Most of my experience has been with our big blocks and Hemis. I have messed with a some small blocks, but have much more time with the BBs and Hemis which really respond to ignition advance so this is directed primarily at the BBs and Hemis.

I don't really understand the place that manifold vacuum has in this. Our big blocks/Hemis like a lot of lead at idle and the bigger the cam the more lead they like. They also like the advance in as soon as can be, but kept out of detonation. That is the part where for me, the manifold source fails. It drops advance at a time that my experience shows that the engine wants it.

On my street combos, I typically run 25+* of initial. As much as I can and it still not hit the starter on hot days. And then all in by 2,000. On the wedges the total is usually 32* to 34*. On my current Hemi, it likes 31* for what ever reason. As I said before, I use a hand held vacuum pump to experiment with how much it likes at cruise. The change in exhaust note is very noticeable as you change the amount of advance and is a good starting point to set the amount of advance for cruise.

I use a stock Mopar distributor and JB Weld the slots to get the amount of advance that it likes. I also use JB Weld to add weight to the advance weights along with micro springs from a hardware store to get the rate right. It takes some trial and error and multiple disassemblies of the distributor, but after a time or two, it goes pretty quick.

For me, the toughest part is getting the vacuum can stop at the right place. The opening point is easy of course, since Mopar made that adjustable. There used to be a wide selection of vacuum cans with different amounts of vacuum advance. But that is not true anymore, so modifying what we have or can get is the way to go.

So my preferred mix is modifying the weights and springs in the distributor to get the initial timing and the mechanical advance where the engine is happy and then using ported vacuum for a modified vacuum advance. That's the way Mopar did it and the way that has worked very well for me forever.

There was some previous talk in this thread about the PROMAX metering block not having the vacuum port.. and that is what I'm using... about 8 years ago, they (Promax) told me that I didn't need the port for the vacuum advance. They knew my driving -- some street/highway and road course HPDE/HSAX autocross racing... so, yes, it has lots of part-throttle, only occasional flat-out WOT. The car runs great -- and also now with a new SB stroker if still runs great (SB six-pack recently professionally rebuilt).

When I was ordering the Promax setup (base plates, etc, etc), they wanted to know all my specs, etc. They said they would make the metering block to better match my overall setup.... I'm an easy sell.

But... I'm curious if I should put the original OE/Holley metering block WITH the vacuum port back on and see how it would run.

Interesting thread.

(Maybe the Quick-Fuel metering block WITH the vacuum port would have been the better choice?)

Is ProMax till around? Send it back to them or find someone else to add the port. Adding a vacuum port to the metering block shouldn't be a big deal. That may be better than putting an entirely different metering block on.

I was undecided on manifold or port vacuum and i was about to buy a distributor from one of 2 people that set them up on a machine to your combo. I ended up with the one set up for port vacuum but i wish i would have went with the manifold vacuum because the idle stinks like hell with the 284/484 cam and the manifold vacuum may have cleaned that up a bit..

there's no reason why you can't try manifold vacuum if you want. i'm a port vacuum guy, but if engine vacuum is fairly low manifold vacuum use may be the best choice. i have to wonder if your distributor is set-up for proper initial and low rpm centrifugal timing. the 284/484 is fairly aggressive with the 108lsa and a hydraulic tappet doesn't help much.

18* initial sure isn't much with any cam at all.

If it were mine, I would want more initial than that. And it seems to me that the best route would be to shorten the advance slots. Can someone explain why manifold vacuum would be a better solution in this case than more initial in the distributor?

If the engine likes 20+ initial timing, but can't crank with that much, pretty much the only option is to use manifold vacuum to give a lower initial, or some sort of start retard box.

If it does crank fine with 20+, then sure, limit the advance slots, and set the initial at 20+.

Also in general, the engine vacuum at idle needs to be higher than the vacuum in the advance (if using manifold). If they're similar, then the timing will change with vacuum, which changes the vacuum, which changes the idle speed, which change.... It will never settle down and be steady. Similar issue if the step up springs in an Edelbrock are the same as the idle vacuum.

I’m using a Mopar tach drive distributor. I’m going to try this with it.
https://www.cbperformance.com/default.asp

Try what?