Here's the tach drive distributor (small block version) with its original springs.
The first photo shows the loop to loop lengths at rest. That's what I posted early in the thread.
The length effects initial tension, and how many degrees out before the secondary engages.

I follow all that. I would also state the difference between the cardone spring at 45lbf versus 175 is very significant. Granted tighter coils on these secondary springs, but the thickness is not as great as other springs that are in the 35 to 45 lbf range on fuel pumps, let alone a valve spring at 175lbf. I guess I would have to go back to some of my physic course to calculate the maximum force the weight could apply at 3000rpm as an example. Just seems that rate was too high, but I understand what your doing.

As a note, the one picture shows the slot welded up on the inside (closest to shaft), I assume to limit total advance of that 15degree cam stop to something more in the 10 to 12 range. Some have told me it does not matter whether it is welded inside or outside. Which is true as far as total advance is concerned. But it does make a difference on the spring preload. The inside weld puts a preload from the smaller spring which means initial advance as you raise RPM is delayed by that force. That may be what you want if you are heavy on Initial. But is seems to me, it is better to weld up on the outside to limit total advance, while letting the spring combo do its thing.

You understand exactly.

I beleive the idea of limiting the max advance came from the era where the secondary spring was removed, and the vacuum advance pinned.
This worked for drag racing with points. It was pretty easy to do as well.

With electronic amplifiers, this doesn't work well. The first time someone cued me into this was a coworker who had been running stock eliminator. They had tried to improve their times and one of the first things they did was remove that stupid heavy secondary spring from the DC Tach drive distributor. They tried a whole bunch pof different curves with one and two light springs but were not hitting their times and mph. Finally they put that seconday spring back in, and guess what, their mph and times came back.
About 10 years ago Tuner was telling similar stories on Speed-talk, and not just about his own experiences, but noting how all of the factory teams had learned this too.
https://www.speed-talk.com/forum/viewtopic.php?p=201411#p201411 and here
https://www.tapatalk.com/groups/rac...d=5f00c7686ad6e655eaf34962286567e5#p5625
A couple years ago Bill Baldwin posted a video showing this on his distributor machine.
See: https://board.moparts.org/ubbthread...ry-unilite-timing-issue.html#Post2487959
When I tested that tach drive distributor on the sun 404, that was with the Sun amplifier. That's a pretty slow unit - in the same range as the Chrysler ECUs. With a faster ECU, might have seen a slight rise in the timing from 2000 to 4000 rpm. Maybe a degree.

For use with a vacuum advance, and in uses where engines get really heat soaked, the easiest way to get the mechanical advance curve is is shorten the inside and let the secondary spring slow the advance around 2000 rpm plus minus depending on the engine. That's because pretty much all magnetic pickup distributors were made after the implementation of the Clean Air Package. Shortening the inside of the slots makes the curve the way it would have been before CAP and retarded initial timing. If the begining of th e advance is delayed too much because of the increased initial tension, the spring perch can be adjusted.
psst. This is super secret stuff. Only share with people you like.