Modified Mustangs & FordsHow To Engine
Dual Quads - Twin Win, Or Double Trouble? Part 2
The merits of dual quads on a small-block Ford
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!