Hot Rods Flathead Vacuum Advance?

There seems to be conflicting recommendations everywhere.

Just trying to figure out the best place to hook up the vacuum advance for my Pertronix distributor and dual Holley 94's.

The distributor says to use ported vacuum just above throttle plates. (I know I'll need to plug and re-drill new holes in the base plate.) Just the primary carb I ***ume.

Many people say to use manifold vacuum especially running two carbs.

What do you guys think?

On another note, I have taken the draft tube off and replaced with a pcv valve. Where would be the best place to attach given the dual carb setup?

 

The problem with using a manifold vacuum port is you get full vacuum (and advance) at idle. That may not be ideal, or even a good thing. Once the throttle blades begin to open up, ported and manifold vacuum are essentially equal, but at idle they are vastly different. You will have to determine if your flathead can idle smoothly, well or even at all with that much timing advance if you choose to use a manifold port. Your cam profile, compression ratio, chamber shape, etc. all play into that calculation. Flatheads typically prefer more advance just due to the longer flame travel that comes with the design.

Your PCV tube needs to be connected to a manifold port.

 

There was a discussion over on the C2 Corvette forum about this exact thing. To sum it all up, the consensus was manifold vacuum for performance, ported vacuum for economy.

 

Although I agree with most of what you say, this is wrong. flatheads need less advance, in the low 20's, all in by 2500 rpm. This is for a basically stock engine; highly modifed engines can use more, up to 30 degrees in some cases. (For comparison, an SBC likes mid thirties total.)

 

I looked it up while I was booking my HolidayInn Express stay.

That distributor has the option of being run mechanical advance only.
Simple, simple if it works and the mechanical advance is adjustable per the instructions.

Really it’s for modern ish 4bbls.
Since you are running 94s you may want to consider mechanical only advance. IE plug the vacuum at the distributor and plug the 94’s ports.

You can test the advance with a timing light by revving the engine after initial timing is set. This will confirm it works.

I would hook PCV to manifold port if you go mechanical advance only.

 

If it were mine, I'd try all three ways; no vacuum advance, manifold vacuums, and ported vacuum. It's not that hard, and you can let the "seat of your pants" make the decision. I guess maturity does have it's drawbacks; when I was 18, I already would have been driving up and down the street trying it out.