Distributor advance

[JHHeiger]

I installed a SVDA on my T4 to get over the hesitation and it works pretty well. And I understand how the mechanical advance part works with engine speed. But I'm not sure about the vacuum part and how that advances and retards based on engine vacuum.

Can someone 'splain' it to me.

[luftvagon]

The Single Vacuum Distributor Advance works off a ported vacuum on the throttle body.



In my simple words, as you apply the throttle it provides a vacuum, but then the vacuum stabilizes, you loose vacuum. So when you apply the throttle, it provides vacuum for a brief period, and as you maintain steady throttle, it stabilizes, and looses vacuum. Technically, it only works as you "accelerate".

[Piledriver]

The "ventuuri" vacuum mostly concerns vacuum only distributors used on older engines.
A few carbs had both vacuum and ported manifold vac taps.

Later VWs (and most other cars) primarily use ported vacuum, which comes from a drilling just ABOVE a closed throtlle butterfly on the rising side. (assuming a downdraft carb)

This provides a variable % of manifold vacuum as the butterfly passes the tiny drilling, and can possibly provide a tiny bit more due to the Bernoulli effect in the tiny gap.

You can also "fake" ported vacuum with manifold vac using a throttle operated cutoff valve at idle, Ford at least did this on several vehicles into the 80s.(trucks) and it was probably common prior to that.

The dual vac can setups had a retard port that ran off manifold vac.
This is great for emissions and heating up your heads at idle...

Explanation:
The way it works is you have NO additional advance at idle, at low throttle, it opens manifold vac to the drilling>vac can, and advances to max vac advance (as it is NEEDED at low MAP settings, the A/F mix burns very slowly) and as you give it more throttle, the manifold vac decreases as does the advance. At WOT/max MAP it provides no additional advance.
It is a load sensitive advance mechanism.

[JHHeiger]

Wow, Jacob Bernoulli! Of nonlinear differtial equations fame, or the guy from the pizza place in town?

[Piledriver]

Pretty sure it's named after the former.

[rayjay]

Here is some great info. There is nothing more critical to making power and overall engine efficiency than lighting the fire at the right time. I used to do distributor recurves and the gains could be amazing.

http://www.hotrodders.com/forum/distrib ... 59033.html