Dizzy Vacuum - Ported or Manifold?

[T25Convert]

Right, a theoretical question for you all...

I have a new set of twin carbs and have been looking at vacuum ports for connecting the dizzy advance to. There is a port on each carb body, with small hole into the venturi immediately above the throttle plate. My understanding (mainly from TheSamba.com) is I 'T' these together and run the dizzy off these. However, comparing to my old single weber I noted that the vacuum port is below the throttle plate.

Foolishly I then searched the internet to try and understand why they were different.

It seems that manifold vacuum is actually preferable and results in a lower heat idle (but higher emissions) than the ported vacuum. Ported vacuums are an emissions led compromise. However, it seems that the dizzy advance on the T25 is more suited to the ported vacuum.

So which is better, and where is the Pierberg vacuum taken from?

Cheers,

Alex

[itchyfeet]

Sorry don't know whats best but i can tell you that the pierburg is connected below the throttle plate, rhs port on this pic but they are both in the same location

Standard ignition advance here, you could check this with a ignition strobe with adjustable timing
https://club8090.co.uk/wiki/Pe ... nce_curves" onclick="window.open(this.href);return false;



Inside view

[tobydog]

I would think that the vacuum is created between the throttle plate and the head, the other (out) side is at atmosphere pressure?

Mick.

[itchyfeet]

I would think that the vacuum is created between the throttle plate and the head, the other (out) side is at atmosphere pressure?

Mick.

Air filter side of throttle plate cant be at atmos pressure because the secondary throttle is opened by vac takan from above the throttle plates on both barells, the vac will of course vary depending on revs and throttle position.

[tobydog]

Seems like the Samba idea is to tee them together and sort of get an average of the two vacuums. If the vac is taken after the throttle plate, is that a more definitive measurement?

Mick.

[T25Convert]

Thanks all - confirms that I need a manifold vacuum to get the carbs/dizzy running right.

Not a massive problem, as I need to boss one manifold for the brake servo vacuum take off, just drill another hole in my lovely shiny manifolds (gulp)...

Cheers,

Alex