Don't forget though, RD, that pretty well all distributors have vacuum advance as well, which has the big advantage of retarding things when you stick your foot on the pedal under load. Quite a lot if needs be. So the curve can be longer, and the full advance position can be a bit further out in terms of engine rpm - Maybe?
In fact, 34-36° isn't so different from an ATD with say 16 or 18° of movement, as you have to double up for crank degrees obviously, so total advance isn't all that far off is it?
Presumably, the makers of things like Boyer and Rita systems and more modern equivalents sussed all this out and made things which might suit better than an ATD - but as an old fart I don't know. When I've had electronic ignition on things I've usually ditched it in favour of a magneto (as I have done on my P11), cos that's what I understand best!
ATDs are pretty basic, and because they are controlled only by rpm, are not really a very smart way to manage a motor in any sort of state of tune. But hey, by the standards of the day, when the choice was a sloppy camring, sub-optimal sparking when the ignition was in any position other than full advance . . . not a bad thing overall.
I have often wondered whether there is enough inlet manifold depression to allow a vacuum advance system to operate on a typical vertical twin. Had a half-formed idea to (try to) make something with a stock ATD on the shaft, plus a manual camring controlled by the vacuum if there was enough variation in the manifold to actuate it. However, I have no idea how big a diaphragm might have to be, what complications there would be getting anything to integrate with a mag, miss the carburettor, etc etc etc. So it never got done. Part of me thinks that folk like Lucas, who after all made most UK car distributors at one point, must have thought about it too, as they were much smarter people than critics sometimes allow (- and much much smarter than me . . .) The fact that they didn't go there suggests perhaps that the gains weren't deeemed worth the hassle . . . As folk like Bosch and BTH didn't either (afaik), although the technology was there in their time, seems to say the same thing. And no doubt penny-pinching cost people who wanted their magnetos for 25/- apiece (old GB money for one and a quarter pounds for our younger and/or overseas friends!) wouldn't have liked the idea at all!
