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Add Horsepower with a Holley HP Main-Body Swap

More airflow out of the same-size carb is easy when you swap perform a Holley HP Main-Body Swap.

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The Test
We figured the best way to test this was on our 466ci big-block Ford that we had on the dyno for our big-inch Ford story elsewhere in this issue using Jim Grubbs Motorsports' SuperFlow engine dyno. We have to admit that the 750-cfm carb was probably a bit small for this free-breathing Ford, so while this is still a good test, increasing the airflow potential through the carb with a cleaner main body would certainly deliver more of an advantage than you might see on an engine with less airflow and power potential. As an example, if we had used a 400hp 302 small-block Ford as our test mule, the difference in power between the two carb bodies would probably have been less.

With that said, we were impressed with the results we saw on our big-block Ford. A quick peak-to-peak horsepower evaluation revealed a minor 4hp difference--literally less than 1 percent. But once we compared the overall power curves, we saw major increases just past peak torque including a solid 14 lb-ft of torque at 4,600 and as much as 17 hp at 5,400 rpm. But the best news is the average power increase of 12 lb-ft and 10 hp across the entire test range.

To be fair to both carburetors, we did not use the air turbine to measure airflow since the particular unit JGM was using offers a slight restriction to airflow. Without the aid of cfm numbers, we looked more closely at the fuel flow and discovered that the power increases achieved by the HP conversion in the midrange could be attributed to roughly 5 percent less fuel than with the standard Holley carb. We had already increased jetting on the original 750 in order to improve power, yet the HP carb made more power with less fuel in the midrange. We'll attribute that to better mixture distribution in the engine from the HP's radiused inlet into the venturis of the carburetor. This may also have improved mixture distribution in the intake manifold as well. Either way, the HP made more power with measurably less fuel.

Test 1 was the baseline test on the Ford 460 with the PN 0-4779 750 carb, jetted to make best power. In Test 2, the only change was to the HP main body. The fatter jetting included with the HP carb was already in place so this effectively leaned out the air/fuel ratio on the HP conversion slightly.

3,800 534 386 546 395 12 9
4,000 540 411 550 419 10 8
4,200 544 435 555 444 11 9
4,400 538 451 549 460 11 9
4,600 523 458 537 470 14 12
4,800 518 473 528 482 10 9
5,000 503 479 520 495 17 16
5,200 492 488 508 503 16 15
5,400 472 485 489 502 17 17
5,600 468 499 468 499 0 0
Avg. 513 456 525 466 12 10
Peak 544 499 555 503 11 4
Power/ci 1.16 1.07
Parts List
Holley 750-cfm carb mech. 0-4779C $397.88
Holley main body 134-300C 125.39
Holley 750 HP carburetor 0-80528-1 629.88
Holley 650-cfm carb mech 0-4777C 327.88
Holley 700-cfm carb mech 0-4778C 349.88
Holley Performance Products
1801 Russellville Rd.
Bowling Green, KY 42101
KY  42101
Jim Grubbs Motorsports
28130 Crocker Ave.
Unit 331
CA  91355
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