It's a no-brainer that if you start with a wimpy two-barrel carb and intake, an enginehas nowhere to go but up, but is the aftermarket carb and manifold really worth it if your engine is already factory four-barrel equipped?
To find out, we compared a GM cast-iron four-barrel intake and Rochester Quadrajet spread-bore carburetor against two popular Edelbrock dual-plane intake manifolds—a standard Performer EGR and a high-rise RPM Air-Gap—both fueled by an Edelbrock 750-cfm Performer square-flange carburetor. We used the EGR version of the Performer (PN 7101) because there was one in-shop at our dyno facility, Westech Performance. Its EGR boss was blocked for the test. A non-EGR Performer (PN 2101) is about $50 cheaper.
The tests were conducted on Westech's SuperFlow engine dyno using an 8.67:1 355ci small-block Chevy equipped with aftermarket aluminum cylinder heads, headers, and a healthy performance flat-tappet hydraulic cam (0.472/0.472-inch lift, 220/228-degrees duration at 0.050). The engine ran Union 76 92-octane unleaded street gas and 35-degrees of ignition lead.
The tests showed the Q-jet combo was a pretty good combination for making low- and midrange torque. Below 3,800 rpm, torque production was higher than the two aftermarket designs, and its peak 401.9 lb-ft output occurred at only 3,700 rpm, 600- and 700-rpm lower than the Performer and RPM intakes, respectively.
But the Q-jet's 361.6 peak hp at 5,300 rpm was not up to the Performer's 379.2 hp at 5,700, or the RPM's stout 392.1 hp at 5,600. In terms of overall average numbers throughout the entire 2,000-6,200rpm test range, the Air-Gap RPM and Performer carb were up by 6.6 lb-ft and 9 hp on average overall. Over 3,800 rpm, the Edelbrock intakes and carb really came into their own and began to pile up the numbers. In the 4,100-6,200rpm range, the Performer gained 11.5 lb-ft and 11.7 hp on average, while the RPM doubled this to 22.4 lb-ft and 22.6 hp.
A pronounced dip in the torque curve was recorded from 2,000-2,700 rpm. Part of the dip was due to this engine's low compression, which reduced effective low-end cylinder pressure. However, the dip was less pronounced with the stock intake and Q-jet than it was for the aftermarket combos. At this point the carburetors' secondaries are still opening. Apparently, the Q-jet's small primaries combined with its spring-loaded secondary air-door are particularly helpful in smoothing out full-throttle tip-in response.
On this engine combo—with its good heads and fairly hot cam—the RPM Air-Gap outperforms both the stock Chevy Q-jet and intake as well as the Performer. But fully realizing this intake's potential requires paying attention to the entire combination. This engine's healthy cam and high-rise intake perform best over 3,500 rpm, so you'd want to use at least a 2,500-stall converter and 3.73 gears, as well as pay careful attention to accelerator-pump tuning and secondary opening rate.
