You can make a useful curve with a genuine GM parts HEI using a 375 cam and 41 weights with the two big springs which come on MSD billet distributors out of the box, (approximately) .046” wire diameter, .286” OD, 4 and 1/2 coils. Trim a bit of the end of the eye loop off each end, about ¼ of the loop and close the loops up until the inside free-length of the springs is about .010” or .015” shorter than the length you measure on the outside of the weight pivot pins and cam mounting pins, the outside distance of the spring mounting pins, so the springs are pre-loaded equally
---- It is very important that these heavy springs are pre-loaded equally ----
The curve will be close to this, use 18° initial, start: 1100-1200 RPM, 26°-28° @ 2000-2200, 32°-34° @ 3000 RPM, 34°-36°@ 5000 RPM, 36°-38° @ 7000 RPM, 38°-40° @ 8000 RPM. This is contingent on the weights and pivot pins are not worn or loose and the cam shows no wear.
Again, it is very important the springs in an HEI advance mechanism are exactly alike and installed with exactly the same preload. Measure the pin span and the relaxed springs’ eyes internal lengths to be sure the springs will be stretched equally when installed.
This type of advance curve with the flyweight and spring mechanism in control of the timing at all RPM, instead of bottoming the mechanism on a mechanical stop by welding the slot or using a screw or roll pin in the rotor mounting bar, it isolates the torsional noise – oscillation - in the distributor shaft caused by driving the oil pump. This is especially useful when a high-volume or high-pressure pump is causing spark scatter because the advance is welded solid or bouncing on and off a solid stop.
In restricted classes, such as circle track where everybody has the same size 2-bbl carb, using this curve with the initial set to wherever makes the best power, depending on carb size 350, 500 or 390, etc. but typically advancing from 36 at 3000 to maybe 44 at 8000, is almost an unfair advantage in a field of engines which are “all in at 2500” because the locked or all in early timing retards as RPM increases (because the electronics in modules or CD boxes have a constant slew rate) and engines actually need more timing because of the high intake vacuum at high RPM. This curve compensates for the retard and allows timing for best torque and then advancing to best power at high RPM.
Each engine’s best timing will depend on intake vacuum at peak RPM which will depend on carb size and engine power. Engines with small carbs which have a large increase in intake vacuum from peak torque to peak power may want more timing at peak power than is best to avoid detonation or is even safe at peak torque. This advance curve allows satisfying both requirements at each end of the RPM range.