Testing 1, 2, 3...
Our testing was simple. All we did was swap each carb onto the engine, warm it up, make a few dyno pulls, and then note the air/fuel ratio and the power numbers the dyno spit out. Because we used Westech Performance Group's Superflow dyno, we were also able to measure the airflow into the carburetor and fuel flow to the engine. These two parameters are what really told the story behind each carburetor's performance. instead of assuming why each carb did what it did, we were able to find out the truth.
The truth was the exact opposite of the misconception that overcarbureting would lead to pouring too much fuel down the intake runners into the engine. What really happened was the larger-cfm carburetors actually made the engine run too lean, consequently killing the power. Why? Because the larger venturi diameter and booster design of the 750- and 850-cfm carburetors slowed the velocity of the air being sucked into the engine. The slow air speed killed the signal to the booster, so not enough fuel was pulled through the main jet and booster into theventuri at slower engine speeds.
Sure, at high rpm the larger carbs worked OK, and the engine benefited from large doses of air, but at moderate rpm, from 2,500 to 4,000, the engine was down on power because the air/ fuel ratio was too lean. Check out the dyno charts, and you'll see that the 850 carb flowed more air than the rest by a large margin. But, the speed of that air was so slow that the fuel flow into the engine was down by an even larger margin, from 2,500 to 3,200 rpm. in fact, the 525-cfm Road Demon carburetor helped the engine produce 135 hp at 2,500 rpm, while the 850-cfm carb made just 108 hp. Up top, the 850 was good for 360 hp with an air/fuel ratio of 12.1:1, but at 2,500 rpm the air/ fuel ratio was 15.1:1, which was way too lean and almost undriveable. The engine bogged down hard and ran rough in the midrange, which is where, in a truck, the motor would cruise at. in the midrange, the 525 Road Demon maintained an air/fuel ratio of 12.7 to 11.3, which, in the minds of many engine tuners, is an acceptable range for building horsepower.
The 750 Speed Demon wasn't much better. Power peaked at 357.2 hp at 6,100 rpm with a 13.1:1 air/fuel ratio. At 2,500 rpm, the air/fuel ratio improved considerably, compared to the 850-cfm carb. This time the engine ran smoother with a 14.2:1 ratio, which is still too lean, and the 750 carb made 124.9 hp. The 650 Speed Demon offered the best balance of power and air/ fuel ratio across the entire rpm band. At 2,500 rpm, the engine was making 135.1 hp with a slightly rich 12.4:1 air/ fuel ratio. At 4,000 rpm, the air/fuel was 12.6:1, and the engine produced 261 hp and peaked at 6,000 rpm with 356.9 hp and a 12.5:1 air/fuel ratio. The 650 carb proved to be the best all-around performer for our smaller, 305-cid V-8.