What is the difference between a vacuum secondary and a mechanical one and which brand makes the best carb?

Thanks

 

Mechanical secondary carbs..The secondary throttles open when the accelerator pedel is moved beyond a certain point. IE- when the pedal is mashed at a dead stop, all 4 throttle blades are open. Mechanical secondaries tend to use more gas.

Vacuum secondary carbs- As the motor RPM goes up and vacuum signal drops ( i think when it drops) the secondaries start to open. When you mash the gas pedal from a dead stop just the front blades open and as you accelerate the secondaries slowly start to open. They tend to be more gas friendly and are usually used on low to mild performace cars and automatics. Mechanical secondaries tend to make better usage on radical motors.

Hope this helps.

Aaron

 

Vacuum secondaries rely upon engine vacuum to "pull" open the secondary barrels. Usually they have some type of spring that the engine has to work against to draw the secondaries open.

In the Holley vacuum secondary models the secondaries only open via vacuum in the carburetor working against a spring to allow the secondaries to open. In the QJet and Edelbrock versions, the throttle cable opens the secondaries in a mechanical fashion, but they have an upper air valve that stays closed until the engine can produce enough of a draw through the carburetor to overcome the spring trying to hold it closed. As the secondaries open on the QJet and Edelbrock, they move metering rods that regulate the amount of fuel allowed in. In the Holley they just have a venturi and a set jet size that regulate how much fuel the engine sucks in.

Mechanical secondaries are only on the Holley carbs or the Holley clones (Barry Grant, Demon). They use throttle linkages to progressively open the secondaries. Again jets regulate how much fuel is drawn into the air-stream going in to the engine. They also have a second set of accelerator "squirters" to give an additional shot of fuel to cover up the lean mixture when the throttle starts to open.

As far as which is best, that's an opinion matter.

I think that for a street driven car the vacuum secondaries are the preferred choice. They offer better mileage and use engine demand and not throttle position to determine if the secondaries open or not.
My opinion is the QJet is the best for the street because it offers a set of smaller primaries for general use and large secondaries for when you really need it. My second choice would be the Edelbrock. I like the air valve design for metering the mixture much better than the set up the Holley vacuum secondary carbs use. The Edelbrock also seem to need less adjusting and general attention.

Holleys by far offer the largest amount of parts and adjustments, but that also means you may be adjusting and fixing more often.

 

This is actually a very educational thread. I don't know too terribly much about carbs but i will say that when I switched from an Edelbrock to a Demon I gained some power that I could definitley feel. I know a bunch of guys here are using Demons and seem to be pretty satisfied with them. :thumbs: