Everyone knows the classic four-stroke engine cycle: intake, compression, combustion, exhaust. While all four play a part in performance, it is the combustion step that relies on the critical sequence of idealized ignition timing. Late model EFI engines use complex mapping to implement proper timing; before the days of electronic spark control, distributors performed this task.
We will look at three critical ignition timing areas: initial timing, mechanical advance, and vacuum advance. Together, they form the overall spark curve the engine uses over its entire RPM and load range for optimal performance. The important part to remember is that all three work together on a street engine not only to maximize power but also to enhance drivability and mileage.
It is also important to note that all distributors spin at half of engine speed. In this discussion all the specifications will be given in crankshaft degrees. Some shops that use a distributor machine may report an ignition curve in distributor degrees, which is half of crankshaft degrees. So, if you see a spec on a distributor, it is worth asking if the specs are in crankshaft degrees.
Let’s set some definitions so everyone understands the terms we will be using:
Initial Timing
Sometimes called base timing refers to the position of the distributor that creates the crankshaft position where the spark plugs fires at idle. This and all other timing specs refer to the position of the piston as it approaches the top of its stroke, called Top Dead Center (TDC). Engines tend to idle best with a small amount of ignition advance to begin the combustion process. Advance means the spark plug fires before the piston arrives at TDC, referred to as the number of degrees Before TDC (BTDC).
A popular misconception is that when the spark jumps the gap on the plug, there is an instant explosion inside the combustion space. This is not entirely accurate. When the spark occurs, the combustion process requires a short amount of time to create the flame front that builds pressure. Conventional wisdom holds that lighting the fire a few degrees BTDC even at idle allows the combustion process adequate time to create maximum cylinder pressure just as the piston and rod assembly apply maximum leverage on the crankshaft. This occurs roughly at around 15 to 17 degrees After TDC (ATDC). Engines run best with initial timing that fires the spark plug somewhere between 4 and 12-14 degrees BTDC. At peak power at high RPM, total timing numbers generally are 32 to 38 degrees BTDC. This additional timing is especially important because as engine speed increases, there is less time for combustion to occur.
Mechanical, Or Centrifugal Advance
A distributor is controlled by a simple, weighted device like the speed governor weights and springs on an old steam engine for ignition timing advance. The weights pivot outward as shaft speed increases. The rate of change for these weights is controlled by the springs. The total amount of advance is generally controlled by a pin that travels in a slot in the distributor advance mechanism. The longer the slot, the more advance is created.
This centrifugal mechanism is on the same shaft as the trigger wheel that, on a V8, will have 8 poles. As the advance weights move outward with RPM, the shaft moves the trigger wheel forward, advancing the spark. Mechanical advance systems generally add between 20 and 30 degrees of timing. Adding the mechanical and initial values together generates the total timing. As an example, 10 degrees of initial and 24 degrees of mechanical advance creates 34 degrees BTDC of total timing.
Vacuum Advance
Vacuum advance is a separate system that uses a small diaphragm assembly attached to the side of the distributor. Not all distributors are equipped with vacuum advance, but for a street engine it can be very useful. Engines operating at part throttle require more ignition timing to light the fire in the chamber because the cylinder contains only a portion of the total charge that is present at wide-open throttle (WOT). With a reduced density charge in the cylinder, more timing is required for the engine to run efficiently.
Intake manifold vacuum is a very easy way to judge engine load. We need to know about vacuum because this is the force that will ‘pull’ on the diaphragm on the vacuum advance canister, which in turn moves the baseplate tied to the magnetic pickup in the distributor to advance the timing. Keep in mind that as engine load increases, vacuum is reduced, and vacuum advance will also be reduced.
Part throttle restricts the volume of air into the engine, which creates a vacuum in the intake manifold at a pressure that is much less than atmospheric. Atmospheric pressure can be expressed as pounds per square inch (PSI), millibars, or inches of mercury (inHg). The most familiar atmospheric pressure standard is 14.7 PSI at sea level. This same pressure can also be expressed as 29.92 inHg.
Most automotive engine vacuum gauges are scaled to read inHg. A common automotive vacuum gauge reads 0 (no pressure) when exposed to atmospheric pressure. When the gauge is measuring manifold vacuum on a running engine, it will be reading negative pressure. It is just not expressed as -20 inHg.
Engine manifold vacuum can be as high as 16-18 inHg at idle on a stock engine. As the throttle opens and load on the engine increases, this reading will move closer to 0 inHg on the gauge. These readings are referred to as PSIG or PSI gauge pressure readings. Yes, it is a little confusing since we referenced 14.7 PSI as atmospheric. It takes a little effort to get comfortable with science, but as you will see, it is necessary.
