[T3] 72 FI simplified

[Dave Hall]

I'll have a closer look - not sure how different the '71 twin carb dual vac
situation might be.  It's timed at TDC with hoses, just to be different from
the '72 FI.  Sometimes it's better to accept the truth than try to
understand it!

Dave.
UK VW Type 3&4 Club
===================

-----Original Message-----
From: type3-vwtype3.org-bounces at lists.vwtype3.org
[mailto:type3-vwtype3.org-bounces at lists.vwtype3.org] On Behalf Of Jim Adney
Sent: 22 May 2012 14:12
To: type3 at vwtype3.org
Subject: Re: [T3] 72 FI simplified

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.

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

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