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.
7 Virginia Classic Mustang...
7 Virginia Classic Mustang is a great source for OEM-style hardware. We sourced our carb studs, nuts, heater hose water necks, and various other necessary fasteners and dress items from the company in one easy stop.
8 As soon as the Trans-Am/dual...
8 As soon as the Trans-Am/dual 390s combo fired off, it was clear we had a vacuum leak. In the end it was traced to some PCV passages under the carb base plates, which overhung the small manifold carb pads. We solved the problem by running extra thick gasket, such as Holley's #108-12, which we’d advise for the Blue Thunder intake as well.
9 If there's a rub to running...
9 If there's a rub to running dual-4s, it's the more complicated tuning that's involved in assuring the combination is optimized. Testing on the Blood Enterprises Mustang dyno was critical to our project, and we'd strongly recommend finding a similar shop with extensive carburetor experience in your area.
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.