One more point to keep in mind is that distributors that have a vacuum advance unit usually have a smaller range for the mechanical advance.

Last point to keep in mind. Hopefully this will answer your questions as well. The vacuum advance and mechanical advance do different things to create the advance. The mechanical uses weights and springs to advance the rotor beyond where it would normally be, in relation to the driven shaft. The vacuum advance actually moves the breaker plate, or in electronic ignitions, the pole plate, where the magnetic ring is. Both forms of advance work independent of each other, and must be added together when determining your total advance.

Want me to take some pictures for you?

I'm gonna need the pictures, I know how it works, except the part where you say that the spark port keeps pulling harder as the air moves faster, wouldn't this mean that both advance mechanisms are doing the same thing, increasing with rpm?
The spark port pulls a vacuum that is proprtional to the load so it'll pull it's hardest under the least load once the throttle blades are opened. I also have an article in HOT ROD that says the exact thing as the FordMuscle article and I think if it was incorrect, it wouldn't have made it into print.

Gees man, just because its in Hot Rod , don't mean its true!
I've seen LOTS of stuff in there thats wrong.

Oh and by the way. This explains ALOT of timing problems I've had over the years. Good to gets these out of the way before I hit 30!

Everything PKRWUD is saying is correct as far as figuring total advance, but , Vacuum advance doesnt apply to WOT.

This is from your article:

Have you ever driven up a steep hill? Your pedal is to the floor, and your manifold vacuum is at zero. The last line is completely false. If manifold vacuum and spark vacuum were in fact the same, then it would be correct, BUT THEY ARE NOT. They are the opposite. if they were the same, it wouldn't matter where you got your vacuum from for the distributor.

This is simply false. Check the vacuum at the spark port when you are idling. There is none. The vacuum increases as the throttle plates are opened.

I'll take some pictures for you. Do you understand the venturi principal? You need to in order to understand this. I'll be back in a few.

This is from the side bar on that same webpage. Read it, and you will see how they contradict themselves.

This picture shows where the vacuum advance port goes to. It is above the throttle plate, which means that manifold vacuum does not apply to it. As the throttle plates open, the air rushing by creates a vacuum in the carburetor. This is how the fuel is sucked from the bowls, and is how vacuum is created at the spark port. The more air that rushes by, the higher the vacuum.

If this doesn't explain it, I quit.

Take care,
~Chris

I dont think you are getting it now PKRWUD, those articles sound correct to me. At WOT it is the same vacuum above or below the throttle blades. Vac advance is only there to bring in more advance at part throttle wheras mech advance works all the way to WOT. Same total advance with or without vac advance at WOT.

Okay, let me clarify something, for the purposes of explaining this, I have referred to WOT when I should have been saying 2500 rpms, or whatever engine speed the full mechanical advance is tuned to comes in at. To be specific, yes, at WOT, the ported vacuum source is zeroed out, but prior to WOT, at common driving speeds, both the vacuum advance and the mechanical advance can and do simultaneously reach their peak, so if you don't account for this, you will have detonation problems. That was the whole point I was trying to make here.

I disagree about the article, though. They are mistaken.

I went to the carbed ford message board and the corral and so far I've got about a dozen people that say the same as I do and I've even got a test for ya. Hook your vacuum guage up to the spark port with a T and go for a drive. 0 at idle, goes up cruising at 2000 and a little more as you go up in cruise rpm. Put the throttle down a bit and the vacuum will drop to 3-4" meaning it's dropped some of the advance, go to WOT and it's at about 1", go ahead and try it.

PKRWUD, I already had my vacuum advance adjusted so that it wasn't pinging at part throttle, we could've ended this debate a long time ago. I did in fact know that the vacuum advance added timing at part throttle but that wasn't my problem, it was top end track performance at WOT that I was concerned with. Oh well, just glad it's settled.

yep ... now let the RPM come up and watch it go back up again ... its load dependent, its affected by both throttle position and RPM. Either way, whatever vaccum advance you have is added to the mechanical advance you have which is added to the initial advance, so you can't just discount vaccum advance. Well you can but you're gonna end up with more advance than you bargined for :eek:

Looks like everyone is on the same page I guess our wording and interpetations of posts just werent on point. Oh well.

Great! As long as you understand that you need to include your vacuum advance when calculating your total timing, my job here is finished.

:)

Take care,
~Chris