[Jack21]

Well, here goes my $.02. I'm an Environmental Engineer (aka jack of all trades, master of none). But have been working with this stuff since a "Program" was what was playing on TV.

(There will be a quiz at the end of the class, so take some notes.)

A fuel/air mixture burns when ignited. This burn occurrs over a period of time to exert a push on the top of the piston. Ignited too soon, and the mixture pushes down, while the piston is still moving up. This is not good. Ignited too late, and the piston is already on its way down before it gets its push. This is not good. There is an optimum time based on 1) RPM of the engine, and 2) load on the engine, when this burn must start. Burn start is a function of the spark plug. Timing of the burn start is a function of the distributor. With me so far?

Distributors are dumb devices with springs, weights, and diaphrams that we adjust and set based on engine power output and behavior at different RPM, and load.

There are two devices in a typical distributor that alter the timing (advance, retard, lead) of the burn start sequence. Centrifugal, and vacuum.

The centrifugal advance senses engine RPM. The more RPM, the more advance, up to a pre set limit. This is what the springs and weights do. As RPM increases, the weights move out, and move the rotor tip forward relative to its direction of rotation. This times burn start earlier or advanced.

The vacuum advance senses engine load. The little silver cannister on the side of the distributor with the hose on it does this. It pulls vacuum either from the intake manifold directly (below the throttle blades), or from the carburetor (just above the throttle blades). It has a rubber diaphram inside that moves back and forth with engine vacuum, and is connected to the distributor with a lever. It rotates a plate inside the distributor which changes (advances/retards) the timing of the burn start.

An engine (an air pump), draws air (& fuel) in through an inlet restriction (carburetor). Between the inlet restriction, and piston, (intake manifold) there exists a vacuum. An engine with more load is using more throttle opening to overcome the load, and subsequently will produce less vacuum. An engine under light load, such as while cruising at a steady speed, will use less throttle opening, and will produce more vacuum.

(As a point of confusion, diesels, have no such inlet air restriction, have no vacuum, and operate at wide open throttle (air) all the time.)

The time it takes for a fuel/air mixture to burn is a function of the ratio of fuel and air within limits. More fuel in the air will burn faster, such as under high load conditions. Less fuel in the air will burn slower, such as under low load conditions. The vacuum advance senses engine load and times the burn start earlier or later to coincide with the engine load (vacuum) demands.

Both of these systems, vacuum and centrifugal, work to optimize the burn start timing for optimum power output, and optimum fuel economy. (And have given hot rodders and performance enthusiasts trying to achieve this optimum setting grey hair for years).