I rebuilt the engine but confused on the vacuum line going to the vacuum distributor. Looking at the front of the 4 barrel quadrajet, there is a 'Ported vacuum port' on the passenger side, and a 'Full-time vacuum port' on the driver side. Which one should the distributor connect to? Right now I have the 2 speed auto transmission modulator hose connected to the 'Full-time vacuum port' and the distributor to the 'Ported vacuum port'. Does anyone know if this is correct????

Francis

 

You can use either port. Ported is the way the factory had it, however it will run cooler hooked to full. All you need is a T for the line that the modulator is hooked to.

 

I appreciate the help on the vacuum lines. I was not sure if the full vacuum would alter the advance on the distributor. appreciate

Francis

 

Full vacuum will cause an increase to full advance at idle which will increase your idle speed over ported vacuum. You will have to readjust your curb idle speed after connecting it that way.

 

If you decide to use manifold vacuum instead of ported vacuum, you will need to reset the initial timing.

 

Why?

 

Because the timing at idle is the sum of initial timing and vacuum advance. Ported vacuum is zero at idle, so total timing at idle is only what is set at the crank. If you change to manifold vacuum for the advance, that is maximum at idle. Depending on the vacuum canister, this adds between 8 and 12 degrees of timing at idle. Couple this with the mechanical advance, this can cause pinging under light loads.

 

Once you leave the idle circuit it drops. The reason to use manifold vacuum is to get the higher advance which helps the engine run cooler. If you drop your initial, you also drop your total defeating the purpose entirely.

 

Agreed, but if the car starts pinging, you have to change something. The reality is that the engine neither knows nor cares whether the timing is controlled by initial, vacuum, or mechanical. It only cares that the timing is correct for a specific cylinder pressure and RPM. This is one of the advantages of a totally electronic distributor - you can create the optimal advance map vs. RPM, throttle position, and any other variable that matters. With vacuum and centrifugal systems, you are limited in what you can do. Often you have to make compromises in one operating condition to stay within bounds at another due to the constraints of the system.

 

As long as you generally stick to the stock initial timing spec and fuel octane requirement, running the stock vacuum can will not cause any pinging what so ever. As far as points and HEI systems, you can fine tune these systems infinitely to accommodate most engine setup performance requirements.

This is a good read and lists many of the stock vacuum can specs:
http://www.crankshaftcoalition.com/w...ance_Specs.pdf

 

The problem is the cat urine that is passed off as gasoline today...