[T3] 034 on Squareback

[Jim Adney]

On 20 Jan 2014 at 17:52, Max Welton wrote:

> At a steady 67 mph at an altitude of 6163', I had 6" of vacuum. Now granted I
> have a slightly larger than stock 1776, but that should give an idea of how much 
> advance was being added at that point. Maybe someone with a vacuum pump 
> could tell us what 6" at the canister yields in degrees of advance.

It's very important to understand that this is the vacuum port vacuum 
and not manifold vacuum. They are completely different. Manifold 
vacuum is what the FI Pressure Sensor and the AT Vacuum Modulator 
get, but the Vacuum Advance gets this different vacuum which is 
created by the Bernoulli effect of air flowing across the vacuum port 
opening.

Manifold vacuum is max at idle and min at WOT. (True for all engines)

Dist port vacuum is zero at idle, min at WOT, and max somewhere 
toward the idle end of the throttle opening. (True for engines that 
came with distributors that also have mech adv, because vac adv only 
dists often used manifold vacuum.)

6" (of mercury, Hgk you can see that on the dial scale) converts 
directly to about 152 mmHg. The tables for the stock FI vacuum 
advances are given in the Bentley, and 150 mmHg gives 2-3 degrees of 
distributor shaft advance, or 4-6 degrees of crankshaft advance, for 
the most common, #265, FI vacuum advance.

If you look again at your vacuum gauge scale, you can see a second, 
inner scale in what must be cmHg, which is just 1/10 of mmHg. The 
needle sits at about 16 on that scale.

It would be interesting to see what your reading would be when closer 
to sea level. The extra air friction would mean extra load on the 
engine, but you might not have to open the throttle valve any more 
since the air would be denser. I don't know enough to predict whether 
the vacuum would increase or decrease.

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