Distributor Revelations

By using a caliper and a mity-vac it is possible to measure the vacuum advance curves. The range of mechanical advance can be measured with a dial indicator or a digital caliper. The math of converting measurements to degrees is simplified to multiplication and division when small angles are considered. I will not go into all that, I do show at bottom of calculation sheet

The vacuum advance in a OEM mopar moves the point plate, or the pickup plate in an electronic VR distributor. The plate pivots on a pin on the far side of the advance can connection point. The sweeping action moves the points in relation to the distributor cam, resulting in timing advance. I think the main reason of the sweep motion instead of adding a center bearing was economics and simplicity. The Mallory distributor has a center bearing, however they are fairly expensive.

A fine tipped Sharpie can be used to mark a reference point on the distributor body, to line up with a point on the pickup module on the advance plate. I show and example in the distributor picture. The mighty-vac is used to slowly apply vacuum until the plate just starts to move. Record the vacuum in inches of Hg (Mercury), it is the minimum vacuum of the plot. Below this vacuum the advance will not have an effect on timing. Then increase vacuum slowly just to the point the plate stops, record the vacuum value. The measurement points may be more accurately determined by repeating the tests, and using a dial indicator to measure travel. Mark the full travel point, and measure the length to the starting mark.

To convert to degrees is simple. The circumference (measurement around) of the distributor is about 10.6" for both the Mopar and Mallory with diameters of 3-3/8". For those with math skills Circumference = pi x diameter. The distance around a distributor is 720 degrees (remember distributor turns 1/2 crank speed). So if the distance between the marks is 0.2", vacuum advance range is (720 x 0.2) divided by 10.6 or 13.6 crank degrees.

On an adjustable advance, the set screw can be rotated to change the both the starting vacuum for movement, and the total advance. The setting changes both, so you do not have a choice. Counter clockwise results both advance limit and more vacuum to initiate travel.

Similar method may be used to measure mechanical advance range by holding shaft, and measuring rotor angular movement. A pointer can be super-glued to an old rotor to help with measurements.