First, you need to understand that lean mixtures, like at idle and light throttle pressure or low speeds, burn slower than rich mixtures. These lean low load mixtures are a result of lower volumetric efficiency and the scavenge ability of the exhaust port to clear the spent exhaust gases from the combustion chamber this dilutes the oxygen levels and slows down the burn rate. This condition requires the flame front to be ignited earlier in the compression cycle (more advance), allowing more burn time so that peak cylinder pressure is reached at about 12-15* after TDC.
If your engine doesn’t have enough initial timing in it your Exhaust Gas Temperatures (EGT) will rise dramatically due to the excessive fuel that remains unburned in the combustion chamber and tries to complete the process in the exhaust runner and into the headers.
So we’ve determined that a lean mixture burns slower so on the other side of the coin a rich mixture will burn faster. So if you have a rich mixture you need to fire the plug later or retard the timing slightly. Remember we’re trying to achieve maximum cylinder pressure at 12-15* after top dead center, before that point we’re into detonation after that point results in drastic power loss.
The second timing event is what we refer to as “Mechanical Advance”.
This part of the advance curve is controlled by Engine RPM only and has nothing to do with load on the engine or cylinder pressure, it’s simply RPM activated through the weights and their retention springs. Sometimes this is referred to as “Centrifugal Advance”. When we add the static or initial timing to the amount of Mechanical timing we get what we refer to as Total Timing.
So if we set the initial at 10* and allow the weights to pull in another 25* of timing we would have a total of 35* of timing at the RPM where the weight travel is maximized or they hit what we refer to as the Advance Limiters.
The third and final timing event and probably the most misunderstood is the “Vacuum Timing”. Vacuum timing has nothing to do with total under load timing or performance tuning. Vacuum timing is controlled by “Manifold Vacuum”, as soon as you accelerate the vacuum in the manifold drops and the Vacuum Timing is released and the timing events are then controlled by the Mechanical and Initial timing settings.
So previously we determined that a lean idle mixture requires more timing to get a complete and efficient burn in the combustion chamber than the under acceleration or full power richer mixtures as delivered by the main circuits.
So to achieve this, the vacuum unit is connected to a “MANIFOLD or CONSTANT” vacuum port and can be set to add anywhere from 5 to 20* more timing in the motor when high vacuum readings or low load conditions are present. So now to calculate the total timing at idle or low load we add together the static or initial, the Vacuum timing and whatever mechanical timing we have based on RPM. Going back to the previous example you would add the 10* initial setting and say 15* of Vacuum timing for a total of 25* at idle or low load, low RPM cruise. Because the vacuum timing is progressive to and directly related to manifold vacuum as the manifold vacuum increases (Light Load) or decreases (As you accelerate) the timing in the motor changes accordingly to keep the efficiency at optimum. The mechanical timing will come and go as engine RPM increases or decreases.
So let’s look at an actual car rolling down the road at 65MPH on a nice long flat stretch.
We need to realize that a car in cruise mode at 65 MPH only needs to make about 50-75 HP (Depending on weight and wind resistance) to maintain a steady cruise speed.
We start with the 10* initial add to that the mechanical of 25* at say 3000 RPM and Vacuum timing of another 15* totaled up we see that we have 35* of total timing in this engine example and 15 in the can. Pretty standard for a near stock application, but probably not the best for a Muscle Car trying to burn unleaded 93-octane fuel.