I realize this is an old topic, but the info is important, and I am new here.
There are a few things to consider when running vacuum advance, and especially when running a NON-emissions engine. Most people just plug the vacuum advance feed hose into full manifold vacuum, and go bonkers when they get 40 plus degrees of IDLE timing.
So, what is what with all that? Well, engines really like IDLE timing to be between 18 and 24 degrees, but, that can cause some serious starting problems. So, how to do it right? Well, restrict the number of crankshaft degrees the vacuum advance delivers, to between the INITIAL used, and the 18/24 degrees.
OK, how do we do all that? Simple, put a stop on the vacuum advance pull pin to restrict its travel down to the degrees you want to ADD with full manifold vacuum.
On a ZZ4 series crate engine, we see 11 degrees of INITIAL timing, with 22 more in the mechanical advance, for a TOTAL of 33 degrees. unfortunately, that vacuum advance is connected to the incorrect ported vacuum source, and does not help IDLE, nor cooling issues, but it can be changed, very easily.
How? Well, the 11 degrees initial for that engine is just fine, but doesn't come close to shat the engine really wants for idle, because GM has to sell the engine as an emissions spec engine, and cannot tell someone to connect the vacuum advance to full manifold vacuum.
So, what to do? Either use a Crane 99601-1 vacuum advance degrees stop plate, or, make a home made one from thin steel, mounted on the advance side of the vacuum advance pull pin to stop it when the correct extra degrees are needed.
In the case of the ZZ4, we need to stop the vacuum advance pull pin travel at:
Home made stop plate:
.086 inch travel for 8 crankshaft degrees
.110/.112 inch travel for 10 crankshaft degrees
.135 inch travel for 12 crankshaft degrees
Crane 99619-1 stop plate,
4th serration, 8 crankshaft degrees
5th serration, 10 crankshaft degrees
6th serration, 12 crankshaft degrees
What then mixes is
11 INITIAL + 8 vacuum = 19 deg/BTDC IDLE
11 INITIAL + 10 vacuum = 21 deg/BTDC IDLE
11 INITIAL + 12 vacuum = 23 deg/BTDC IDLE
In the case of the ZZ4, the mechanical advance was allowed down radically, starting @ 1,300 rpms, limiting well past 5,000 rpm's, so conflicting vacuum and mechanical advances wouldn't cause engine damage.
When the vacuum advance is set correctly, and sourced to full manifold vacuum, the mechanical advance speed can be made faster to work with a correctly applied vacuum advance, changing one spring to start the curve @ 800/850 rpms, limiting @ 3,000 for that series engine, with no detonation, no pinging, no engine overheating, and a lot better operation.
If anyone wishes a complete set of pictures and instructions on making a stop correctly, or using the Crane stop correctly, send an email ask ask for them, they are free, no strings, no ads, only info on how to do it the right way.
gmvacuumadvancemodifications@gmail.com
BTW, the way Crane says to mount the stop plate, on the rear advance screw, does not make the vacuum advance work correctly. The way I outline, DOES, on both Crane and stock Gm vacuum advances.