We jetted each carb to get...
We jetted each carb to get the most out of it. Time-consuming? Yes, but it's the only way to get a meaningful comparison. Output surged more than the small difference in rating would suggest.
Just to see how all this theory works out in practice, we rounded up an assortment of new Demon carbs, ranging from the vacuum secondary 625-cfm Road Demon to the professional-level 950-cfm annular discharge booster Drag Race Demon and put them through the paces at the Westech dyno facility. Our test engine is a stout '71 street 440 Six Pack engine, bored .030-inch over and fitted with replacement TRW forged Six Pack pistons, out-of-the-box Edelbrock heads, Tube Technologies Inc. (tti) 2-inch headers, a Competition Cams 280/286 duration street roller cam, matched Comp valvesprings, and a Comp 1.5:1 roller rocker valvetrain (PN 1321). The engine is capable of output in the 550hp range, while maintaining 9 inches of vacuum at idle. To minimize intake restriction in this test, we decided on a single-plane intake and settled on Mopar Performance's M-1. The M-1 ranks as the highest-flowing, single-plane intake we've tested to date for a standard-port, big-block Mopar, at an average runner flow of 289 cfm out of the box. This one was port-matched and ported in the plenum for an average runner flow of 316.5 cfm.
Bolted onto the engine, the...
Bolted onto the engine, the Race Demon looked racy with no choke horn and radiused air entries.
For our baseline test, we bolted on the baby of the lineup, the Road Demon 625 carb. The Road Demon is an entry-level street-performance piece-as the name suggests-set up for street use. Featuring an automatic electric choke mechanism and vacuum secondaries, the Road Demon makes a nice street piece, but has amenities usually found on racier carbs, such as four-corner idling and replaceable main jets at both ends. On this racy big-block, however, it seemed way too small.
Though we would test a wide range of carb sizes, our numbers would be meaningless unless we optimized each carb to provide an ideal air/fuel ratio. As we would do with each carb, the Road Demon was first run at a static state on the dyno at near-peak torque rpm. Dyno operator Steve Brule analyzed the air/fuel ratio based on feedback from the dyno's air and fuel flow meters; lambda sensor; and calculated brake-specific, fuel consumption figures. Following the static tests, a number of partial pulls were made and the jetting changed to optimize the carb's air/fuel ratio for our 440's specific combination. In other words, we made sure each carb's mix was bang-on for making as much power as it had in it.
The Demon 625 turned in surprisingly good numbers, with peak horsepower coming in at 526.7 at 5,700 rpm and peak torque showing 539.7 at 4,500 rpm. Our 440 was making good power, but the manifold vacuum numbers showed the 625 carb was restrictive on this large, powerful engine, with 1.7-inch Hg recorded at 6,100 rpm. Based upon the intake vacuum readings alone, it was clear there was power to be had by going bigger.
Our final move was to the...
Our final move was to the 950 Race Demon (left). This carb is equipped with annular discharge boosters, which help torque and low-end response with the large carb. Note how much larger the boosters (left) are than on the 850 Speed Demon (right). Annular boosters have a series of discharge holes all around the inside diameter, rather than the single discharge nozzle in a conventional booster.
It's Called the 650
Next up was the 650 Speed Demon carb, a small vacuum secondary carb unit similar to the Road Demon. The Speed Demons are street/strip carbs, coming with or without a choke, but with the choke horn cast in place on all of them (the choke can be added as an accessory). Vacuum-secondary Speed Demons feature the choke installed and a cap on the vacuum diaphragm housing, allowing for easy access for changing the secondary opening spring.
We didn't expect much change from the 625 carb by going to the 650, but the numbers were surprising, with power moving up to 543.2 hp at 5,800 rpm and 546.4 lbs-ft at 4,500 rpm. The manifold vacuum told much of the story, with WOT intake vacuum dropping to .9-inch Hg at 6,100 rpm-a drastic reduction-for a gain of 16.5 peak horsepower. With this carb, the air consumption reading on the dyno showed the 440 was taking in an additional 21 cfm of air. Is it possible that the actual flow difference between the two carbs is much more than the 25 cfm implied?