I know this has been talked about before, but I would like to know ya'lls preference. Do you use ported or manifold vacum for your vacum advance? After reading previous posts I cant come up with a clear reason for or against either.

 

I'm not an expert by any means.
My rule of thumb is to go for recommendations by make of vehicle or carb.
Both my Ford trucks use ported vacuum.
I think also if you use different dist. you may have to use mainfold vacuum.

Just my opinion. Trav.

 

Ported. I cannot get the same idle every time with manifold vacuum. It seems like any little change in operating condition makes a big change in RPM. Drop it in gear and the load reduces the vacuum which reduces the advance which reduces the RPM which reduces the vacuum which reduces the advance which reduces the RPM which....... Or, set it in gear and the RPM in neutral is way too fast. Let the A/C compressor come in and you'd better have the idle solenoid working well.

But, if I put it to ported vacuum all those things go away.

 

Thank you, Gary.
I never tried mine on manifold, but makes sense.
BTW, on the other deal your working on; Save all the shavings. Scrap is about ten cents per lb..Trav...

 

You pretty much have to use a ported vacuum source if you have a automatic. Unless you found or modified a dist that didn't have as much vacuum advance, and I don't know if that would even help.

But there is no harm in trying it, you will quickly find out if it works for you or not.

 

If your engine is stock then use ported.

If it has an aftermarket cam with enough duration to bleed off a bit of compression at idle, then you can try full manifold vacuum because it can increase idle quality and basic drivability.

 

Hey, this is unrelated to the issue, (If ya'll don't mind )but what is Manifold vacuum and ported vacuum, and how does it affect the distributer?

 

Manifold vacum comes from the intake manifold, ported comes from carb above the throttle plates. You have lots of manifold at idle, and none under wot. Ported is very little if any at idle but increses as rpm goes up.

 

A ported vacuum source will pull it's vacuum from slightly above the throttle blades within the venturi of a carburetor. When the throttle is closed, at idle.. next to 0 vacuum will be drawn through it.Then when you push on the throttle, the blade will be above the hole allowing the engine to draw vacuum from it.

On the other hand, full manifold vacuum is.. full manifold vacuum. It can be drawn from the intake or below the throttle blades (usually in the baseplate) of the carburetor. You will have vacuum even at idle, while with ported vacuum you will not have vacuum at idle. Once you push on the throttle, exposing the ported source.. both manifold and ported vacuum will have nearly the same reading.

It affect/effects? the distributor because you have a vacuum hose running to it if you have vacuum advance. The vacuum advance is to add ignition timing to the engine when it is under light load. The lean mixture under light load burns slower, so by advancing the timing it sparks the mixture earlier to potentially increase the engine efficiency. Neither full manifold vacuum or ported vacuum will show a vacuum reading at 100% throttle. Vaccum advance does nothing at full throttle.