To say vacuum retard replaced vacuum advance is wrong. Vacuum advance did
not disappear, and is still in use today, albeit usually electronically.
Nor is it uncommon to find both vacuum advance and vacuum retard on a single
distributor. Frequently it's nothing more than a tap on both sides of the
diaphragm on the same can. Some TR6's came this way for example. As did
one of my Spitfires as I recall.
Nor can one correctly say vacuum advance connects manifold vacuum and retard
connected to port vacuum. They are mixed. It depends on the design. Nor
is all port vacuum the same. I've seen ports tapped into the main venturi,
and into various areas alongside the throttle plate. All will produce
different vacuum signals.
Some of you might want to read this, to follow along Triumphantly.
http://www.tr6web.com/Documents/tr6/advretard.html
You can take a look at Paul Teglers chart of Lucas and Delco Remy
distributors and see the many differences in the vacuum advances. Amounts
of advance, vacuum for that amount, and the different port or manifold
sourcing of that vacuum signal. As well vacuum retard.
http://www.teglerizer.com/mgstuff/advance_curves.htm
Many of you are placing the advent of vacuum retard much too late on the
timeline. It showed up around 1966 in California, was nationally common by
the late 60's, and virtually universal by 1970.
If you wish to read a fairly decent 101 primer on ignition timing, you might
enjoy this one.
http://www.corvette-restoration.com/Tech_Papers/Timing101.pdf If you want
to go beyond that, Dave Vizard's book How to Build Horsepower, Volume 1 has
an excellent section on ignition timing, covering vacuum advance
particularly well.
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