....and was wondering "This won't work with a Holley vacuum secondary". Correct me if I'm wrong, but the diaphragm that operates the secondaries works by creating a vacuum in the diaphragm chamber. Atmospheric pressure pushes on the "vented" side of the diaphragm, pushes it up, and opens the secondaries.

With boost, this process would not happen since there would be boost in the venturi (this is where the vacuum diaphragm gets it signal from) and the boost would keep the diaphragm closed. So one can only run a double pumper carb with a "boost hat" on top.

Is this a correct statement?

 

it mite work in a box

 

It might, but I never cared for totally enclosed carbs.

 

I have TurboChargers by Hugh MacInnes somewhere and it tells exactly how to set it up.. lemme find it for you guys.

 

Well i found that quicker than i thought I would but its still quite a bit to type...here goes.

Diaphram-operated carburetors usually have an interlock to help pull the secondary throttles closed as the primaries close. The is type of actuation is used on the Holley 4150/60-series carburetors and is easily modified for turbo use.
The interlock between the primaries and secondaries is removed completely. The diaphram assembly is taken apart and the spring moved to the opposite side of the diaphram. the linkage is reworked so pressure on the diaphram opens the secondary throttle and the spring closes it.
With this modificaiton, the secondaries open only when the engine is supercharged. Consequently, it is possible to select main jets and power valves for the primary to give the ideal fuel/air ratio of naturally aspirated conditions. Secondary system jets can be sized for a rick condition more compatible to the high intake-manifold pressures, or boost.
When a carburetor is modified this way, there is no mechanical linkage between the primary and the secondary butterflies. The engine operates using only the primaries until the intake-manifold pressure overcomes teh spring in the diaphram. this opens the secondary butterflies.
If no other modifications were made, the secondaries would not close when the driver's foot was removed from the accelerator pedal; the engine would continue to run supercharged on the secondaries alone. To prevent this, a small three-way Mead Valve is placed in the sensing line and mounted on the carburetor.
The Mead Valve is actuated only when the primaries are fully opened. As soon as the primaries close, the valve vents the secondary diaphram to the atmosphere; the secondaries close immediately. This Mead valve is the same size and shape as a standard microswitch and is actuated in the same manner.
This setup gives a different feeling to the car than the standard setup. the secondaries will not open until manifold pressure reached about 1 psig. when this happens, there is not only a sudden jump in manifold pressure but a definite surge in power.

Forgive any typos guys, and if anything doesn't make sense, its on page 57-58 and I can fix any errors.

Cheers

 

check out the blow thru list on yahoo and/or the carb section on turbomustangs.com

lots of successful combos out there

 

you can make a vacuum secondary carb work, but its alot of modifications and is usually not recommended. Its much simpler to modify a double pumper since it doesn't require vacuum. And when you have boost, vacuum goes away.

 

Why not use a mechanical secondary carb?