[T3] New Distributor Same Results, 034

Max Welton max_welton_2k at yahoo.com
Thu Jun 12 17:01:32 PDT 2014


A few things about vacuum advance.

I've done my own testing with 32mm PDSITs, Kadrons and a 34pic3 / DVDA setup. Why? Because I have trust issues with most of what I read on the internet, that's why. ;-)

It seems to work best with single carb or TB (such as the type-3 FI TB). The vacuum profile is fairly smooth with all four cylinders contributing. It is less smooth when pulled from one single-throat carb feeding two cylinders.

So we did this test by putting the car in gear, holding the brake and briefly opening the throttles. Takes two people to do it this way, one person to hold the brake and one in the back to operate the throttle and watch the gauge. But since the car isn't moving it tends to be a really repeatable test. When I've done a similar tests with a standard transmission, I ran a long hose up to cabin where I could see the gauge under various driving conditions.

This is my squareback at 67 mph at an altitude of 6163'. 6 inches Hg and steady at about 25% throttle.

http://images.thesamba.com/vw/gallery/pix/1086606.jpg
http://images.thesamba.com/vw/gallery/pix/1086607.jpg
http://images.thesamba.com/vw/gallery/pix/1086608.jpg

In Adams case the carbs are dual throat Dels ... one throat per cylinder. Pretty sure at least one person here missed that. With only one throat providing a signal, the gauge swung rapidly between zero and ~10 inches. Basically there is a pulse as #3 cylinder pulls, then nothing until #3 comes around again. It was rapid but very regular (to me) and synchronized to engine RPMs. I bet if we had measured advance on each cylinder we might have observed different timing at each. That assumes the vacuum advance mechanism could react quick enough to follow the oscillating vacuum signal. Something to consider, eh?

So these Dells on this engine at 7500 feet could generate 10 inches of vacuum under load and light throttle. That was the upper bound of the oscillations. If the other three throats were modified like #3 has been (and the lines used together), it would be a pretty smooth signal. And easily strong enough to be useful. If we were to use an accumulator to smooth the one-throat signal, the resulting signal might be more of an average and of lower magnitude.

One thing we didn't do is to measure the advance deflection of the distributor for several amounts of vacuum. It's an easy enough test with my little hand-pump. I just forget. So sue me.

Something else I forgot when you were here Adam ... I have the tool to remove and reindex the pinion drive. So we don't need to do anything clever about the condenser bumping into the shroud. :-D

Max Welton
http://www.maxwelton2k.net/



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