2. Holley HP 750-cfm mechanical secondary

The next carburetor we bolted on our 383 was a Holley 750-cfm unit with mechanical secondaries and both front and rear accelerator pumps (hence the term double-pumper. And while the extra accelerator pump and mechanical secondaries of this unit won't necessarily help peak dyno numbers, the 750's larger cfm rating means our big-block should get more air and fuel, and make more power than the 650 did. Our first pull with the 750 showed definite improvement, and the best dyno pull with the 750-cfm carburetor installed netted 391.6 lb-ft of torque and 387.6 horsepower.

3. Holley 780-cfm vacuum secondary

Stepping up a little in size, we next bolted a Holley 780-cfm 3310 series four-barrel onto our 383 to see if it would further improve our engine's torque and horsepower. Vacuum secondary carburetors are known to be more economical than carburetors with mechanically actuated rear throttle blades, and arguably improve drivability as well. And while neither of those factors matter much while attempting to achieve the best power and torque on the dyno, they can be important once the engine is installed in a car. We figured the 780 would perform about as well as the 750 and it did, but at 381.3 lb-ft torque was down by ten. Horsepower was also slightly lower, at 380.5, so our engine liked the 750-cfm Holley HP carburetor a little better than the 780. Are we verifying that you can over carburerate an engine? With 30 extra cfm for this portion of the test, it seems so. But one drawback of this carburetor is the lack of a rear metering block, making precise air/fuel adjustments more difficult. So without the tunability of rear metering, we'll have to see what happens when we go larger.

4. Holley HP 850-cfm mechanical secondary

Taking the next logical step, we decided an 850-cfm carburetor may just be what we needed to break the 400 horsepower barrier with this engine. Bolting a Holley HP series 850 to our engine, we made a couple of pulls and adjusted jetting for the proper air/fuel ratio. This carburetor improved torque and horsepower at every rpm, and sounded crisp and responsive. Even so, it fell a little short of our 400 horsepower goal, but it reached the one horsepower per cubic inch benchmark by making 393.2 lb-ft of torque and 388.2 horsepower. Our data indicated that this engine liked the 850-cfm unit, and we'd already gained more than 30 horsepower over our baseline with the 650 installed.

5. Holley 850-cfm mechanical secondary

We had another Holley 850 on hand, so we decided to see if there was much difference between two models of the same size. With a list number of 4781, this 850 is a non-HP version, and has a choke horn installed whereas the 850 HP doesn't have a choke horn. Otherwise, these carburetors are very similar, both featuring dual metering blocks and accelerator pumps. After tuning this carburetor's main metering circuit through a series of jet changes, we found that 87 jets in the front and rear, with the power valves blocked off, made the most power. On Auto Performance Engine's dyno, our 383 big-block with the Holley 850 double-pumper made a best pull of 392.5 lb-ft of torque at 4,500 rpm, and 401.4 horsepower at 6,100 rpm. Since our 383 seemed to like it each time we bolted a larger carburetor on, we decided to see if we could over-carb the engine with a Dominator.

6. Holley 1050 mechanical secondary (modified by Quick Fuel Technology)

Knowing that we'd likely reached our point of diminishing returns, we decided to test a 4500 series Holley 1050 Dominator that had been modified by Quick Fuel Technology just to see what would happen. For our mildly modified, relatively small-displacement big-block, a 1050 Dominator is a lot of carburetor, but since our results so far showed this engine liked larger carburetors, we decided to give it a try. Of course, the 4500 series has a larger flange than our intake, so we utilized a two-inch tall tapered spacer to make it fit. After a little tuning, it was apparent that although the 1050 ran well on our engine, it didn't make quite the power of the 850. On its best pull during this test, our 383 made 393.4 lb-ft of torque and 398.9 peak horsepower, indicating that while the 1050 is a good choice for many big-blocks, it was just a little much for our 383.

Results

The results of our testing make it apparent that the 383 we tested liked the 850-cfm size carburetor better than the others tested. And while the 1050 Dominator did seem a little much for the 383, it certainly performed well, making nearly the same horsepower, and slightly more torque, than the 850, but is a substantially more expensive carburetor. As a result of our testing, we determined that larger carburetors generally do make more power on a big-block like ours, and that proper carburetor tuning is an important factor as well. And, while we were testing for peak power and torque, our results show that even with a smaller carburetor, the engine made good power. Since unleaded gasoline is still over three bucks a gallon and smaller carburetors are known to be more economical for daily driving, you may want to choose the carburetor for your car based on economy, not peak power. So when is too much carburetor too much? When the dyno numbers stop going up!

Costs

PartCost
Carburetors Tested$100 (used) to $879 (new)
Dyno Time$650 Per Day
93 Octane Unleaded Fuel$3.56 Per Gallon (varies daily and by location)

SOURCE
Auto Performance Engines
1479 Berkley Rd.
Auburndale
FL  33823
863-967-8781