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#p5625A 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#Post2487959When 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.
