Eaton vs. Kenne Bell vs. Vortech vs. HP Perf Boost Curves (14 psi)
With the exception of the twin turbos, the boost curves tell the power story. The Roots and twin-screw blowers produced similar boost curves, the major difference being where the Roots fell off after 5,000 rpm, the curve offered by the twin-screw continued to climb. The boost curves offered by the centrifugal supercharger and turbos were decidedly different than the pair of positive displacement superchargers. Check how the boost curve offered by the turbos ramped up after 3,000 rpm. Judging by this boost curve you'd be inclined to think the turbos offered similar power to the blowers once the motor reached 4,000 rpm, but the power curves hammer home the fact that all boost is not created equal. The 14.5 psi from the roots produced 247 hp less than 13.6 psi from the turbos. Obviously, the centrifugal supercharger cannot compete in torque production down low and the boost curve shows why. With just 1.9 psi compared to 12.3 psi, the centrifugal must rely on engine speed before things really start to happen.
Runner Length Ruminations
While running this comparison, I saw an opportunity to once again illustrate the difference in power supplied by intake manifold runner length. Here it is, one more time for the record: Intake runner length is one of the primary design features that determine the effective operating range of the motor. Longer runners will produce peak power at a lower operating range than shorter ones. This tuning effect is present regardless of whether the motor is turbo or supercharged.
The presence of boost pressure does not dictate a shorter runner length. Shortening the runner length in every case will reduce power at a lower rpm, but has the potential to increase power at higher engine speeds. The terms long and short can be misleading, as a 19-inch runner should certainly be considered long, but is 14 inches still long, or how about 11 inches? A runner length of 11 inches is certainly long compared to a runner length of just 4 inches, but just where do you draw the line?
Stepping down off my soapbox, I can now get to one of the hundreds of dyno tests that support the runner length discussion. The Vortech Cobra replacement kit came equipped with an intake designed to utilize the factory air-to-water intercooler and lower intake. Unfortunately, the factory lower intake on the '03 Cobra featured almost no runner length, just radiused openings leading into the head. Talk about short runners! This system was utilized by Vortech in an effort to retain the factory air-to-water intercooler and does not fall under my category of specialty intake manufacturers, as Vortech is fully aware of the change in the power curve produced by this short-runner intake. Their decision to build the intake was one of cost, as they wanted to retain the factory air-to-water intercooler. Adding a factory '01 intake (like the one tested) and one of their Aftercoolers to the mix would likely drive the price of the Cobra replacement kit up beyond what the market would bear.
To illustrate the power gains offered by the change in runner length, I ran the Vortech supercharged '03 Cobra motor at 15 psi with the adapter/intercooler system and then again after installing the long-runner '01 factory NA intake. Naturally this required removal of the intercooler and '03 Cobra intake. The pulley ratio, air/fuel and timing were all kept constant for the two tests. Check out the difference in power offered by the runner length in the '01 intake. From 3,500 to 6,600 rpm, the long-runner intake upped the torque production by a solid 50 lb-ft, with gains as much as 72 lb-ft occurring at 4,400 and 5,500 rpm. The '01 factory intake offered as much as 75 hp over the short-runner version at 5,500 rpm and carried a 50hp gain right through 6,600 rpm.
 In an effort to demonstrate...  In an effort to demonstrate the effect of the change in runner length, we replaced the fabricated intake used on the Vortech kit with the stock '01 (NA) intake. |  Combining a Paxton and Vortech...  Combining a Paxton and Vortech discharge tube allowed us to connect the T-trim to the (now repositioned) throttle body. The increased runner length provided by the stock '01 intake increased the power output of the 4.6 from 3,500 to 6,600 rpm. Don't be tricked into thinking that a short-runner intake works best with a blower or turbos-they don't. |  Effect of runner length-Vortech...  Effect of runner length-Vortech '03 Cobra Blower Upgrade. |
When choosing a manifold, choose the one that provides the best average power production in the rpm most used while driving. If this is a drag race motor that runs from 5,000 to 8,000 rpm, then by all means select a short-runner intake designed to optimize power in that rpm range. But for nearly any type of street engine, stay away from anything with less than 10 inches of runner length, or your average power will suffer.