Played around with the Sun machine today. Made an interesting discovery.
I'm working on an Olds HEI, but it's the same rotation as Pontiac, so the results should be the same. No guarantees on Clockwise rotation distributors--I'll screw with them later. (Chevy, Cadillac, Buick are CW. Olds and Pontiac are CCW)
I gutted the centrifugal weights and center plates out of my "core" distributors. For today's post, I used the same set of weights (130) and springs (aftermarket lightweight stuff) but I installed a small variety of center plates. I put each center plate in with the numbers "up", and then re-installed each one with the numbers "down".
Using the same weights and springs, the center plates made a major difference in the overall advance curve--just as you would expect from the table in the first post. Both the amount of advance, and the RPM of start-of-advance, and full advance, changed.
Here's the scary part. In every case, when the center plate was installed with the numbers "down", the advance curve would seem to be "normal". It would begin advancing similar to the others, it would seem to quit advancing similar to the others. You'd think it was just another advance curve. BUT!!! after the advance "seemed" to be all in, the advance held pretty steady until the RPM doubled, AND THEN IT ADVANCED A BUNCH MORE. So with my lightweight springs, I was getting full advance at about 2000 engine RPM, give or take a few hundred. And it would hold steady at that figure--perhaps even retard a degree or two because of the pickup coil effect--until about 3500--4000 engine RPM. From 4K to 8K it was into the secondary advance curve which might add another 10 to 20 degrees!
If you didn't check the mechanical advance ABOVE 4K, you'd never know this extra advance was happening. Think a bunch more "mystery" advance could cause engine problems at six grand? I do!!
On each of the center plates I have, one edge of the center plate has a smooth curve, while the other edge has a "dogleg" curve. The weights must act on the dogleg curve, NOT on the smooth, gradual curve.
You might want to verify how your center plate is installed, if you're running an HEI.
I'm still looking for part numbers or a source for the center plates.
I ordered some 139 weights from NAPA yesterday. They claimed there are only four left, and "they have to go to the manufacturer" to get them 'cause the warehouses don't have any. The only way this makes sense to me is that the weights are discontinued and I got the last ones. I hope I'm wrong.
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