I think that the shape of the hole(s) are also as important as the sizes.

For example, if I theorize way above my pay grade, I would imagine the shape of the distributor advance goes like this... They wanted a very narrow spike of vacuum oat a specific pedal position, so they used a narrow slit. And I can affirm (from driving around with a vacuum gauge T'd into that line) that the vacuum spike is very narrow and very high (vacuum).

The EGR ports on the other hand, appear to be placed to provide vacuum over a much broader pedal position. I'm guessing the smaller hole in the back is used to "extend" the EGR vacuum value deeper into the throttle.

So my read on the whole thing is... They optimized the ignition advance to kick in only at one narrow pedal position (light cruise), but the EGR is activated over a broader range and deeper into the pedal (medium cruise).

But, neither of those ports will produce any significant vacuum once you get above medium cruise.

25 minutes ago, Captain Obvious said:

I think that the shape of the hole(s) are also as important as the sizes.

For example, if I theorize way above my pay grade, I would imagine the shape of the distributor advance goes like this... They wanted a very narrow spike of vacuum oat a specific pedal position, so they used a narrow slit. And I can affirm (from driving around with a vacuum gauge T'd into that line) that the vacuum spike is very narrow and very high (vacuum).

The EGR ports on the other hand, appear to be placed to provide vacuum over a much broader pedal position. I'm guessing the smaller hole in the back is used to "extend" the EGR vacuum value deeper into the throttle.

So my read on the whole thing is... They optimized the ignition advance to kick in only at one narrow pedal position (light cruise), but the EGR is activated over a broader range and deeper into the pedal (medium cruise).

But, neither of those ports will produce any significant vacuum once you get above medium cruise.

Interesting.  My question then is, since the hole is pretty tiny, am I losing vacuum advance movement on my distributor since the EGR is now taking some of that vacuum or is there enough vacuum for everybody?

I think that since everything is essentially dead ended (or at least should be), there's enough vacuum for everybody.

Why don't you "T" a vacuum gauge into the line and drive around like that a little. You'll get a feel for where the vacuum peaks. And if it peaks somewhere other than light cruise, you might need to come up with some sort of alternative. I believe the ignition advance tops out somewhere around 15-20 in Hg.

I don't know anything about the big bore TB. Other than it's got a big bore.

3 minutes ago, Captain Obvious said:

I think that since everything is essentially dead ended (or at least should be), there's enough vacuum for everybody.

Why don't you "T" a vacuum gauge into the line and drive around like that a little. You'll get a feel for where the vacuum peaks. And if it peaks somewhere other than light cruise, you might need to come up with some sort of alternative. I believe the ignition advance tops out somewhere around 15-20 in Hg.

I don't know anything about the big bore TB. Other than it's got a big bore.

Interesting, I very well might give that a shot.  Thanks for all the info.

14 hours ago, Captain Obvious said:

 

I've zip tied gauges to the driver's wiper blade and ran my coolant temp tweak inside the cabin to adjust while driving but I'll be damned if I can figure out how you got that gauge to hover like that. Are you some kind of space alien?

9 hours ago, siteunseen said:

Are you some kind of space alien?

So we've known eachother for how many years... And you're just figuring that out?

Na-Nu Na-Nu.