As your engine rpm and load varies, so does the need for your timing. At idle, a spark occurs on a piston’s compression stroke a few degrees before it reaches TDC. At this point, the fuel mixture is ignited beginning the combustion process. The act of combustion remains fairly constant but because the piston is travelling at a much higher speed, the initiation of the combustion process needs to occur sooner. Therefore the spark must occur more earlier in the compression stroke to generate the best combustion and power results. To meet these demands, distributors are equipped with an advance mechanism that operates through centrifugal force.
This mechanical advance assembly is made up of two weights that are pushed out by the spinning force of the distributor. Springs are attached to these weights to control the rate at which they are forced out by the spinning motion. The weights are mounted on an advance plate which is attached to the trigger assembly of the distributor so it moves forward resulting in the ignition being triggered earlier (advanced).
By swapping different tension springs you can alter how quickly the timing advances. A spring with less tension lets the weight spin out easier to advance the timing quickly. Many companies offer advance kits that consists of different weights and springs to fit a variety of distributors so you can dial-in an advance curve to match your engine’s needs.
The mechanical advance is an important element is tuning your engine. A slow advance can hinder performance while too much advance can cause pre-ignition resulting a loss of power or even engine damage. This is why it is important to check the timing at idle and at higher rpm so you know what the total advance is at.
The advance assembly is mounted on top of distributor shaft on all of MSD’s distributors which makes changes even easier. To stop the amount of advance, there is a stop bushing under the advance plate that can be replaced. This way you have better control, and an easier time adjusting, over the total timing.
There are three timing terms you’ll hear during bench racing sessions; Initial timing, mechanical (or centrifugal) advance, and total timing.
Initial timing refers to where the timing is set at an idle. This is also where you position the distributor when setting the timing. Generally, this will be 6° or 12° or something lower, depending on the application.
Mechanical timing refers to the amount of advance that the weights, or advance assembly add to the initial. For instance, a distributor may be set up to supply 21° of centrifugal advance.
Total timing is the term given to the highest amount of advance. This is when the initial timing is added to the mechanical timing. For instance, if an engine has 10° of initial combined with 21° of mechanical, the total timing is 31° BTDC. Note that Total timing never includes vacuum advance.
Vacuum advance is primarily used in an effort to improve economy, therefore you won’t hear much about vacuum advance when discussing performance and racing. There is little to no vacuum during wide open throttle which means no vacuum advance any way!
A vacuum advance canister provides a way to advance the ignition timing during moderate and part throttle conditions. This is when the load on the engine is less and vacuum is higher. There is a diaphragm inside the canister which is connected to a linkage connected to the pickup plate in the distributor. When vacuum is applied, the plate is pulled which advances when the trigger signal is created. When the engine accelerates, vacuum drops so the advance returns to the original position.
When connecting the vacuum advance, most applications connect to a port above the throttle plates. This is called ported vacuum compared to manifold vacuum which comes straight from the intake manifold. The difference here is that manifold vacuum is there constantly while the ported source provides vacuum only when the throttle blades are open. The amount of advance that occurs varies by application but generally ranges from 10°-15°. Some companies even offer an adjustable vacuum advance canister. This lets you set the exact amount of vacuum advance that your engine requires.