Eric English
June 20, 2012

Those of you who read Part 1 of our exploration of dual quads on a small-block Ford (June '12) know that first installment was really more about preparation than actual testing. More to the point, we didn't run any of the dual-quad combinations, but instead tested a couple of popular single-four combinations to set a baseline for comparison. Both an original '66 G.T. 350 Cobra-lettered, high-rise and a modern Edelbrock Performer RPM were evaluated, each with the same Holley 670 Street Avenger carburetor. As well, the tests on Blood Enterprises Mustang MD250 chassis dyno were run with the engine timing set at a consistent 34 degrees, a number that had previously been determined to work well on this hot street 331.

The results showed the engine's FRPP GT40X heads and modest Comp hydraulic roller cam were still making power at our self-imposed 6,000rpm limit, and as expected, the Performer RPM proved the better piece in the end. With monitoring via Blood's wideband oxygen sensor, we strove for a 12.5:1 air/fuel ratio on all test combinations. In the end, peak numbers for the Cobra were 243.9 hp at 6,000 rpm and 252.5 lb-ft of torque at 3,750. The Edelbrock turned in 256.6 hp at 6,000 rpm and 262.1 lb-ft at 4,000, good for 12.7 additional horsepower and 9.6 lb-ft at the peaks. As addressed in our previous installment, the Mustang dyno reads roughly 20 percent lower than the more oft quoted Dynojet, so if you'd like to add that back into the equation plus a 15 percent reciprocating loss for the manual transmission drivetrain, this engine is arguably around 340 flywheel ponies.

But enough single-four focus, let's dig into what we really want to address here--how do dual quads perform on this same engine? Rather than accept recent magazine wisdom that has shown 1,000 cfm or more of two-fours on a 350-inch or smaller engine will result in 10-15 horsepower less than a modern four-barrel combo, we set out to try more reasonable cfm totals. To this end, we turned to Holley and Carl's Ford Parts, the latter being a well known purveyor of factory-style, multiple-carb induction systems for muscle-era Fords.

Carl's typical small-block kits use 465-cfm Holleys that are quite close in configuration to the 460-cfm Holleys that came in Ford's over-the-counter offerings back in the day. That still seemed like a lot of cfm, particularly for those playing with honest 289- and 302-inch engines, so we also investigated the possibility of running a pair of Holley 390s as well. Both carbs are vacuum secondary 4160s, as were most Ford dual quads of the '60s. We discovered that Holley 465s come with a Ford-style throttle shaft/endplate, whereas the 390s use a universal piece that isn't compatible with factory Ford dual-quad linkage. That said, Carl's main man, Carl Binius, offered to reconfigure the 390s with the Ford shaft, followed by the standard prep for dual-four use--removal of the secondary carb choke plate, installation of ported vacuum secondary diaphragm lids, and several other odds and ends. In short, there were no real obstacles to testing the 390s, so we did.

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While bouncing dual-quad ideas off various experts in the field, we found an array of opinions. Some thought we were nuts to think duals could run well on a modest small-cube engine, while others thought we might be onto something with the smallish carbs. Whatever the reactions, we were hardly proceeded blindly. Your author has long remembered digesting a pair of Hot Rod magazine articles from 1968 (July and October), in which Ford Performance advisor Ak Miller and intake manufacturer Fred Offenhauser teamed on a project to explore the street potential of Ford's then new 302. Various intake and carb combinations were run on a 302 with nothing more than a Hi-Po 289 cam and headers. The champ was a dual-plane Offenhauser intake and 600-cfm Holley, though of note, a dual-quad Carter/Offenhauser combination ran neck and neck throughout, even outpacing the single-four by 15 horsepower at 4,000 rpm.

The edit explained that dual quads would be a poor decision on such a motor since they cost much more and resulted in similar overall performance.

Miller and Offenhauser were back at it a couple months later, this time with the 302 attired in 351W heads, pop up pistons, and a Shelby solid cam with 0.508-inch lift. In this guise, the single-four champ was a 715-cfm Holley on the same Offy intake, but the dual quads now won the battle by 20 horsepower at 6,000 rpm, while giving up little down low.

Armed with this background, we had reason to be optimistic, yet were tempered by the limited availability of Holley-flanged 2-4 intakes for short-deck Windsors. To our knowledge, there is just one available new--the Blue Thunder reproduction of Ford's first dual Holley, over-the-counter dual-plane from mid-1966. We were also encouraged to test Ford's second dual-Holley intake that seems to have debuted with the '67 Trans-Am teams--we'll call it the Trans-Am intake for the remainder of this discussion, though it subsequently appeared in Shelby's parts catalog beginning in 1968. Unfortunately, this manifold isn't currently reproduced, but the bottom line is that both designs are more than 40 years old. We felt it likely that the dual quads would be superior to a same era four-barrel combination, but how about a modern single-four design?