The Race Demon is a pro-level...
The Race Demon is a pro-level carb with changeable air bleeds for the low- and high-speed circuits (arrows), making tuning choices nearly unlimited.
With the 650 carb cutting the intake vacuum in half, a big chunk of the potential gain from reducing intake restriction had been realized, but we felt there was more to be had. Our next move was to step up to the popular 750 size, a favorite carb configuration for street/strip engines and the size closest to the cubic feet per minute given by the formula. Again, we went with a Speed Demon series of carburetor, but this time used a mechanical secondary unit (a vacuum secondary version is also offered). Unlike the 650, the choke plate is omitted from the mechanical secondary 750, but the carb has provisions to add a choke.
The 750 did its job: added horsepower over the entire test range, although the intake vacuum was down slightly from the previous test with the 650. The stats came in at 552 hp at 6,000 rpm and 552.5 lbs-ft at 4,500, pushing our engine beyond the magic 550hp number with a gain of 9 hp and making peak horsepower 200-rpm higher. Airflow readings showed the 750 was allowing the 440 to ingest 4-cfm more air than in our previous test. Output down near the bottom end of the test range was never significantly off from the numbers turned in with the smaller carb, and after 3,500 rpm, the 750 had a decided advantage.
So far, our 440 responded favorably each time the carb size was stepped up. Next to find its way onto the manifold was the Speed Demon 850 mechanical secondary, a carb size that has long been a favorite of more serious street runners and drag cars. The 850 is the carb that most closely fit the manufacturer's recommendation for a high-output big-block such as ours. The 850 mechanical secondary had been our choice for carboration in previous dyno tests of engines in this performance range. How would it stack up against the smaller carbs?
We dialed in the jetting and it wasn't long before the big carb showed its worth, belting out 560.4 hp at 5,900 rpm and making 557.3 lbs-ft of torque at 4,600 rpm. The bigger carb bagged another 8 hp up top, while not materially detracting from output at the lower part of our test range. In fact, midrange torque was up compared to the 750. Manifold vacuum was down significantly with the 850 carb, and according to the flowmeter, the 440 was able to pull in another 8 cfm of air. The 850 is a good match for this engine.
The Race Demon
Our next carb was the 950 drag race calibrated Race Demon, a professional-level carb. The Race Demon isn't cheap, but it has the features to make it a serious performance piece. Unlike the Speed and Road Demons, the Race Demon has replaceable air bleeds on both the high- and low-speed circuits, for super-fine-tuning of the fuel curve. The version we ran also had annular discharge booster venturis, which use a series of discharge holes around the inside diameter of the large booster, dramatically increasing the booster signal and atomization for such a large carb. This helps boost low-end torque, where the large size would often penalize output and response.
The 950 boosted peak output to 565.7 hp at 6,100 rpm and torque to 564.6 at 4,600 rpm, both sets of numbers up on the 850's results. The engine was indeed taking in more air with the 950 carb, with air consumption edging up by another 3 cfm. We were clearly reaching the practical limits on carb-size requirements for this street combo, but it seemed the engine would take in more air with a spacer, better using the large carb. With the 950's annular boosters, we figured even with the larger plenum volume, the 440 should retain its outstanding torque output at the lower end.
We sandwiched a Barry Grant 1-inch laminated spacer between the Race Demon and our M-1 intake and prepared for another pull. The engine definitely liked it, with power now climbing to 571.8 hp at 6,000 rpm and torque also going up, with 568.5 lbs-ft now on tap at 4,500 rpm. In fact, with the spacer, our 440 actually gained torque throughout the curve; credit the Race Demon's annular boosters for this.