5.4L Four-Valve Stock Navigator vs. Custom Intake
The great thing about building an adjustable intake is that it's possible to find out exactly what the engine combination responds to. Using this principle, we were able to adjust the custom intake to produce not only 456 hp, but also the highest peak torque of the bunch, a solid 433 lb-ft of torque. True to form, the custom intake did not match the torque production of the factory Navigator intake below 3,600 rpm. From there on up, the custom intake offered both more power and torque. Torque production from this design exceeded 400 lb-ft, from 3,800 rpm to 5,950 rpm.
We were anxious to see the battle of the factory Fords, with the Navigator intake slugging it out with the Australian Boss 290 intake. The Boss 290 intake featured slightly smaller ports than those on our ported Navigator heads, but the intake design offered longer runners than the Sullivan or Kar Kraft manifolds. This extra runner length showed in the results, as the Boss 290 produced the best torque number of the bunch (until running our final competitor) with a peak reading of 420 lb-ft.
Basically a mini Cobra R manifold, the Boss 290 from Australia was a factory Ford piece. Too bad we don't see this combination on any Mustangs in the states. The cross-ram design looked trick and made impressive power-always a good combination. Equipped with the Boss 290, the 5.4L produced 457 hp and 420 lb-ft of torque.
We ran into one difficulty with the Boss 290 test, namely a suitable throttle body. The Boss 290 intake was supplied by the great guys at Sullivan Performance, but the stock throttle body was nowhere to be found. To cure this problem, we clamped in place an aftermarket throttle body designed for an LS1, and were off and running. Vacuum readings in the manifold indicated that the LS1 throttle body did not represent a restriction at this power level. The cross-ram Boss 290 intake demonstrated its worth by producing 457 hp and 420 lb-ft of torque, bettering the Navigator intake from 4,500 rpm to 6,500 rpm. Even down as low as 4,000 rpm, the Boss 290 equaled the power output of the Navigator, though below that point, the torque production of long-runner Navigator clearly held the advantage.
The final intake to be run on the 5.4L was of the author's own design. This intake would not likely fit under a production Mustang hood, but the same could be said of some of the others. The tall deck height of the 5.4 combined with the intake height makes for a tall package. In truth, this was not one intake design but many, as the adjustable nature of the manifold allowed us to dial in the optimum runner length for this particular application. Though it was possible to improve both low and high-speed power production, this combination provided the best overall powerband. Running dual plenums joined at the rear with a 3.5-inch crossover tube, dual 75mm throttle bodies, and full-radiused air horns inside each plenum, the custom intake offered 456 hp and 433 lb-ft of torque. This combination bettered the power output of the Navigator intake from 3,600 rpm to 6,500 rpm but lost out from 3,000 rpm to 3,600 rpm. It should be obvious from the supplied power graphs (peak numbers tell only a small portion of the story) that the right intake can make a major change in the shape of the power curve. The trick is to dial in the effective operating range and install a manifold that will run well in that rpm band.
Since we didn't have the factory throttle body, we were forced to run an aftermarket LS1 TB on the Boss 290 manifold. The longer runners in the Boss 290 intake (compared to those from Kar Kraft and Sullivan Performance) improved torque production throughout the rev range. As with all the others, the Boss 290 could not match the torque production of the Navigator intake below 3,900 rpm. | 
The guys from Sullivan Performance came to the rescue in a big way by offering us a factory Navigator lower manifold to hack up to produce this custom (adjustable) intake. Using the Navigator flanges and custom plenums, we were able to dial in the optimum runner length for this 5.4L combination. Equipped with the custom intake, the 5.4L produced 456 hp and 433 lb-ft of torque. This optimized combination bettered the Navigator intake from 3,600 rpm to 6,500 rpm. | 
Boss 290 intake manifold with upper plenum in place. |