[T3] 72 FI simplified

[Jim Adney]

On 22 May 2012 at 12:26, Dave Hall wrote:

> My head's spinning thinking about it - doesn't the 'vac advance' actually
> reduce the advance when the throttle is opened, as the manifold vacuum
> pressure rises?    At high rpm and small throttle openings or closed
> throttle, the manifold vacuum is greater, and so is the advance - isn't that
> right?    

None of these dists (all the later ones which also have mech adv) use 
direct manifold vacuum for the advance, which would change as you 
say. Instead, they get their vacuum from holes drilled near where the 
throttle butterfly valve closes, so that they are dependent on the 
relative position of the valve flap to the hole and the speed of the 
air flowing across the hole (Bernoulli's effect.) For all of these, 
the advance increases as the vacuum increases.

The vacuum retard drilling is positioned in a location that the valve 
flap passes across as the flap opens. Thus that hole gets full 
manifold vacuum at idle, when the flap is closed, but that changes 
quickly to very little vacuum once the flap is opened a bit.

I believe the retard side of the dual cans has a spring with much 
more preload, so that it really doesn't move much at all until it 
gets the strong idle vacuum. Then it moves to the end of its travel. 
On these, the retard increases as the vacuum increases.

The dual vac cans are expensive ($150!) and have poor reliability on 
the retard side, so if someone sends me a bad one to rebuild I 
generally advise them to "upgrade" to the '73 dist, MAP sensor, and 
brain, because VW & Bosch figured out a better way to do this the 
following year. I recently managed to buy several NOS '72 dual vac 
cans, however, so I can now replace them at a reasonable price.

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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