Hot rodders are an odd bunch, at once enthralled by the latest highfalutin technological advance, yet also bonded by years of tradition with the old-school "the way we used to do it." One example of the way things used to be is the dual-quad carburetor setup. Theoretically, it makes sense. In the quest to get more fuel and air into an engine, why not use two carburetors instead of one? In reality, though, it was often a troubled arrangement. Making two carbs designed to work separately then work as a team was problematic, and the manifolds that held these two four-pot mixers were compromised designs at best. To make a long story short, advances in carburetor technology led to advances in intake manifold design, and dual-quads were rendered all but obsolete for the street. We say "all but;" when hot rodders worship at the altar of tradition, the altar is sometimes topped with a set of dual-quads.
Functional considerations aside, dual-quads just plain look cool on a street car. On a race car, dual-quads and a tunnel-ram intake may be the system of choice for maximum performance. On a boulevard cruiser, however, twin four-pots are mostly done for looks. Even Edelbrock, the only company we could find that still makes a streetable dual-quad manifold, states that its more modern single-carb/manifold setups will >> make more power than the old-school system. But let's not get too carried away in sounding the dual-quad's death knell. With a proper engine combo (cam selection is especially important) and correctly sorted carbs (which we'll be covering here), dual-quads are still a viable intake system for those who've just gotta have the look.
Edelbrock sells two variations of its small-block dual-quad setup. The C-26 manifold, topped with twin 500-cfm mixers, is the basis of both. The primary configuration, if you will, is similar to that found in many mildly built customs and Edelbrock's own Performer 350 crate engine. The Goodwrench 350 block runs Edelbrock PN 60909 heads, which feature a 64cc combustion chamber, 2.02/1.60-inch valves, and a 9.0:1 compression ratio. The cam, Edelbrock's PN 2102, is exceptionally mild, and ground at 0.420/0.442 lift and 204/214 degrees of duration, both at 0.050-inch lift. Along with the dual mixer's calibration, which is designed to obtain the best power possible with good throttle response, a Performer 350 dual-quad crate engine comes in at 315 hp and 372 lb-ft of torque--not monster numbers, but certainly more than enough for a strong, torquey street motor.
The other cam that Edelbrock recommends with this manifold/carb combo is a Torker-Plus unit, which is more aggressive: 0.488-inch lift (intake and exhaust) and 232/234 degrees of duration, again, both at 0.050-inch lift. Edelbrock doesn't have power numbers for a combo with this bumpstick, though R&D man Curt Hooker made a couple of suggestions. The most critical is that a dual-quad setup with this cam needs to be atop a more aggressive block, something along the lines of a ZZ4 and its 9.5:1 compression ratio. Of course, carb tuning would be a must, and though this is something Edelbrock can help with (the company will even send out jets and metering rods), chances are that there'll be a lot of trial and error. The payoff? Somewhere in the neighborhood or 400 rumpity horsepower, Hooker tells us, and probably a similar torque level. That, friends, is about the limit of the ol' C-26 manifold and its pair of four-pots. If your street machine needs more power than that, you'll need to be lookin' at more modern single-carb dual-plane manifolds.
Of course, some of us aren't after the latest and greatest ultimate power-making combination; a few of us traditional types can live with a merely acceptable power level as long as it comes with a very cool old-school look, and as long as it runs well enough to get us out on the boulevard. With that, here's how it's done.