[TR] Vacuum advance/retard

[Nolan]

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.