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".....but then how would you know which ones worked for the proper curve?"
You would have to obtain a set of springs from a distributor assembly with a known advance curve (given equal centrifugal weights), then have the spring company replicate the physical dimensions and spring rate.
This may not be as accurate as one would like, even with replacement "factory springs". Even an ever-so-slight error factor (manufacturer's tolerances) could result in a less-than-desireable advance curve.
Perhaps a kit with a mix of springs labeled "normal tension" as well as "+1", "+2", "-1", "-2" might work for the owner (or tuner) to mix and match.
or..... tackle this from another direction and somehow add weight to the existing centrifugal weights to change the advance curve. As you know, the shape and weight of the mech. advance weights also determines the advance rate.
In each case, a distributor advance machine would be ideal to verify the modifications. Short of that, a test procedure with an advance-type timing light could be devised to verify and record the resultant advance curve.
Ideally, a preliminary spark test on a chassis dyno will reveal the maximum advance/rpm for your particular engine (all engines are not the same, even factory engines). With that data in hand, one can proceed to change the distributor advance curve per the dyno test.
Sounds like a viable project.
Sherwood
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