My inclination is to connect the vacuum advance to the timed spark port...but I'm open to opinions/suggestions on where to start.

 

vacuum at idle is at 12...230/236 @.50, 525 lift cam, 401...etc, if that makes a difference.

 

Thanks,

 

Steve

 

Just to be a little more specific. The factory actually used both manifod and ported vacuum (at least during the '70s).  If look at a TSM showing the vacuum hose routing, you will see that the factory used a spark CTO (coolant temperature overide switch) in the thermostat water outlet.  There were two vacuum lines into the CTO.  One supplied manifold vacuum and one supplied ported vacuum.  A third line ran from the CTO to the distributor vacuum advance.  What the spark CTO did was supply full manifold vacuum to the vacuum advance upon startup and until the CTO reached 160 degrees.  Once the CTO reached 160 degrees, the CTO switched from manifold to ported vacuum and supplied ported vacuum to the distributor vacuum advance thereafter.  The factory did this because the extra advance at startup makes the engine run hotter and hence warms it up quicker.  In the '80s, I ran a 401 in a Jeep truck and had to restore all the smog gear (and figure out how it worked--not hard really) to get the motor to pass California smog.

 

I'm going to disagree on this point.  A big cam that produces minimal vacuum at idle may cause the vacuum advance can to dither the timing in and out at idle if connected to manifold vacuum.  Unless your cam makes enough vacuum at idle to pull the can all the way in and hold it there, you'll have trouble getting your idle speed to stabilize.  With a mild cam and good vacuum at idle, you could probably get away with manifold vacuum without affecting the stability of the timing at idle.

 

I fought this problem for a couple of months on my AMX, even tried an adjustable Crane vacuum can, but was only able to get a stable idle after I moved the vacuum advance to the timed vacuum port.

 

Then was pretty straightforward to get the rest of the timing curve "right" by playing with the adjustable can, weights and springs, and initial timing setting.  (I put "right" in quotes because I've got it "good enough," and have no dyno, timeslips, or other hard evidence to prove otherwise -- just good driveability).

 

You can set it up to run with no vacuum advance, but IMHO, you'll give up some economy on the street/highway at the middle rpm ranges/throttle settings.

It's THE reason Chrysler and then Ford went with ported in the late 30's.  (I'm a student of ICE history and development)

And clear up into the 70's, it was a reason. However, to deal with emissions and other little details, it got more complicated........... vacuum is an indicator of load. Timing needs to change with load and speed.

There is ANOTHER, besides ported or timed and that's VENTURI vacuum - some companies used that. It is taken from higher up, in the venturi area. What happens with MANIFOLD vacuum is that it DECREASES with high load and speed. Venturi vacuum INCREASES with speed and doesn't drop with load. It's typically used in combination with other controls but not always.

 

(whizkidder hit on a topic too - unstable vacuum at idle with a big cam.....)

 

Steve

You may want to just do away with the vacuum advance all together on your car.  I just put two sets of Pertronix III electronic dizzy inserts in two distributors for the A&B tag team SC/Ramblers and I am setting them both up for a straight 12 degrees initial advance and 36 total so the engine can go up to a total of 38 degrees with a 14 iniitial setting or 34 degrees with a 10 initial setting. 

 

Your engine should run fine without any vacuum advance being used if you decide to go that route. 

I think he's on the right path, try things out.....

Mine ain't worth a crap without the VA and things setup the way they are.

I have a mild cam and it's mostly street.

No way I'd run mine sans vacuum advance and those settings. I'd be fighting demons constantly.

 (I know as I tried)