This whole thread doesn't make any sense.
You're running manifold vacuum to vacuum advance. In other words, you're forcing the timing to advance at idle and low end, but it's also advanced all the time at the low end. So then you're readjusting the distributor timing at idle?
Vacuum advance is an economy measure, nothing more. At WOT, the vacuum drops, the timing retards. At cruise, vacuum is high, so vacuum advance kicks in advance to insure full burn of a relative low volume of air/fuel.
Hooking up vacuum advance to manifold vacuum is adding in advance, but it's doing it at a point in load/RPM where you should be altering springs on the advance weights, or dialing in the distributor twist.
Adjusting the screw in the vacuum cannister only adjusts the
rate of vacuum advance, not the total amount (the analog when using manifold vacuum goes right back to when you should be altering the springs). So when you improperly use tuning vacuum advance settings to improve low end performance (via manifold vacuum), you're going to lose economy and increase ring wear due to fuel wash at cruise, because you're using the vacuum advance in place of properly tuning the distributor and not when it's supposed to be used.
NONE of this talks about the actual vacuum stop limits in the advance canister. If you don't know this number from your canister, then someone elses' experience with tuning using manifold vacuum is potentially very different from yours.
Ping Pong covers the base of this, but the magazine articles were much more in depth.
TLDR:
If you're seeing improvements in low end performance by going to manifold vacuum, that means you need tune your timing in the distributor, not use the vacuum advance as a crutch.
