I was looking at Holly and EdelBrock carbs and some say vaccume 4bble secondaries and other are not what ar the pros and cons about these????

 

TTT

 

The only "true" vacuum secondary carbs are made by Holley & Demon (and I think Motorcraft built some for Fords). They have a vacuum diaphragm that actually pulls the secondary throttle blades open. The "pro" for them is on automatic tranny cars, mashing the throttle wide open won't cause a bog like it would on a similar mechanical secondary carb on the same application. The point at which the secondaries open can be adjusted by changing the spring that sits on top of the secondary diaphragm.

Edelbrock, Carter, q-jet are a hybrid - the throttle blades themselves are opened by the accelerator linkage, but actual secondary flow is controlled by an air valve, which opens wider as demand increases. They are very tolerant to WOT as well when properly adjusted.

 

I switched from a vacum secondary to a mechanical and I'm glad i did! The advantage to the mechanical is that you know that as soon as you a hit a certian throttle position the 4bbls are there. With the vacum secondaries if the vacum wasn't just right the 4bbl would never open. But remember this is just my personal experience.