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?

Blue Thunder

The Blue Thunder intake is surely a nice factory looking casting, but caused immediate concerns for performance when turned upside down. Port dimensions were the smallest of any intake in our test, measuring 1.812-inches tall by just 0.913 wide. We checked this against an original first-gen Ford two-4 intake we had access to, and found the Blue Thunder has indeed been reproduced accurately in this area. In comparison, both the Cobra and Edelbrock as-cast ports were a much more generous 1.88 by 1.04-inches. Considering the port handicap, we honestly didn't expect much from the Blue Thunder, and figure most customers would prefer it come cast/machined with a port comparable to the others.

No matter, with the twin 390 Holleys bolted down with hardware from Virginia Classic Mustang, we initiated our first dyno pulls with Blood's technician James Leahy at the controls. Were we ever shocked to find the 390s spot on right out of the box! Call it dumb luck, considering the engine variations that are possible, but with their 51 primary jets, 0.054-inch secondary metering plate orifices, a 6.5 power valve and plain secondary springs, the 390s on the Blue Thunder really rocked!

It dusted the same period Cobra intake, and was on par with the Performer RPM every step of the way. Peak numbers for the Blue Thunder and Performer RPM were virtually identical at 256.1 vs. 256.6 horsepower at 6,000 rpm, 259.6 vs. 262.1 lb-ft of torque at 4,000. Below the six-grand redline, some horse trading of around 5 hp and 5 lb-ft existed depending on engine speed--the Performer RPM with a slight advantage from 3,200-4,000, and the Blue Thunder similarly superior from 4,400-5,400. Really, there was little discernable difference overall, and we were super impressed with this intake/carb combo considering our initial concern.

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As good as things had been with the 390 Holleys, the 465s fired up lean everywhere. Off idle, part throttle, wide open, you name it. That's no knock against this setup, it just happened to be wrong for our combination. Leahy went to work with the fix, which involved much more than fattening up with bigger jets. Leahy made use of the MD250's part throttle loading capabilities, and ran the '65 Falcon test mule just off idle, as well as throttled up to where the secondaries were just shy of opening.

Having noted AFRs at these key points as well as wide open, Leahy's initial move was to jet up one primary size to No. 58s, augmented by drilling the power valve restrictor holes from 0.026- to 0.036-inch, opening the idle circuit fuel jets from 0.025- to 0.028-inch (effective for right off idle), and drilling the secondary metering plate jet orifices to 0.057-inch. Another dyno run revealed AFRs to be much better, but still too lean wide open.

With the carbs off once again, Leahy drilled the metering plate jets to 0.059-inch, which pretty well nailed a 12.5 AFR at full throttle and a decent fuel curve across the board. The final numbers were a bit off the 390s, though not by much. Throughout the pull to 6,000 rpm, the 465s were 3-5 hp and 3-5 lb-ft shy of our prior bests, indicating we simply had too much carb for this combination. Were the results indicative of a restrictive intake, or an engine that just couldn't use the extra airflow? We'd know better after testing the Trans-Am intake, but in the meantime, we can say that the 390s look like the hot ticket for modest strokers and stock-cube, small Fords using the Blue Thunder intake. Imagine how this combination might do with better ports!

Trans-Am

The Trans-Am dual quad intake came to us from our friend John Vermeersch, and is an entirely different design than its predecessor. You can see some of it from the exterior, but you really need to study the inside to fully understand. Whereas the Blue Thunder/Ford first-gen is a true 180-degree design, the Trans-Am intake is a considerably different breed--still a dual-plane, but with distinct airflow paths depending on rpm.

At low revs, the flow is much like a regular dual-plane intake, but at high flow, there is a direct shot at each port--somewhat mimicking an independent runner arrangement. This was the lone intake in our test that had previous port work, but beggars can't be choosers when seeking hard-to-find parts. Fortunately, the work was nothing more than a gasket match, and with the GT40X heads being bigger still, we don't think this amounted to any significant advantage. The measurements showed 1.96 x 1.12-inches, up from the 1.88 x 1.04 spec of an unported version of the same manifold. At this point, we expected the Trans-Am to top all others in terms of max power, but weren't certain by how much or how it might perform across the board. Initially designed for road racing, would it really prove streetable? Running the 390s first, we were ready to find out.

Initial dyno pulls indicated the Trans-Am was a superior flowing intake, as the AFR was lean with the 390s still configured as they had been on the Blue Thunder. Leahy fattened up the primaries one size to No. 52s, then drilled the power valve restrictor holes from 0.038 to 0.046-inch. Another pull found the 12.7 AFR nearly on target, and some pretty impressive power numbers. The Trans-Am and 390s accounted for the best power numbers so far, 268.8 horsepower at 6,000 rpm, and 265.3 lb-ft at 4,250--which are peak gains of 12.7 hp and 5.7 lb-ft over the Blue Thunder/390s. Better still, this combination proved very good down low, then bested all other combinations from 4,000 rpm on.

We thought the Holley 465s would come into their own with the extra flow capabilities of the Trans-Am manifold, but it didn't pan out this way. Nobody was surprised when the carbs started off lean again, and Leahy compensated with several richening techniques that got the mid-12 AFR we were seeking. Still, the 465's overall power and torque curves lagged slightly behind the 390s--some parts of the curve were a statistical draw, while in other areas, the 465s trailed by similar 5 hp/5 lb-ft numbers we saw in the Blue Thunder tests. In the end, the healthy 331 simply ran better with the Holley 390s no matter the manifold, pointing to a combination of cubes, cam, and cylinder heads that were happy with 780-cfm. With this in mind, we figure it would take a healthy 347- to 408-inch engine to really take advantage of the 465s.

