg BTDC.  This base timing figure is called the static timing.  This figure was chosen by the Isuzu engineers for low emissions, however the engine will perform better if this value is increased (advanced).  An old theory that works pretty well is that when idling and you advance the ignition timing, if the engine speed increases, then the engine likes the extra timing and is more efficient at that setting. 

If there was no such thing as ignition advance, then the timing would be static all the time.  However the Gemini distributor houses a mechanical advance mechanism that uses centrifugal weights and springs to move the ignition points so that it advances the timing as the revs increase.  This is to make the spark happen sooner as the revs increase, because there is less time for the mixture to burn, so it must happen sooner to get peak cylinder pressures at the right time.  By around 3500rpm, the timing has increased from 6deg BTDC to about 30deg BTDC.  This means there is 24deg of mechanical advance being applied.  It looks a bit like the following graph, but I am going on my sketchy memory, so the RPM points may not be accurate.

As load is increased, the ignition timing needs to be retarded and vice versa.  The Gemini uses a vacuum advance 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 by spinning the breaker points base plate.  As the load increases, the vacuum drops and the pressure increases to atmospheric pressure, where the diaphragm stops advancing the timing.  

The vacuum advance diaphragm is referenced from a pressure source above the butterflies in the carburetor.  This is called ported manifold vacuum and it means the only time a vacuum signal is present is when the throttle is lightly to moderately applied.  This was done for emission reasons again, and a smoother idle and better throttle response can be obtained by connecting the vacuum advance directly to a manifold vacuum source.  This means that at idle, you will now get more ignition advance, and you will probably notice that the revs increase, which means the engine likes it!  So you will have to reset your idle speed with the carburetor adjustments.

Total timing is the amount of ignition timing advance that the engine needs to make maximum torque.  This is a sum of the static timing and the mechanical advance.  Keep in mind that when aiming for maximum torque, your throttle will be at Wide Open Throttle (WOT) and there will be no vacuum present (hopefully), so the vacuum advance does not play a part.

A stock Gemini G161 will be running around 30deg total advance, consisting of 6deg static and 24deg mechanical advance.  This may be good for a stock Gemini engine, but if you alter the compression ratio, increase the amount of air flowing into and out of the engine (bigger cam, extractors, etc), or turbo the engine for example, the amount of total timing required will change.

Usually, anything that increases cylinder pressures will mean less total timing is required, and vice versa.  So a compression ratio increase would require less timing overall (total timing), but the engine might like more static timing, and if you set static timing higher, you will get more total timing, right?  That is right, unless you modify the mechanical advance unit.

If you get this far, you have disassembled your distributor and you will find the mechanical advance unit.  You will see the unit can advance until the pin hits the end of the slot.  The size or length of the slot determines the amount of mechanical advance you will get.  The size of the weights and springs determine when the advance will occur, ie. at what RPM.  This mechanism spins the base plate that the points attach to, since it is when the points open that actually determines the ignition timing.

By making the slots smaller, you will reduce the amount of mechanical advance the distributor supplies.  For example, if your target is to achieve 28deg of total timing, and you want to set your static timing to 16deg, then you need to make the slot smaller so that 12deg of advance is delivered (16deg static + 12deg mechanical = 28deg total).

If you want the distributor to advance sooner or later than it currently does, then you need to change the springs, which is something I have not played with before, and I suggest you contact an ignition specialist as they will have to equipment to select the springs correctly for your application, and measure how your distributor is behaving.

The ignition curve would now something like the following.