Can someone keep me from guessing. What's the difference & what should be hooked up to which?? PCV, PB, vac adv? At this point I'm guessing that to access manifold vacuum you need a plug into the manifold??

Thanks
Len

 

Brakes and pcv always get manifold vac. The distributor gets whatever works best for your combination imo. There was a lengthy thread on manifold vs ported for the vacuum advance a week or so ago...

Mike :cheers:

 

The Chilton's repair manual has diagrams for your vacuum routing if you want to set it up with stock configuration. You may want to take a look at the Tech-Tips where I've posted an image of a quadrajet with arrows pointing to the ports where the vacuum lines were originally connected before I did some clean up.

basically, manifold vacuum is high at idle, and decreases as you open the throttle. Ported vacuum is low at idle and increases as you open the throttle. The different smog components require different amounts of vacuum signals at different times. If the components are connected directly to the carb (without any TCS) then PCV and PB require manifold vacuum. The distributor vacuum advance requires ported. There are several ports available...I tested them and determined that the one in this image gives me the best advance signal:

 

Search under my name (hudman) both current and archived posts.

I have numerous questions posted and some great answers from the Forum. Most of mine are related to the vacuum advance issue and which vacuum source to use.

My car runs better using ported vacuum.

To answer your question, you can get manifold vacuum from the carb. You do not have to get it from the manifold.

 

While most after-market carbs have both ported and manifold vacuum sources, some smog carbs only have ports for ported vacuum. Pre-smog era carbs and many race carbs don't have ports for ported vacuum. Post some information about your carb and someone will probably be able to tell you what ports it does have and where they are.

Ported and manifold have the same source (manifold) and they function the same except ported doesn't employ until the throttle is opened. Beyond that point, there is no difference. When emissions regulations came into being in the mid `60's, fuel air mixtures were leaned out to reduce hydrocarbons at idle and initial timing was cut back to reduce nitrides of oxygen (I think I have that right) at idle. Vacuum advance was still benefitial to economy and drivability so they just delayed it so that it didn't employ until after idle by using a ported source.

IMO, there is no downside (other than idle emissions) to using maniflod vacuum but there are some upsides. More advance at idle means more torque which, in turn, makes for a smoother idle. This is especially benefitial with a large cam. In many cases, the greater advance at idle will also result in lower temps.

 

Vetterodder:

That's a good description of what emissions did in the 60's...but was that for CCS (controlled combustion system) or AIR (air injector reaction)? You'll probably recall the CCS really leaned out the mixture, robbing the vehicle of power. AIR permitted richer burns and therefore accompanied the special high performance vehicles of the period.

 

I'll be using a Holley 830 DP, no choke, mech sec. right now out of the box it has no Ports! :eek: I ordered a base plate from Proform which should have the ports I need. They'll all be in the base, so does that mean they'll be "manifold"??? :confused:

 

You may want to post a new thread asking for someone with the same carb to see how they have their system set up. "manifold vacuum" should be a source which has an opening in the intake manifold and not in the carburator venturi. "ported vacuum" should be a source which has an opening in the carburator venturi. The main difference is that the manifold vacuum is not effected by air flow through the venturi while ported does.

With throttle plates closed at idle there will be the largest amount of vacuum available in the intake manifold. At the same time there will be a high velocity/low volume flow of air through the carb venturi. Openings on the inside of the venturi which are positioned below the throttle plates will have an initial vacuum equal to the manifold vacuum; however, the air flow past the openings will modify the amount of vacuum present in the port depending on how the opening is shaped and angled in orientation to the air flow and venturi wall and is controlled by the velocity of the air flow past the opening. As the throttle is opened the manifold vacuum will drop; however, the air flow velocity will drop as well. In this way a vacuum source can be customized by it's position, angle, and orientation in the venturi.

So, just because the port is located in the base of the carburator doesn't mean that the vacuum signal will be plain manifold...I'm sure the base plate will have a useable ported vacuum signal for distributor advance. You may also want to get a vacuum gauge and test the ports until you find one that has little or no vacuum at idle, but increases when the throttle plates are opened.

 

Probably. Your 830 is engineered as a race only carb and that's why there are no vacuum ports. I don't know if the Proform base will be the same as Holley bases that have vacuum ports but Holley bases will have manifold vacuum ports on the base. The ported vacuum source is on the primary metering blocks.

 

Thanks for the great info guys. I'll keep you posted once I get the base plate----it's B/O till May 5. :cry