vac sec or double pumper?

[HPaddict]

hi, is a d/p carb really needed on a hot street car with an auto box, what if any gains are to be had with a d/p on a automatic street/strip car? i can understand maybe a manual car liking it through the gear changes but an auto you generaly hold flat to the floor all gears, i mean once all 4 barrels are open their open right?. i see alot of people go straight to a d/p holley on their streetcars i think they might be going the wrong way, surely a properly set up vac could go close to matching the double pumpers performance?. am i thinking along the right lines with this? thanks.

[shrinker]

hello; I assume your talking Holley here, The differences in the design of the boosters and the emulsion wells in a vac sec and a DP is a main reason for performance change. The carbys can be modified of course to be anything but they are designed from Holley with an intention in mind.

[HPaddict]

hi shrinker. yes im talking holleys. i am also talking modified vacs eg. down leg or annular boosters, air bleeds and circuts and pumps, squiters all the usual mods etc.

[shrinker]

Oh well then, at times on a dyno I grab the vac secondary shaft and force it open the power allways goes up so I guess DP is the way to go then. Its not really a fault of the carby but the tune of the primary barrels. I was working on a twin 4 barrel setup on a t bucket rod once and it was all wrong. The motor was all wrong etc and sometimes you could get 85 BHP at the wheels without the secondaries opening and other times you could get them to open at 35 BHP. Thats without doing anything to the carbys just driving it on the dyno and thats at the same revs and load. It was wierd. So at least a mechanical carby will give you full open power.
However a correctly setup vac will give good results for street use. DP is probably easier to do though.

[rookie]

Originally Posted by HPaddict

hi, is a d/p carb really needed on a hot street car with an auto box, what if any gains are to be had with a d/p on a automatic street/strip car? i can understand maybe a manual car liking it through the gear changes but an auto you generaly hold flat to the floor all gears, i mean once all 4 barrels are open their open right?. i see alot of people go straight to a d/p holley on their streetcars i think they might be going the wrong way, surely a properly set up vac could go close to matching the double pumpers performance?. am i thinking along the right lines with this? thanks.

I have minimal info on this but it may help plus here is a link to David's thoughts on some of the carb's on the market. The 100 mpg carb - does it exist?
My early drag racing with a 355 Chevy approx. 400 horse 255degree at .050 thousands cam 3000lbs. car 3.73 gears and a 2800 converter.
Regardless of what I tried with a 750 cfm Vac. or Dpmp. my times where identical.
On the other hand my friends 355 blower moto with twin 750vac's pikced up a good bit with dP's I dont have eggzac. #'s buthe was very

[Cammer]

Vacuum secondary carburetors are indicated for most street vehicles, off road vehicles, and other applications where excessive wheelspin is to be avoided. Trying to carve your way through the mountains in a 4X4 with a mechanical secondary carburetor could prove frustrating and dangerous.

Drag racing vehicles will generally be best served with mechanical secondary carburetors. Mechanical secondaries allow vehicle to be tuned precisely to overall setup.

Oval track and road racers use mechanical secondaries to allow best performance when coming out of corners.

Vacuum secondaries allow street vehicles to use a bigger carburetor and also provide excellent driveability.

Many feel they are smarter than carburetor designers and disconnect vacuum secondaries and stick a screw in linkage to "improve" performance. This is a mistake as vacuum secondary carburetors lack secondary accelerator pump circuits. Use a carburetor as it was designed!

My rule of thumb is to use vacuum secondaries on most street vehicles and mechanical secondaries on track vehicles.

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