Originally Posted by oldbogie
Vacuum advance goes away as the throttle is open because the vacuum in the manifold reduces toward atmospheric pressure unless you're running a super charger of some sort.
With an idle vacuum of only 8 inches you wouldn't be getting much vacuum advance unless the advance can is adjustable or has been reworked with a different return spring pressure.
With 8 inches of idle vacuum, this is pulling a pretty good cam, big cams like a lot of initial advance because the effects of late closing the intake valves reduces cylinder pressure which causes a slower burn which needs more time, thus advance to get across the chamber. I don't think dialing back the base timing is the way to go, at least not if you don't put it into the centrifugal and perhaps bring it up faster.
Generally Vortec type chambers like about 34-36 degrees max. Aluminum often likes more advance and more compression to make up for the cylinders faster temperature loss because of aluminum's much faster heat transfer rate into the cooling system than iron. You can also stand to run the coolant temp 15-20 degrees to slow the heat transfer rate a little.
All this is tuning for the new conditions, it can take a while to sort out what works best.
The engine is pulling the VAC to the stop at idle. The can is adjusted to its limit, which is to say that it's delivering full vacuum advance of 14 degrees at 7.5 in-hg (the engine idles at 1100-1200, or 9 in-hg, so there is SOME breathing room here before a stall condition develops due to feedback if idle vac drops below 7.5 in-hg). The can is adjusted so that vacuum advance goes to zero by about 4 in-hg.
The block's water jacket is 3/4 filled, to 1" below the decks, and runs an oil cooler. This takes a LOT of cooling load off of the radiator, which, as you know is marginal, even when brand new. There was VERY little extra cooling capacity built into them from the factory. Because of this, I'm able to run a 160 degree t-stat, AND ACTUALLY HAVE THE COOLANT STAY PEGGED AT 160............EVEN ON 100 DEGREE DAYS IN AUGUST!
I'll play around with the spark timing and, I agree with what you say about the big cam and cylinder pressure. When I was trying to dial it in with the iron heads 2 years ago, the thing was a dog until I took the initial up past 17-18 degrees. If I can hold initial at 20, or better, then I'll rework the advance slot in the distributor to limit WOT advance to what the combustion chambers like......which will (probably) be somewhere between 34 and 38 degrees, total.