Vacuum Advance

[30ccpickup]

In theory, with a Model A you advance your spark retard lever one notch per 5 miles per hour. In reality you advance it by sound and feel. Many on the various V8 sites claim with today's gas setting, your vacuum advance on your V8 is not necessary and there is no need to set it at all. Is it all about "the ping'' or is there still an advantage setting it, in order to take speed and hills into account? I am looking for a good technical explanation to understand it. Thanks

 

[38 coupe]

First we need clarification. The flathead V8s used two different methods of adjusting ignition timing with vacuum.
1 - the 1932 through 1948 cars used a spring brake mechanism to reduce the mechanical advance in the distributor. Intake manifold vacuum was used to pull the brake plunger away from the advance mechanism allowing quicker ignition advance. Today these are frequently these are adjusted to put just enough braking force to stabilize the ignition timing at idle and no more.
2 - 1949 through 1953 cars use a vacuum advance diaphragm as the only source for ignition advance. This setup uses a calibrated vacuum source from the carburetor that responds to engine speed and load. You must use a factory type carburetor with the vacuum port for this system to work well. Multiple carburetors, high overlap camshaft, etc. can mess up the calibration of this system. When working properly on a well tuned engine this system works very well.

 

[DavidG]

As stated above, the early flathead V8s have centrifugal mechanical advance. The vacuum is used as a brake on the centrifugal advance on deceleration.

 

[cadillac512]

As stated above, the early flathead V8s have centrifugal mechanical advance. The vacuum is used as a brake on the centrifugal advance on deceleration.

Actually, the vacuum brake is used to limit advance on heavy acceleration. At normal throttle openings high vacuum compresses the spring and holds the plunger away from the advance unit. I'm referring to the pre-'49 V8.



Terry