vacuum brake authority

01A79

01A79

Junior Member
According to the specs the mechanical advance is completed at 1900 rpm and 16 degrees advance for the 78 (37-40) - 1200 rpm with 22 degrees for the 11A (41-48) (Ref:Canadian service manual, page 45.) I notice the difference in the ramps or slots that the bearings of the weights ride in to effect this. Where does the vacuum brake come in? During cruise at say 2500 rpm do the weights relax and upon acceleration (lower vacuum) the brake comes in and prevents advance? I understand why by reading JWL's great book "Flathead facts" but do not understand the mechanics.
 
The front plate of the advance mechanism is the brake disc...sort of free floating. When the pad on the brake piston drags against it, it is dragged back to restrain the rate of advance.
Engine running at low load/low throttle opening, like cruise on a level road, produce high vacuum, and engines at high load/full throttle get down around zero vac.
High load requires less advance than light load.
High vac LIFTS the brake and lowers its drag.
Low vac allows the spring to press brake more firmly into the disc.
The effect is of course a continuum, not on-off, and spring pressure is adjusted by the bolt above it...OUT lowers the pressure.
The Service Bulletins give (in several different and complicated formats) give curves for high and low vac, though of course these would vary by spring adjustment.
Tuning is by releasing spring pressure till pinging comes on...modern gas is much higher octane than 1939 gas and often is happy with screw nearly all the way out.
I hope I had enough coffee to get all that right...
 
Yes. More detail is this: Ignition is advanced by the centrifugal cams and retarded by the vacuum brake. The brake pad that rubs on the centrifugal disk is actually pushed on there by a spring, not by vacuum. Vacuum lifts the pad off of the disk to allow greater advance by the centrifugal cams. When the adjusting screw at the top of the distributor is loosened, such is loosening the pressure of the spring on the brake pad such that it takes less vacuum to lift the pad off of the disk and allow advance. Tightening the screw in puts greater pressure on the spring pushing on the pad and requires greater vacuum to lift the pad off of the disk, delaying the advance.

Hope this helps.
 
Thank you, I understand the why. What I don't understand is at say, 1900 rpm ,the centrifugal force has the weights all the way out on the cam, fully advanced. Now the vac is decreased with more load. What effect does the brake have when it is already fully advanced? Once at cruise do the weights relax?
 
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Let's say at 1900...floor it, vac drops to nothing, spring presses brake into disc and vac retards.
Cruising on the level at 1900, throttle opening low, vac high, piston is retracted against spring, brake releases and full centrifugal advance is what you have.
Actual amount of retarding would depend on spring adjustment.
On other old distributors in general, say 1950's-60's, the centrigugal advance gives the full throttle curve and the vac ADDS advance at part throttle
On the early Ford, which is based on Mallory patents, the centrifugal advance is the longer part throttle advance and is retardes when vac drops indicating full throttle~!
 
And vac brake is not meant to pull advance back to nothing, unless really screwed down tight perhaps...it drags against the efforts of the weights to advance and so slows the rate of advance in accordance with the stiffness of the spring adjustment.
 
So, Bruce, the Vac brake is only effective below 1900 because full centrifugal advance is already in. At rmps higher that 1900 for 78 and 1200 for the 11A the brake does nothing.
correct?
 
No...the centrifugal is always trying to reach (or hold, when it bottoms the slots) advanced position, contact between brake puck and disc tries to move the slots back and retard things.
With brake loose, weights will reach full advance at the specified RPM. Weights will reach full advance at higher RPM (or maybe never) as brake tightens.
Grab a set of innards and twist watching movement, then twist that forward plate to resist.
 
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