Vacuum advance is completely independent of mechanical advance. We like to describe vacuum advance as load-based timing since timing is added when there is a light load on the engine. Conversely, timing is reduced when load is applied. Mechanical advance is strictly determined by engine RPM and nothing else. When load is applied and manifold vacuum is lower, less timing is added because the cylinders are more densely packed with air and fuel, and require less ignition advance to create maximum power.
At part throttle initial, vacuum, and mechanical advance are all added together. Here is an example: Let’s start with 10 degrees of initial timing at idle. We add 18 degrees of mechanical advance at 2,400 RPM for a total of 28 degrees of advance. Now let’s add 12 degrees of vacuum advance with engine vacuum at 16 inHg. This would create a total ignition advance of 40 degrees of timing at that specific point.
If we add load to the engine by stepping on the throttle in high gear, maintained the same 2,400 RPM (climbing a hill, for example), and engine vacuum dropped to 10 inHg, vacuum advance would be just 4 degrees. Total advance at 2,400 RPM would then be 32 degrees.
Checking these numbers on an engine in the car is relatively easy with a dial-back timing light. You may have read that race engines do not use mechanical or vacuum advance since they always run at WOT (wide open throttle). This is true in most cases because these engines operate at extremely high engine speeds where fixed ignition timing is all that is necessary. One problem that crops up is starting an engine with 34 degrees of initial timing is very difficult. That is why many race ignition boxes have a start retard feature to reduce timing to a level the starter can handle.
A very quick timing curve is also not always necessary for a street engine. You may have heard about advance curves that have total advance all in by 2,000 RPM. It is possible, but the weak springs required to do this start the mechanical advance at or below idle speed. Stabilizing the idle speed low enough to set initial timing becomes difficult because centrifugal advance has already started. It is a much better plan to start the mechanical advance curve at around 1,300 to 1,400 RPM and have it fully advanced by 3,000 RPM.
It is also important to mention that in most applications, the vacuum advance canister is connected to what is called ported manifold vacuum. That means there is no vacuum sourced to the canister at curb idle. With ported advance, vacuum is applied once the throttle is opened past curb idle. Some tuners prefer to add vacuum advance at curb idle on engines with big camshafts to improve idle quality. If you choose to do this, be sure to remove the vacuum advance line from the vacuum source before checking initial timing. If the initial timing numbers appear excessive (20 degrees or more), it could be that manifold vacuum is being applied to the advance canister.
Most engines use a timing tab, located on the front timing cover, which references advance. In this case, the tab reads in degrees BTDC above the zero mark with the mark set here at TDC.
Centrifugal or mechanical advance works based on centrifugal force. As the distributor shaft spins faster the weights pivot outward, which moves the eccentric-shaped shaft in the center. As the weights move the shaft, this advances the position of the spinning 8-pole piece, advancing the ignition timing.
This vacuum gauge reads both manifold vacuum and positive pressure. The scale we are using for manifold vacuum is the inside row expressed in inches of mercury (inHg).
This graph is representative of a typical mechanical advance curve on an engine. Advance begins with timing added after 1,500 RPM and reaches full advance at 3,500 RPM. This curve starts at 10 degrees because this is the amount of initial timing. This curve can be quickened slightly but is a good place to start. Total mechanical advance is determined by the length of the slot in the distributor advance plate while the rate (steepness of the curve) is determined by the weights and springs
Mechanical vs Vacuum Advance
Mechanical advance uses the weights and springs in the distributor to rotate and alter the timing sensor shaft’s position relative to the engine’s position. Vacuum advance timing uses a manifold vacuum that reacts to the rotation of a position sensor mounting plate located in the distributor. As the throttle opens during higher engine loads, the vacuum diminishes, enabling the timing advance to move back to the base, or original advance.
Speed
Mechanical advance activates only during high engine rpm, which causes the distributor to rotate quickly enough to activate the advance. Vacuum timing senses changes in the engine’s load, or the demand placed on the engine for power, and offers greater timing advance during low loads.
Advantages
Vacuum advance offers greater better fuel economy and engine performance due to raised timing during periods of low speed such as gear shifting or stopping; it extends the combustion mixture burn cycle. Mechanical advance timing offers better engine performance in high-speed applications such as race car driving.
Finally
Advancing the vehicle ignition timing ensures the spark plug ignites the combustion mixture at the best time for optimum engine efficiency and performance. Mechanical advance and vacuum advance both advance ignition timing but vary in function and results.
There are many more nuances and little tricks of the trade when working with mechanical and vacuum advance distributors, but hopefully we have shed a little light on this three-tiered timing arrangement. Tuning will make you a little bit like an orchestra conductor, getting all the instruments to play the same tune at the same time. When that happens, your engine will make some beautiful music!
