Re: 396 Timing
Oh, PLEASE, timing specifications are UNIVERSAL, there is NO "set the total and don't worry about the initial", not back then, not now, never in the future. EACH area MUST have a definate setting that works, NONE of them can fall to fate, the fate of not being set right, and then falling where they will.
The Pontiac reference was simply to show that when a curve has 40 degrees in it, and the total is set to 36 deg/BTDC, the INITIAL will NOT be where one really wants it to be on an engine.
I AM NOT WRONG.
Lets use your settings above
And a target of:
Initial of 15, plus vacuum of 10, on full manifold vacuum: 25
Initial of 15, mechanical of 21: 36
Basis is good, and ISN'T the disaster the Pontiac was saddled with. MOST GM HEI's were set at an arbitrary 18 degree mechanical curve, for emissions use. Later small blocks used very conservative initial of 8 deg/BTDC. Total for one of them was 6 + 18, for a total of 26
Now, why did that work for those engines? Well, because they were also set up to have as much as 24 degrees of PORTED vacuum advance, to add a second acceleration ignition setup. Depending on the specific cruise vacuum level on ported, any level of vacuum advance degrees, from 0 to 24, could be in use. Assuming the initial was the conservative 8, and the mechanical was to the rpm point it gave 12 of those 18 degrees, (20), and, the vacuum advance was being run at a vacuum that gave it 16 degrees, the combined total would be 36. 8 + 12 + 16 = 36.
Remember, GM HEI was NOT designed as a performance distributor, but as a universal emissions unit, with vastly different curve setups, just at 437 of them, with the ONLY performance curve coming from the late 70's/early 80's trucks and some few suburbans, that went on to become the ZZ series carbureted crate engines. 41 weights were found in Buick, Olds, Pontiac, 375 center, different Buick, Cadillac, Pontiac, Olds, and a few AMC that ran large HEI. Where were those same weights and centers run together, other than those Chevy engines listed above? ZZ crate engines only, NOWHERE ELSE.
How many degrees are delivered with the ZZ set? 22 crankshaft degrees. ZZ4 initial, 11 degrees. ZZ total: 11 + 21 = 32, 11 + 22 + 33. ZZ4 vacuum advance can deliver up to 12 crankshaft degrees, and GMPP orders us to use PORTED vacuum, so, idle stays 11, totl stays 32/32, but, as much as 6 more vacuum degrees can be added at set cruise rpm's, but upon acceleration, up to 11 + 22 + 12 = 45. They run hot, aren't especially drive able, don't accelerate well set up like that. Which subtle changes, the problems go away, 11 initial, 22 degrees of mechaiical, and making a 9 degree stop for the vacuum advance, changing one spring to make the mechanical advance faster, and connecting the now stopped down vacuum advance, those engines wake up, don't over heat, and get reasonably good fuel economy. Some people idle their ZZ4's at 12 degrees, no problems. The springs on the mechanical, as GMPP set the curve up, start point is 1,400 rpm's start, well past 5,500 rpm's limit. Chancing just one of the stock springs, or, using a spring set from a kit, to get start at 100 rpm's over out of gear idle rpm's, and limiting to 3,000 to 3,100 rpm's, does the trick on those setups.
Same setup is used in the ZZ430, which is a ZZ4 with a hotter cam, the above is only off in the mechanical advance speed.