Vacuum Secondary Versus Mechanical Secondary Carburetors

[toobroketoretire]

You may have noticed every car in the world came from it's factory with an AVS or vacuum secondary carburetor. The reason being those styles of carburetors produce the most power thru the entire rpm range that a street engine operates in. Mechanical secondary carburetors (aka double pumpers) on the other hand are intended solely for high-rpm track racing where engine speeds never drop below 4500 to 5000 rpm. When used on a street engine a mechanical secondary carburetor becomes extremely inefficient at lower rpm's because the venturi velocity drops so low. In other words they produce "too much too soon" and kill the power output. But yet they have become immensely popular over the last few years among those who don't understand how carburetors work. By limiting the amount of throttle opening the AVS and vacuum secondary carburetors keep the venturi velocity at it's highest speed which produces the finest fuel atomization and power. The same reason why every super bike uses CV (constant velocity) carburetors.

 

[toobroketoretire]

Track Racing Carburetors On Street Engines

I wish Holley would say "Not Recommended For Street Use" in their advertising of their Double Pumpers so the buyers would at least know the truth about them. Double pumpers are solely intended for track racing engines that run 4500 to 5000 rpm all the time and they have "4 corner idle screws" for those racing engines that idle at 1500-2000 rpm. The reason being to idle an engine that fast without getting into the main jet circuit an engine needs all four butterflies slightly open for enough air and therefore need 4 corner idle screws. Street engines that idle below 1000 rpm have no need for 4 corner idle screws so they require standard Holley carburetors.

People continually put track racing carburetors onto their street engines then wonder why their engines runs so bad.

 

[toobroketoretire]

If you are hell-bent to run a double pumper carburetor I suggest looking in E-Bay as there are literally hundreds of listings for them at about half their original prices. People buy them on the recommendations of their uh "racer" friends then find out the hard way how poorly they perform on street engines. An engine can only swallow just so much air at low speeds and that's why none of the world's engines ever came with mechanical secondary carburetors.

 

[BigJimZ16]

If you are hell-bent to run a double pumper carburetor I suggest looking in E-Bay as there are literally hundreds of listings for them at about half their original prices. People buy them on the recommendations of their uh "racer" friends then find out the hard way how poorly they perform on street engines. An engine can only swallow just so much air at low speeds and that's why none of the world's engines ever came with mechanical secondary carburetors.

Except all of the Quadrajets... Everyone of them has mechanical secondaries! By definition...

 

[toobroketoretire]

Except all of the Quadrajets... Everyone of them has mechanical secondaries! By definition...

Before the advent of the AVS and vacuum secondary carburetors all of the 4-barrels had "mechanical secondaries" but they all had an air valve above the secondaries to prevent "too much too soon". Now days the 4-barrels use the latest "air valve" technology that prevents too much too soon.

Have you ever noticed how ALL dyno tests involve "ramping the throttle up" very slowly? Dyno's provide a load for the engine to work against and if you suddenly "snapped" the throttle open the engine would fall on it's face. So if you look closely at the carburetor during a dyno test you'll see the throttle is only 1/4 to 1/3 open at the 3000 rpm level.

Dyno tests do not duplicate actual driving conditions as none of us "ramp our throttles up" when increasing our speed in drag or track racing. Instead we stand on our throttles and that's where the mechanical secondary carburetors fail at low road speeds in a higher gear. The AVS and vacuum secondary carburetors will tolerate a full throttle at a low rpm/higher gear but a mechanical secondary carburetor won't and that's why none of the world's production engines ever came with them. The Holley "double pumpers" do have an advantage in off road high rpm track racing events but not on street engines operating over a wide rpm range.