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Final Analysis

In the end, we think our tests advocate strongly for the case of dual quads in the 21st century. When well chosen and tuned, two-fours have plenty of performance to crow about, and the looks are clearly a huge plus. Depending on what numbers you want to compare, our tests indicate dual quads can be at least the equal of a modern four-barrel setup, and with the right intake, even better. At the same time, the 25hp and 13 lb-ft advantage, which the Trans-Am/Holley 390 combination delivered over the Cobra/Holley 670, is quite impressive.

To top it off, the right combination can deliver good driveability; car owner Randy Dunphy has nothing but positive feedback on the throttle response and performance the 331 in his '65 Falcon now exhibits. Key to your combination will be finding a dyno shop such as Blood Enterprises, that can test and tune what is assuredly a more complicated arrangement than a simple four-barrel. As well, the golden ticket for many small-block Fords will be Holley's impressive 390-cfm carbs, which Carl's Ford Parts will gladly revise in order to put them in full Ford dual-four dress.

Carl's existing Holley 465 kit goes for $1,795 with manifold, progressive linkage, fuel log, and carbs, but Carl says the 390s would come in at $1,595 since the carbs themselves are somewhat less expensive. Air cleaners and gaskets/hardware are not included. Carbs alone are $1,350 for 465s and $1,150 for 390s, prepped for dual-quad duty.

While not cheap, these prices are considerably more affordable than many other pseudo exotic/high tech options on the market, and surely worth considering for those looking for something beyond the norm. And about those dual-quad intakes themselves…It seems obvious there is room for a higher-performance option in the world of currently available Holley flanged dual quad manifolds, either by improving what's already on the market, reproducing the impressive Trans-Am intake, or starting with a clean sheet design. As a line in a popular movie once said, "Build it, and they will come."

Beyond the normal cast of characters who went above and beyond for this story (mechanics, dyno operators, manufacturers, and more), we also want to thank John Vermeersch, Cory Hitchcock, and Dennis Chandler for their help in procuring and measuring several rare vintage intake manifolds. Without the cooperation of all, this effort would have been much more difficult to accomplish.

Triple Twos

When discussing multiple carbs for Fords, one can't ignore the six-barrel setups, which were optional on assembly line 390s and 406s in the early '60s. Ford offered a similar over-the-counter arrangement for small-blocks, which Carls has reproduced as well. We didn't test any triple-twos, but believe them to be strong performers. That said, Ford had embraced dual quads by the time it developed really impressive cylinder heads, so the factory six-barrel pieces are of a moderate performance, low-rise design. Both carb configurations use a progressive linkage, meaning you're cruising on just two-barrels until you roll hard into the throttle.

For what it's worth, the 1966 Ford High Performance parts catalog advertised a 12-15 horsepower improvement for the small-block, triple two-barrel Holleys (C4DZ-6B068-B), 25-30hp for a dual-quad Carter kit (C40Z-6B068-E), and 30-35hp for the first dual-quad Holley setup (C6ZZ-6B068-A)--all compared to a production iron four-barrel intake and Autolite carb. When the Trans-Am induction appeared in 1968 catalogs, it was advertised to be worth 45-55 horses "depending on the camshaft your engine utilizes."

Lest we leave a blanket impression that dual quads are better than triple-two-barrels, Mopar undoubtedly knew what they were doing when they simultaneously rode the fence with dual-quad Hemis and Six Pack 340s and 440s at the peak of the muscle era. Its six-barrel manifolds were decidedly high-performance in design, and by all accounts, the results were impressive. Fortunately, today's Ford picks involve some intriguing items never offered back in the day--for example, the Dove FE Tunnel Wedge triple-two intake, a hardcore performance piece which would be absolutely wicked on an early '60s big-block Galaxie!

Vintage Vibes

Randy Dunphy's 331 was in transition during our testing, morphing from a '90s underhood look to something right out of the Total Performance era. Of course, the two-fours are a big part of the transformation, but so are the Holman Moody valve covers, which Dunphy installed during our test. Available brand-new at www.holmanmoody.com, these covers are a great alternative to some of the more commonly seen vintage-themes. We can't promise clearance on every combination, but can say that the HM units easily cleared the FRPP bolt down roller rockers on this engine, and they come in two versions--street oriented covers seen here ($260), and a competition version with twin comp style breathers ($355).

Duals for All

We're pleased to report that almost every '60s-era Ford engine can be fitted with factory-style dual quads, using readily available brand-new components. While FEs were the only '60s Ford engine to get multiple carbs on the assembly line, FEs and small block multi-carb setups were heavily promoted by Ford through its parts department. The 351W and 429/460 arrived too late on the scene to get similar billing, thus it's interesting that applications for these engines are available today as well. Price Motorsport makes the 351W piece you see here (on right), while Blue Thunder makes the 429/460 application (on left)--both of which are available through Carl's.

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14 Not every combination will perform as ours did, but we think we've pretty well debunked the idea that dual quads are losers compared to a modern four-barrel. On the contrary, there may be power to gain with the right combination and patient tuning.