turtlevette

I've been through this whole thing with Barry and bigblok before. Its a waste of time.

Tim H

yes a waste of time.
Just hook it all back up and don't fool with it or you will have the car in a million pieces and driving the SUV on cruise night!

...Roger...

I read the article but I dont think it is correct.Ported vacuum(on the carbs I have worked on)comes on as you open the throttle to a point and then its the same as manifold vacuum.It even drops off when the throttle goes to wide open.If it was true venturi vacuum it would continue to climb with the RPMs and would be uneffected by wide open throttle.One other thought-if you set your total timing to 36 and your vacuum had 15 and you were hooked up to true venturi you would have 51 degrees.I honestly dont believe its truely venturi vacuum.Maybe Lars can chime in and explain it to all.
edit-Here is an article that I found that Lars wrote that explains the different vacuums.
http://www.lbfun.com/Corvette/Tech/v..._explained.pdf

73, Dark Blue 454

A ported vacuum source is pulled from above the primary throttle blades.

A manifold vacuum source (or full time) is pulled from below the TB's.

If you're not sure which port on the carb is which, hook a vacuum gauge to it. At idle, you'll have a very low reading with a ported source and if you've accessed a manifold source, you'll have a very high reading on the vaccum guage.

The vacuum signal goes down, not up, as you open the TB's (step on the gas). This will reduce the timing in the distributor to prevent pinging or knocking on acceleration.

As stated above, manifold vacuum is the preferred source,..you'll have better off-idle performance and a cooler running engine, especially in stop and go traffic. Also, if you suffer from dieseling, manifold vacuum will allow you to shut your TB's more. Dieseling is almost always caused by the primary TB's too far open at idle.

Hope this helps,..pretty simple system,..but often misunderstood.

TheSkunkWorks

Does this sound right, BarryK? Hook your advance can to manifold vacuum for best performance, unless that causes ping. If that ping can't be cured by changing cans, then go with port vacuum, which will result in less vacuum advance.

noonie

You can also control the amount of vacuum advance the can gives. Going to ported won't help pinging off idle.

Using a dyno is the ultimate solution for setup, but to help understand this vacuum and how it affects your car you can run a gauge inside as you drive under different conditions (ported or vacuum).
I've used one inside for years and it's a valuable tool at times.

BigBlockk

Ported vacuum is not created by the venturi. Ported vacuum is taken from just above the throttle blades. At idle there is no vacuum at this port. As you open the throttle the ported (timed) vacuum port is exposed to manifold vacuum. Ported vacuum is a delayed form of manifold vacuum. This vacuum is created by piston displacement in the cylinders.

Venturi vacuum is created by air flow moving through a restriction (venturi). Even in the most sensitive carburetors the main venturis don't begin to function (produce enough vacuum signal to begin to meter fuel) until about 25 of 30 MPH. Up until this speed the engine is running on the idle and transition circuits. If you're chugging along at 1200 RPM in high gear you have ported vacuum but the main system venturis have not started up yet. There is no venturi vacuum yet. Also, in a Quadrajet carburetor venturi vacuum only does one thing, provide the power to meter fuel.

THERE IS NO VACUUM PORT FROM THE VENTURI TO THE OUTSIDE OF THE CARBURETOR! IT IS IMPOSSIBLE TO USE VENTURI VACUUM FOR ANY OTHER PURPOSE IN A QUADRAJET CARBURETOR!

Ported (timed) vacuum is created by piston displacement, not the venturi. No matter how much some of you may want it to be the venturi, it isn't!

Get a book and study up on it.

BigBlockk

Later.....

BarryK

usually full manifold vacuum will give the best results but again some cars still seem to prefer ported for whatever reason. Best to try both and see what works better for YOUR car but in the majority of cases full manifold vacuum works better and it works better on both of my cars.

Usually pinging is from too much mechanical advance and a surging while cruising is caused by too much vacuum advance.

fl_rider

My 350 /290 with an Edelbrock 1806 starts, idles and runs fine with the vacuum advance line hooked up to full vacuum.

Chris B