Vacuum advance adjustments

Don C · Jan 8, 2021

The effects of adjusting a vacuum advance have been discussed many times in the forum, but without much documentation. After a discussion in this thread I decided to see just what happens.

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I connected a vacuum pump to my distributor with a Crane adjustable advance, to see what the effects of vacuum versus adjustment are.

I started with no adjustment, so maximum vacuum advance at lowest vacuum would be achieved. I took looked at the amount of advance at 5 inch increments required to achieve the maximum advance. I then did the same thing with the adjustable advance retarded (turned counterclockwise) by 2.5, 5, 7.5, and 10 turns.

Because the results were buried in another thread I decided to include it in a new easier to find thread. The results are in the attached

View attachment 40048

SteveO_71 · Jan 8, 2021

Don, nice grid of vacuum advance vs retard adjustments and breaker plate movement.

So, I'm assuming your vacuum advance module is the allen wrench adjustment type? Are you starting at one end of travel and then turning the number of recorded retard turns from there (no retard, 2.5, 5, 7 etc)?

Don C · Jan 8, 2021

Yes, it adjusts with an Allen wrench.

Full advance with minimum vacuum is achieved with the adjustment screw in the full clockwise position. Retarding is achieved by turning the adjustment screw counterclockwise. Each vacuum increment test started with the adjustment screw turned to the full clockwise position.

Kilgon · Jan 8, 2021

Good info. Will help take some of the guess work out trying to dial your timing in. At 15 degrees add on max for the average module you should be able to get in the ball park pretty close with your info. Based on your chart I would figure about 8 degrees add on when pulling 25 inches with about 7 to 8 turns.

SteveO_71 · Jan 9, 2021

Kilgon, realize that some vacuum advance modules do not adjust with an allen screw. The advance for my 71 doesnt and I thought the factory made them non adjustable starting with the 70's.

The aftermarket ones typically will.

Don C · Jan 9, 2021

The vacuum advance I have was made by Crane, came with a kit that includes springs, other brands are likely different.

Kilgon · Jan 9, 2021

SteveO_71 said:
Kilgon, realize that some vacuum advance modules do not adjust with an allen screw. The advance for my 71 doesnt and I thought the factory made them non adjustable starting with the 70's.

The aftermarket ones typically will.

Thanks for pointing that out. I am aware of that. I should have stated that most adjustable offer up to15 degrees. Most provide approximately 1 to 1.5 degrees per turn. The nice thing with the info Don provided is that you have a good idea of how much advance you have at different vacuums based on how many turns you gave the vacuum module.

Carolina_Mountain_Mustangs · Jan 12, 2021

When I picked up my 1973 Q code 4 speed Mach 1 I left the dealership drove about 1 1/2 miles to the interstate and opened it up. It would not get to the 120 mark so I drive right back to the dealer. I went in and complained, lol. The service manager in the shop knew I was tough on my cars he saw me at the parts counter a lot. He suggested to the manager that they put in a kit Ford had that eliminated the vacuum advance. It was all mechanical. They set me a date to come in after the parts came in and they made the change. Had to have new shaft with different cam and breaker plate. You cut the arm off the vacuum and put it back on the distributor since it clocked the cap. They used a Sunnen distributor machine to determine which springs to put in it came with like 3 or 4 different sets. It gave 5 mph more on the top end. It is still in the car but I have no machine to pull distributor and set with. I doubt if anyone has them anymore.

tony-muscle · Jan 12, 2021

Is the maximum movement 15 "crank" degrees of advance?

Kilgon · Jan 12, 2021

tony-muscle said:
Is the maximum movement 15 "crank" degrees of advance?

Yes - crank degrees. If you are running 36 initial timing figure your max to be around 50 degrees give or take a few depending on the adjustable module make. Most are around 14 to 16 crank degrees total.

Vacuum advance adjustments

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Vacuum advance adjustments

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