Add Horsepower with a Holley HP Main-Body Swap

There are times when bigger is better. The original Holley carburetor has been around for a long time, and while it has benefited from improvements, there are certainly ways to make it better. A classic example is the 750 Holley mechanical-secondary carburetor (PN 0-4779). This veteran of the carburetor wars has long been the staple of more high-performance street engines than most other carburetors with perhaps the exception of its 750-cfm-vacuum-secondary cousin, the 0-3310.

Swapping the HP body onto a standard 750 double-pumper Holley turned out to be quick, easy, and worth some power.

Several years ago Holley introduced the HP series of carburetors that featured no choke housing, a radiused entry into the main body, and screw-in idle and high-speed air bleeds among its many improvements. Rated at a similar 750 cfm, these are highly coveted carburetors but also somewhat pricey. To remedy this situation, Holley now offers the HP main body as a separate, bolt-in conversion. At just over $125, this is much cheaper than buying a brand-new HP carburetor of the same cfm rating, and you get most of the benefits of the better main body. For those of you looking for a great deal, find a less desirable PN 0-4777 650-cfm or PN 0-4778 700-cfm carb and apply this same HP conversion to pump up either of those carbs to 750 cfm. Both of these smaller carbs use the same 11116-inch-diameter throttle plates as the 750 carb, making this swap an inexpensive way to increase a smaller carb up to 750-cfm flow potential.

The full-on HP carburetors also offer stainless steel throttle plates, high-flow metering blocks, and Dominator-style float bowls that are not part of the HP body conversion, but you do get the advantage of the HP main body's screw-in idle and high-speed air bleeds should you wish to experiment with fine-tuning those circuits.

We decided to put this idea to the test by converting a typical 0-4779 Holley over to the HP to see what would happen. The results were certainly positive with a surprising average torque increase of 12 lb-ft and an average horsepower gain of 10. It doesn't get much easier than that.

The Swap
The conversion is simple enough. All you have to do is disassemble the carb and replace the main body. The HP body is boxed with the accelerator-pump squirters already in place along with larger 72 primary and 84 secondary jets to improve the air/fuel ratio based on the increased airflow promised by the HP's contoured inlet. We swapped the main body in about 10 minutes.

The Holley HP main-body conversion comes with all the new gaskets you'll need to swap the body, along with a bowl, metering block, baseplate, bowl-screw gaskets, and a new set of baseline jets.	Both main bodies feature drop-leg boosters and 138-inch venturi diameters. The HP body offers a much cleaner radius into the venturis as well as screw-in idle and high-speed air bleeds.	We had to lightly file the top of the PCV outlet boss on the throttle plate in order to clear the secondary metering block and float bowl, but that was the only thing we had to do differently to install the HP body on the 750 carb.
This is the standard PN 0-4779-9 750 double-pumper complete with its somewhat restrictive choke housing. We jetted the carb to produce best peak power from the Ford 466.	With the HP converted carburetor back on the engine, we saw a measurable decrease in engine vacuum at wide-open throttle from 1.2 inches of vacuum to 1.1. This indicates a slight increase in airflow through the HP-bodied carb.