[hrcollinsjr]

I did a quick google search on vacuum advance helping fuel economy and came up with this:

Vacuum, Timing and Fuel Economy: in Non-Computer Assisted Engines

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Publication:Lousiana State University Date:1993
GOALS:
To understand the relationship between vacuum and ignition timing and to identify the cause of reduced fuel economy.

OBJECTIVES:
For non-computer assisted and some partially computer assisted vehicles, the student will:

1. Learn to identify the ignition system's vacuum sensing device.
2. Acquire the knowledge in order to test for a faulty vacuum sensing device.
3. Calibrate, for optimum fuel economy, a vacuum sensing device (when possible).

LESSON/INFORMATION:
Ignition timing is the single, most critical adjustment for a gasoline engine. Assuming the engine is in good mechanical condition, there are no vacuum leaks, and the fuel mixture is correct, then the ignition timing can affect the net power yield from the fuel. Improper timing can significantly change a vehicle's mpg. Three types of timing are used for gasoline engines. They are:

1. Static timing;
2. Mechanical (centrifugal) timing; and
3. Pneumatic (vacuum) timing - mechanical.

Each has a specific purpose.

1. Static timing establishes a physical relationship between engine crankshaft position and the distributor. This is fixed and should not change. Basic and initial are terms also associated in the static timing.

2. Since fuel burns at a relatively fixed rate (i.e., time to burn is in milliseconds), then the faster the engine runs the earlier the fuel must be ignited so that the millisecond time frame will remain constant. Thus, mechanical timing advances are used. A device, usually a centrifugally activated weight control balanced by a spring, advances the timing proportionate to engine RPM's. The faster the engine runs, the more timing advances. This is true up to approximately 2,700 to 3,500 RPM's. Mechanical timing is critical to acceleration and power response in a gasoline engine. This timing is added to the static timing.

3. The third timing, pneumatic (vacuum) - mechanical, provides an extra measure of timing over and above static and any mechanical advance. The vacuum advance extends the burn cycle even longer so that all the heat energy may be absorbed by the pistons resulting in more power yield per measure of fuel and more mpg. The sensing device is a spring offset, vacuum activated diaphragm. This diaphragm receives its vacuum signal from the carburetor or intake manifold. In many vehicles this signal is modified by the transmission gear selector position, by coolant temperature sensor(s), by vehicle speed sensor, or by combinations of the above. Early computer assisted vehicles retained the vacuum advance system but employed the computer to modify the signal to the diaphragm, thereby optimizing the system performance.

Total advance is the combination of initial, centrifugal, and vacuum.


Vacuum advances are sensitive devices. By design, vacuum advances are spring offset to give zero advance. Only when a strong enough vacuum signal is present will it advance the ignition timing. The higher the vacuum signal, the greater the timing advance. High vacuum signals are indicative of low engine loads at a closed or nearly closed throttle position. At idle, when the throttle is almost completely closed, vacuum is highest; however, during idle, no vacuum signal is present at the vacuum advance. This is accomplished by tapping the vacuum for the advance system above the throttle plate. Excessive vacuum advance results in part throttle ping and a loss in mpg. Insufficient vacuum advance results in poor part-throttle response and a loss in mpg. Notice both insufficient as well as excessive vacuum advance results in a driveability problem and a reduction in mpg. Many drivers try to correct the part-throttle ping by purchasing higher octane fuels. This may fix the ping problem but mpg will still be reduced (See Fuel Octane Selection Guide.) Some technicians attempt to fix poor part-throttle response by advancing or retarding the static timing. This may correct the part-throttle problem, but basic, wide open, and near wide open throttle positions will have excessively advanced or retarded timing. This could result in an extremely hot combustion chamber temperature and long term, severe engine damage may result. Worn cylinders, cracked piston rings, burned valves, worn valve guides, damaged piston ring lands, or holes in pistons are some of the possible long term engine damages that may occur with excessively advanced static timing. Again, owners may compensate for the pinging noises by purchasing higher octane fuel. The result is the same, lower mpg and wasted energy. (See Fuel Octane Selection Guide.) High combustion chamber deposits, low fuel economy, or poor power output can occur with retarded engine timing.

The correction for each of these problems is a complete, systematic, diagnostic approach using the manufacturer's recommended procedures and specifications. Static timing must first be set properly. Mechanical advance may then be checked and finally the vacuum advance checked. If the distributor fails any test, it should be removed for repair, recalibration, or be replaced. Prior to disconnecting a distributor's vacuum advance system, always check hose routing, system sensors, and soundness of the hoses and connections. Ultrasonic vacuum detectors or propane detectors are good diagnostic devices for locating leaking vacuum lines and connections. Improper hose routing, by- passing system sensors, failed system sensors, and/or leaking hoses account for significant numbers of malfunctioning or poor operating vacuum advance systems.

It is important to remember that even a slightly mistuned engine will deliver poor fuel economy. Optimum fuel economy may only be achieved when timing is optimized.