With a 1972 400, What's the difference IF any between the Vacuum and Mechanical advance on a 4spd vs. auto. Also is there any difference with 2bbl vs 4bbl? Thanks

Good question. Basically that is what people say when they don't have a correct answer!
You'd think that there might be. When I am confused about an interchange, I look to RockAuto.com. Trouble is, so many auto parts stores go to a standardized part to cover multiple applications.
A 4 barrel 4 speed should have a different curve in the distributor with maybe less vacuum advance.

The Vacuum Advance Diaphragm Canister unit is (to some degree) adjustable using a 3/32-inch Hex (Allen) Wrench through the vacuum port opening in the unit.

I think some are fixed, and if you go in there with hex key, can puncture the diaphragm if you are not careful.

It is physically different and has electronics wired to it also.
Ron

The electronic one retards when it is energized by the emission controls (same deal as the O tdc timing). Are you wondering about the differences or is your can shot & you are wanting to replace it? If so I'd suggest a regular (non electric) one and CCW with the 3/32" allen wrench slows the curve (doesn't change the total the can offers which is as said the number stamped on the arm) by requiring more vac for the same amt (deg) of advance. You can file the notch in the arm for more advance and epoxy strips of feeler gauge with JB weld on the back of the can to limit the arms' travel into the can to limit the total (either way as needed). I would set the initial then the total (initial+slots/no can) then springs then hookup/adj the can and (A) you want to stay just under the pinging point on your hottest/driest day at a steady (high vac) rpm such as an interstate cruise at or above the RPM where the springs are maxed out (full mech/vac adv) by altering the arms' travel then (B) with the allen wrench let the can come in as fast as possible staying just under the pinging point on your hottest/driest (most likely to ping) day in everyday driving under varying vac levels. alot of work but alot of gain to be had

What I'm trying to figure out is if All the 1972 Distrbutors are all physically the same and maybe different internally(springs or weights) and if they all have same or different vac. adv. units (physically or internally). I have a 1972 400 4bbl auto and am Looking for an original Distributor. Do I need to buy one just for that application or can I buy a 2bbl or a 4spd one and make internal changes if they are different.

The distributor for your Road Runner is going to look the same 2 bbl or 4bbl.
The auto and 4 speed is going to also look the same except for the large vacuum can on 4 speed that I posted already.

Here is a 400 4bbl. auto distributor:
Ron

Unless there is something wrong with your distributor get a tag from Bill Rolick and call it a day.
Ron

That's a nice engine pic. But that distributor is a 1974. Are the internals(weights and springs)of a 1974 the same as a 1972?

Thanks for the responses. I do Not have a Factory Shop Manual or Parts Book. I did look at the auto parts store sites like Rockauto and autozone and found that they don't have a different part for a 4bbl or a 2 bbl and also don't differentiate between 4spd and auto. Anyone know how one of these re-mans are set internally?

I'd go ahead & get the one listed for your app (# on the arm) & adj it with the allen wrench to come in just under the pinging point (which can be anywhere in the slope as your eng/gas/elevation/driving style etc are different from the next guy with the exact same setup) if you want to get it spot on. bottom line NO pinging allowed under any circumstances, You stay fairly close to the line but never over it

please explain how this works:

In addition to the spring rate and advance slots (mechanical advance allowed), adjustments can be made to the initial spring force by turning the spring posts. The /6 folks know this, but everyone else seems to be unaware of it. On the secondary side this will change the point when the long loop engage