Testing 4.6L Mod Motor Intake Manifolds - Ultimate Intakes

In our last issue, we assembled a number of different 5.4 intake manifolds and compared them to the factory Navigator intake assembly on a modified 5.4L Four-Valve motor ("Factory Fast Vs. Advanced Aftermarket). Given the intended application-a heavy SUV or other truck-it's not surprising that the factory Navigator intake performed well at the lower engine speeds. Designed to enhance low-rpm torque production, the Navigator intake demonstrated its torque-producing nature by besting all other intakes up to 3,500 rpm.

Naturally, there's no free lunch, so an intake designed to enhance low-speed torque production will suffer higher in the rev range. Such is the trade-off inherent in any intake design, though this can be overcome, to some extent, with a dual-runner design like the Aviator and FR500 intakes tested on this 4.6L Cobra motor.

The reverse is also true, as short-runner intakes designed to enhance high-rpm power won't provide the same torque numbers as a long-runner design. It's all about efficient cylinder filling and where you want, or need, the engine to perform its best. For those who have forgotten already, this scenario holds true even on boosted applications.

Since last month, when we ran testing on a 5.4L motor, we decided to show some love to the 4.6L Cobra owners as well. As with the 5.4L, we gathered every Four-Valve intake we could get our hands on and ran them in a back-to-back test against the factory '01 Cobra manifold. Unlike the Navigator intake, the Cobra manifold wasn't designed to enhance low-speed torque production to help accelerate a heavy SUV. Instead, the Cobra manifold was designed to promote midrange torque and allow the motor to pull strongly to 6,500 rpm. Please, no letters about running your Cobra motor to 9,000 rpm or more-the Four-Valve motor will certainly spin that high, but the factory intake was designed to produce peak power well below that point. In fact, our modified test motor produced peak power at just 6,400 rpm with the '01 intake. With a few of the short-runner intakes, the power peak was shifted as high as 7,000 rpm, clearly demonstrating that the intake plays a major part in determining the shape of the power curve.

As has been our practice in the past, we've included complete power curves for each intake tested. The graphs are plotted against the factory '01 Cobra intake for comparison. As indicated previously, the factory Cobra intake was designed to maximize midrange torque production, and not one of the intakes tested managed to top the peak torque production of the factory piece. This tuned-for-torque design naturally left power to be found at the higher rpm ranges, something the FR500, Sullivan, and custom dual-plenum design took advantage of.

While the aftermarket intakes all produced more peak power, not one of them (including the author's adjustable design) did so without sacrificing power elsewhere along the curve.This is often the case with a good intake design, something the Cobra manifold can certainly be classified as. Along with the usual Four-Valve suspects, we also got the opportunity to test some adapter plates from Kar Kraft (yes, the guys who brought you the Boss 429) that allowed us to run any 5.0L intake manifold on the 4.6L Four-Valve motor. You'll remember that Kar Kraft also supplied a carbureted upper intake for the Navigator intake we tested on the 5.4L; we hope to run the dual-quad version on the 5.4L once it's ready.

In addition to the 5.0L to 4.6L adapter plates, Kar Kraft also offers an upper intake for the 4.6L Aviator manifold, although it was not available in time for testing. Check out the accompanying results and see how your favorite did, but remember, Ford put a lot of work into the factory Cobra intake, and these tests show the Blue Oval really did its homework.