Thanks for your response! I did pack a fair amount in my post. To start with, it’s necessary to understand how the distributor works on a four cylinder four stroke petrol car. It is an electromechanical device which, together with the coil, fires the spark plugs in sequence when the relevant piston is close to “top dead centre” (TDC) during the compression stroke. At a low speed on the engine, this happens around 10 degrees before TDC as the fuel/air mixture takes time to burn. As the engine goes faster the “advance” before TDC can be around 35 degrees or more. This is achieved by weights in the distributor which fly outwards with centrifugal force and twist the shaft in the distributor. There is also some “vacuum advance” from the inlet manifold but it’s not essential except for fuel economy at low engine speeds. The way it’s done these days is by using a trigger wheel on the crank pulley - in my case with a tooth every 10 degrees except one of the 36 is missing, which allows the processor to know exactly where the crank is. The reference to “wasted spark” is because I’m using a twin coil pack which fires two cylinder spark plugs at once - a cheaper solution than having a coil for each cylinder. The “wasted spark” fires on one cylinder at the right time and another cylinder which is near the top of its exhaust stroke, which has no effect, i.e. it’s wasted. If you are interested, I recommend you google some of these words to find out more 