Vacuum advance
I never didn't believe you said to run ported from the beginning and I said thanks I haven't questioned that. Other ppl have chimed in and made good points and I have a better understanding of the whole process now! Im the type of person I don't want to just know how to do it want to know why I'm doing it that way and what will happen if I don't do it that way. Please don't take offense if I keep asking questions I always heed everything you say you have never lead me wrong im just the type of person a simple how to answer just isn't enough. And that's how you learn and expand your capabilities. It's easy to learn that 2+2=4 and memorized that but what happens when you get 3+1? Its still 4 your just getting there im a different way. But I never once 2nd guessed you or your advice! Sorry if I made you feel that I did.
Naw it's no biggie. It's just that several people made the correct suggestion for your build and this yahoo caint keep his trap outta where it doesn't belong. He's just slap wrong about the timing being the same for each and that's just wrong. It's bad information and here's how.
Initial or idle timing with ported vacuum is let's say....like you have now "around" 20 degrees. Let's call it 20 just to make it simple.
Now, remove the vacuum advance from ported and hook it to manifold. Now your initial timing is 20, PLUS "whatever" the vacuum can pulls in....let's call that 12 degrees. So now your initial or idle timing is 32 degrees. Does that sound right? Does it sound the same? No, because it's not. So you have to back that 12 off of the distributor (initial) timing to get it back to 20. Get it? Now, unplug the manifold vacuum from the vacuum can and what do you have? You have 12 less than that 20 or 8 degrees BTDC. There's no way in hell those two timing curves will be the same in this country or his.
You can run manifold vacuum. You can run ported vacuum. You can run no vacuum. But the best way, IMO for YOUR build, since it's really not too far from stock is ported. It also does something no one has mentioned. It gives you a certain amount of engine braking due to the vacuum falling off when you take your foot off the gas. That can make a difference in mileage right there.....and right now, gas ain't cheap. With it on manifold vacuum, when you lift your foot, vacuum goes up and pulls in total vacuum advance, and reduces that amount of engine breaking. It makes a very noticeable difference.
Ported vacuum also cleans up part throttle combustion, reduces NOX gasses and picks up mileage and gives a little extra advance at cruise. There's no down side. This slant 6 I just put in my car has 6 Hg at idle. 14 at cruise, because it has a pretty sizable flat tappet solid cam. I can tell you it benefits greatly from being on ported vacuum and I know yours will too. But my advice is try it both ways. But in order to get initial timing right hooked to manifold vacuum, do not disconnect it when setting timing, because you WANT the vacuum operational at idle. Set it the same 20 degrees initial as you would with the can disconnected from ported. You can plot the two curves yourself and see the difference. Some people like it better on manifold vacuum. You may and if you do, it won't hurt a thing.....but it ain't the same as ported. lol
