Thanks Jerry. You’re right. The Delco Remy spec for 2403 Is sort of variable at 12-14 Degrees All in at 1750 rpm, so about 3500 rpm. I have put about ten 2403 distributors on my Sun machine. I rebuilt about half of them, one was a nos unit, 4 were not rebuilt. The nos and unrebuilt got to about 11 degrees. The NOS did not come up to specification because the old but original grease had dried up, gummed up, and prevented the weights from working properly. Once I took it apart and cleaned it, it was in at 13 degrees. The rebuilt units did better, most got to 12-13 degrees, one got to 14. So you have anywhere between 24 and 28 degrees of mechanical advance. I have attached a 55 and 59 AEA and Sun tune up cards so you can see the different settings depending on what year vehicle.
So depending on the year, the initial is either 0 or can go to say 5 degrees for a total advance (with out the vacuum advance taken into consideration) 24 to 33 degrees depending on what you dial in for the initial. there were no real changes to the motors, except for 59 oil system changes, so it is somewhat clear to me at least that any 235/261 can take a few degrees of initial even if the specification is for TDC. And frankly, that has been my experience over the years.
Those are factory settings for the 2403. Pat Smith, former president of inliners international, and the long time crew chief for Dyson Racing IMSA cars, had a 261 that think ended up running its best at like 42 total degrees, so 28 at the distributor (14 degrees distributor advance X2 for what the motor actually sees) and 14 initial degrees for a total of 42. That was with decent gas. Pat's method was to keep advancing a degree or two until the motor pinged, and then backed off a couple of degrees.
So really what you end up with depends on what the motor likes. But there is a caveat, because total timing is impacted by the vacuum advance. So with a 2403 distributor you have to balance the mechanical and initial so that the vacuum advance does not create a pinging event. It’s not adjustable so it can be a little trial and error.
Your HEI is going to be different. You can add an adjustable Vacuum can to the HEI to help dial in the timing curve. But first you need to know what the HEI’s mechanical curve and total mechanical advance is, and then make changes with Weights and bushings and sort out what timing the motor likes. Tom Langdon wrote a little article on vacuum advance.
http://www.langdonsstovebolt.com/tech/vacuum-advance-and-why-you-want-it/
