Current Topic:

The Vacuum Advance

This applies only to non-computerized street vehicles.

The function of that little do-dad attached to the side of your distributor is the subject of much confusion and mis-information. The vacuum advance assembly is a simple device containing a diaphram, a return spring and a link to your distributor's breaker plate or magnetic pick-up and a connection to a vacuum source.

As vacuum is supplied to the diaphram it causes the diaphram to pull on the distributor link , rotating the pick-up/breaker plate in the opposite direction of shaft rotation, advancing the ignition timing. As the vacuum subsides, the return spring pushes the plate back into it's original position.

The source of the vacuum used to actuate the device is what causes the confusion over it's operation.

You have 2 sources of vacuum available. Manifold vacuum and timed (also called 'ported' or 'venturi') vacuum.

Manifold vacuum is just as the name implies. Connections to manifold vacuum are either directly to a fitting in the intake manifold itself or to a hose connection on the carburetor (or throttle body) that has its source
BELOW the throttle plate. Manifold vacuum is strongest under engine deceleration or at idle. When you open the throttle, the amount of vacuum dereases. At wide open throttle(WOT) , under load (that is when actually driving on the road) the amount of vacuum in the manifold is essentially zero.

Timed or ported vacuum is just the opposite. Its source is the part of the carburetor throat called the venturi. The venturi area is where the opening in the passage is necked down to a smaller diameter than the area above and below it. It is necessary to create a low pressure or vacuum source to suck the fuel from the float bowls in the carb through the jets and into the boosters where it is dissapated into the air flow, creating the volatile suspended air/fuel mixture to be drawn into the cylinders. The sources for connecting this type of vacuum is always on the carburetor itself, and is always located above the throttle plates. The higher the engine revs, the higher the vacuum.

Timed vacuum and manifold vacuum are exactly opposite. If one is high the other is low and vice-versa.

The obvious question then is 'which type of vacuum should I be using on my hot rod?'

The answer is :'It depends!'

If you are running a stock or close to stock engine with low compression and mild camshaft, the timed vacuum source is what you should be using. When accelerating, the additional timing that occurs as the engine RPMs increase will create some additional power and the throttle response will be markedly improved. Low compression engines should tolerate the additional timing with little or no pinging and the fuel economy will also be improved slightly.

If on the other hand you have a high compression (over 9 to 1) beast with a lot of camshaft you'll be much better off using manifold vacuum. The additional timing will give a big cammed engine a much more stable idle yet the additional timing backs off once you get into the throttle, minimizing the chances of detonation or ping occuring.

Crane Cams offers an line of replacement vacuum advance units for most V8's that offer the additional feature of adjustabilty. The spring rate can be changed easily to tailor the unit to your specific combination. They also come with a set of springs for the mechanical advance and a limiter cam to set the amount of advance.

We carry them in stock for less than $30.00

If you'd like to comment on this article or if you want to suggest a topic for a future discussion, please send us an e-mail by clicking here:

E-Mail Shaw's Auto Parts