Vacuum Advance - Manifold or Ported

[Excaliber]

Ah, 38 TOTAL not 26, much better, perhaps a bit high, maybe not, certainly better than 26. At what rpm did you get max mechanical 26 degrees advance?

Yes indeed the vacuum signal should be SO low (or virtually nonexistant) at WOT that it wouldn't be a factor. The other thing I might ask myself (just for curiosity) is at vacuum signal does the vac advance start to work? Often it takes 15" or so to get any movement of the vac advance.

Lets say your cruise rpm is 2800, full mech advance doesn't come in until 4500 rpm. Vac advance doesn't come in until 15" of vacuum signal strength. Your cruise vacuum is 12", below the threshold. The main reason to USE vacuum advance is to increase timing under cruise rpm for best gas mileage. So it is important to 'work the numbers' to see if in fact you can\are taking advantage of the vac advance under 'real world' conditions.

[Kennyb]

First let me say what a great discussion this is. I was searching on ported/non-ported and found this thread.
Some of my own thoughts on this thread, Vacuum advance is max when the engine is idling/cruising. Regardless of whether its a ported or manifold connection. the moment you stomp on the pedal the vacuum pressure at the throat of the carburetor drops (instantaneously) to 0. Vac advance is now no where to be found. Also, suction/airflow through the venturis is 0 (instantaneously). To make up for this loss of fuel flow we have an accelerator pump that squirts raw gas into the throat of the carb. as fuel is pulled down through the barrels the and the A/F mix normalizes and RPM increases there is still little or no vac advance because we still have throttle plates wide open. centrifugal advance increases with RPM and we start getting on top of it. Once the secondary open and we get max vac centrifugal advance has/should have done what the vacuum advance was doing at idle. Vacuum advance is fine for street cars but useless in racing.
One last thought, this one about run-on. Throttle solenoids kept the butterfly open just a hair and when power was removed the plate slammed closed and suffocated the engine thus eliminating run-on. Unless of course the timing is so screwed up that the engine diesels just from the super heated pistons.
according to what I have read in this thread I'm not too far off in my thoughts... anyone?? i

[sidewaysinthird]

Like Ernie, I made use of "manifold" vacuum to allow for fairly wild cam profiles. Its when Texas started doing emissions testing that I found it be of use getting the Nox numbers down far enough to pass inspection with a healthy 377 de-stroked 400 Chevy. I also had some sucess with keeping temps down in the Houston traffic using manifold sourced vacuum instead of ported. However ported vacuum is what stock engines ran all through the 60's and 70's, and it works very well for most when the max centrifigal advance is limited as the typical stock distributor tend to be.

Accel used to make an adjustable vacuum advance for many applications, which when set up properly can vastly improve idle quality on most mild cam engines be it used in ported or manifold vacuum situations.

Good luck to you getting the tune just perfect.

Mike