16 replies to this topic

[GraemeC]

Just to clarify a bit further - ported vacuum is taken directly above the throttle disc so that at zero opening of the disc (ie idle) no vacuum is pulled. Manifold vacuum is taken just after the disc (or indeed off the inlet manifold itself) and hence there is vacuum even at idle.

ie - “Ported” vacuum allows little or no vacuum to the distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times.

Generally a standard or mild cam will not require any advance at idle to run smoothly - therefore the vacuum advance only comes into play a small throttle openings (ie cruise conditions) where there is an economy benefit from running more advance. Therefore the engines will be set up to use ported vacuum.
Lumpy cams often need advance at idle in order to produce smooth running - this is where manifold vacuum is used.

[Boycie]

Interesting, thanks Graeme.