Distributor with spark advance

[enfieldbullet]

i'm considering making a vacuum advance one. like in the VW 1300

just wondering, anyone ever made one? any takes on it? tips, whatever?

 

[gbritnell]

I'm not familiar with a VW 1300 but I am familiar with vacuum advance mechanisms. Distributors with vacuum advance mechanisms have 3 stages to the ignition timing, initial, vacuum and centrifugal. If you made a vacuum advance mechanism it would work only when the engine was started. As the throttle is opened and the vacuum drops the centrifugal part takes over. Without this part you would actually lose timing back to the initial setting as the throttle was opened.
gbritnell

 

[enfieldbullet]

VW 1300 as in Volkswagen 1300 boxer engine.

it has only vacuum advance. that is weird. i thought it would have to advance the spark as rev's went up as there is no other mechanism in there.

i also thought the manifold vacuum went up until wide open throttle, when it falls back down.

 

[canadianhorsepower]

I'm not familiar with a VW 1300 but I am familiar with vacuum advance mechanisms. Distributors with vacuum advance mechanisms have 3 stages to the ignition timing, initial, vacuum and centrifugal. If you made a vacuum advance mechanism it would work only when the engine was started. As the throttle is opened and the vacuum drops the centrifugal part takes over. Without this part you would actually lose timing back to the initial setting as the throttle was opened.
gbritnell

Not exactly, on Ford you were using manifold vacuum that you describe

On GM were using ported vacuum,like your main jet in your carb and timing will increasse with RPM till you reach 0 vacuum with engine load

hope that explanation makes sense:hDe:

 

[///]

As Luc has said, Vacuum advance is also needed during partial throttle conditions. Lean mixtures burn slower, hence the need for advanced spark.
There is a pretty good article about it here: http://www.camaros.org/pdf/timing101.pdf under the sub-heading 'Controlling Vacuum Advance' on page 57.