There seems to be a lot of discussion on distributor timing on this forum so I thought I would pass along this information. Hopefully it will answer some shortfall of knowledge on this magical instrument.


The centrifugal advance mechanism in the distributor advances spark timing based solely on engine rpm. In most distributors, this mechanism provides up to 20-25 (crankshaft) degrees of spark advance so initial timing can be set accurately without any influence from the centrifugal advance mechanism.

The key point to remember here is that the centrifugal advance mechanism advances and retards spark timing in response only to engine rpm, and nothing else. Its function is to advance spark timing as engine rpm increases; as upward piston speed increases with rpm, effectively shortening the time for the compression stroke, the spark has to fire sooner, as the air/fuel mixture still takes the same amount of time to burn as it does at lower rpm. In effect, the centrifugal advance mechanism handles only the basic physics of lighting the fire sooner at higher rpm so peak cylinder pressure is still reached at the same point just after TDC.

Vacuum Advance: The vacuum advance system consists of a vacuum diaphragm mounted on the distributor body; the diaphragm is spring-loaded in the zero-advance position, and has a rod which connects to a hole in the breaker plate, which is the movable plate the points are mounted on. When vacuum is applied to the diaphragm, it pulls on the rod, which in turn pulls on the breaker plate, rotating it with respect to the 8-sided cam on the distributor shaft which opens and closes the points.

Donít believe anyone who tells you that ported vacuum is a good thing for performance and drive ability Ė itís not. Anyone with a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they donít understand what vacuum advance is, how it works, and what itís for. There are lots of long-time experienced mechanics who donít understand the principles and operation of vacuum advance either, so theyíre not alone.

A typical vacuum advance unit, when fully deployed, will add about 15 (crankshaft) degrees of spark advance over and above what the distributorís centrifugal advance system is providing at the moment, which depends on engine rpm; they are two independent systems, but they work together to provide the correct amount of spark advance.

Controlling Vacuum Advance: Letís look at how the vacuum advance system is controlled.

Referring back again to burn rates, remember that lean mixtures burn slower, and rich mixtures burn faster. Engine load conditions (idle, steady cruise, acceleration) result in how lean or rich the air/fuel mixture is (the carburetor handles this), and the best indicator of engine load is intake manifold vacuum. At idle and steady cruise, engine load is low, and intake manifold vacuum is high due to the nearly-closed carburetor throttle plates; under acceleration, the throttle plates open wider, and intake manifold vacuum drops; it is essentially zero at wide-open throttle. As a result, intake manifold vacuum is a ďfreeĒ indicator of engine load, which correlates nicely with fuel mixture being supplied Ė lean mixture at high vacuum, and rich mixture at low vacuum. At idle, the engine needs additional spark advance in order to fire the lean (and exhaust-diluted) idle fuel/air mixture earlier in the cycle in order to develop maximum cylinder pressure at the proper point after TDC for efficiency, so the vacuum advance unit is activated by the high manifold vacuum, and adds another 15 degrees of spark advance on top of the fixed initial timing setting.

For example, if your initial timing is set at 10 degrees, at idle itís actually 25 degrees with the vacuum advance connected (a properly-calibrated centrifugal advance mechanism will not have started to move yet at idle rpm). The same thing occurs under steady highway cruise conditions; the mixture is lean, takes longer to burn, the load on the engine is low (it only takes about 40 horsepower to cruise at 50mph) and the manifold vacuum is high, so the vacuum advance unit is again deployed, and adds 15 degrees of spark advance over and above whatever the distributor centrifugal advance mechanism is providing at that engine rpm. If you had a timing light connected so you could see it as you cruise
down the highway, youíd see about 45-50 degrees of spark advance; your fixed initial advance of 10 degrees, 20-25 degrees provided by the centrifugal advance mechanism, and the 15 degrees added by the vacuum advance unit.

The Advance Combination: Now we have two different advance systems working independently, but complementing each other, to manage spark timing Ė centrifugal, based on engine rpm, and vacuum, based on engine load and operating conditions. The centrifugal advance system is purely mechanical and is only rpm-sensitive; nothing changes it except engine rpm. Vacuum advance, on the other hand, responds instantly to engine load and rapidly-changing operating conditions, providing the correct amount of spark advance at any point in time, to deal with both lean and rich mixture conditions.