Which port for vacuum advance?

[Omie01]

This is the bottom of my carb, which of the 2 ports on the left do I want to hook my vac adv up to?

 

[Canted 393]

Do you have any ports above the throttle blades?, that's where msd wants to hook up.

 

[TommyK]

The one on the left goes to a channel which is exposed below the throttle blade. That is full manifold vacuum. The one on the right should lead to a hole above the throttle blade. That is ported vacuum.

 

[will e]

It looks like the two on the bottom are both manifold vacuum. It's hard to tell if the one on the right is for sure but the left one should get you manifold vacuum.

Above the throttle blades you will get ported vacuum. The big difference between the two is at idle. There is no vacuum signal from the ported vacuum at idle.

Choosing between manifold and ported is one of those unsolvable internet questions. I would start with ported and see how it works for you.

 

[Omie01]

Yes, the problem is these are the only 2 small ports on the carburetor. Another question, do I have to run vacuum advance? This engine will be a cruiser and a track racing style engine. Not too concerned about gas mileage as long as it runs good!!

 

[Jeff73Mach1]

No, you don't have to, but you are giving up some flexibility. Without vacuum advance, your distributor is advancing based upon centrifugal weights only. Therefore, it does not adjust based upon load. So when you get into the gas and vacuum drops, your timing doesn't change to compensate. Tuning with weights and mechanical advance curves only, for a street driven car will be difficult and you'll be making compromises that ultimately are unlikely to make you happy.

the inside port (closest to the center) is ported vacuum as long as you are using a proper fitting gasket.

 

[mustang7173]

Omie01,

What type of distributor do you have? Vacuum advanced or mechanical? From what I have read lately, the vacuum advance would be the ideal choice for street engines.

mustang7173

 

[barnett468]

Yes, the problem is these are the only 2 small ports on the carburetor. Another question, do I have to run vacuum advance? This engine will be a cruiser and a track racing style engine. Not too concerned about gas mileage as long as it runs good!!

There is no port on the right [passenger] side metering block?

As was mentioned, you do not need vacuum advance . it is not functional under moderate to hard throttle anyway and should not be used on a track car . the 271 hipo didn't have one and it ran just fine.

you should set your timing curve for your particular setup though which is fairly easy and i can tell you one method i use to do that . it often requires changing one the distributor springs if the dist is stock.

.

Yes, the problem is these are the only 2 small ports on the carburetor.

If those are the only vacuum ports then connect it to the one on the right as TommyK said.

Also, make sure that port does not have vacuum at idle . If it does., your front throttle blades are open to far and you might have to advance the timing or open the secondaries a little or drill holes in the front plates etc to get them to close far enough to use it . I would do this even if you don't use a vacuum advance so you are on the idle circuit instead of possibly into the transition area . this will make it easier to set the idle etc.

.

 

[72HCODE]

easiest test when you are still not sure,, hookup a vacuum gauge to the ports. idle the car, the port that gives you a 0" vacuum reading at idle is ported.

the only difference between ported and full manifold vac is no vacuum at idle.

this keeps the Vacuum advance on the distributor from coming on at idle.

no vacuum advance at idle means the timing stays low and on mechanical which should not come on at idle rpms

lower Timing at idle means higher exhaust temperature, this burns off more pollutants when sitting in traffic and is a form of emissions control.

you can run on full manifold vacuum but you need to recurve the distributor and timing a little to adjust it otherwise you will just have too much timing at idle or just off idle and you could have detonation issues depending on engine setup.

 

[Omie01]

I have a motorcraft recurved dizzy w/ pertronix 2 unit. I think I made a series of mistakes during tuning. I talked with Pro Systems yesterday and he walked me through initial tuning, if I still have issues I will get back to everybody. Stay tuned!!

 

[will e]

Stay tuned!!

Pun intended?

 

[midlife]

Speaking of puns...I prefer Madeira port myself...