I have spent the last month or so deep diving into Lars, SWCDuke, and Johnz's writings on all things timing.

Here is my setup: NOM 350 (supposedly from the 1970s) with Competition Cams cam (unknown). I have a complete MSD 8572 with 6AL set up, currently with it's original MSD vacuum advance. I am plugged into the ported vacuum on the Holley 750 double pumper. At 800 idle I am getting (after adjusting the carb) right around 12" of vacuum. I am running the 21 degree stop bushing, and it likes to run around 16 degrees initial - which I just adjusted down to 15 to get to 36 total mechanical. I started with the Heavy/light springs which go all in around 3500 RPM I believe, and switched to the blue and light silver springs where it is now all in at 2400. I think I am going to back off to two blue springs which will make it come all in at 2800. This is a 64 coupe Muncie 4 speed if that makes a difference.

The car runs pretty good, better than it has since I have owned it. But it still has some low RPM issues, and runs a little hot at idle.

I did try Manifold vacuum with the stock can, and it did not go well at all (as expected from what I have read). The MSD Cam goes all in at 15".

I just want to run this by the forum and see if I am thinking correctly.

THE RULE says I should get a vacuum can that is all in around 10 if I am understanding correctly, then I should be able to run manifold vacuum. Would a B28 can work for me here? I have a B26 can also that seems like it comes all in around 10-ish on my mityvac, along with a couple B28s that come all in way before that, maybe around 6-8.

Does this sound right? If I go to manifold vacuum will I have to change any of the other timing, and will that make my idle go up and therefore need adjusted down?

Also, will the B28 Can need a stop spacer to keep it from going the full 16?

If you run a mainfold vacuum on an 8572 I would love to here your experience and setup.

Thank you all! - Rob