Richard Holdener
April 24, 2007
For part two of our Boost Bash, we once again called upon the Ford Racing '03 Cobra motor.

In the movie world, sequels are rarely as good as the original. There are a few noted exceptions, like The Empire Strikes Back, Aliens, and T2. While the high-boost shootout between the four forms of forced induction can hardly be compared to the high-budget Terminator films, much like California's esteemed governor, in part one we did promise we'd be back.

As promised, here we are with the results of the high-boost testing performed on our 4.6 four-valve Cobra crate motor. For those of you who missed the last issue or skipped right over this juicy test, we'll do a brief refresher course. The idea behind the testing was to illustrate the differences in the boost and power curves offered by the four most popular forms of forced induction (Roots, twin-screw, centrifugal and turbos). To illustrate the differences, we ran all four forms on the same test motor (an '03 Cobra mill from the Ford Racing catalog) at the same boost levels.

In part one, we took the Kenne Bell twin-screw supercharger, the Vortech centrifugal supercharger and a twin-turbo kit from HP Performance and compared them all to the factory Eaton M112 Roots supercharger at 11 psi. Now here we are to perform the same test at 14 psi, and we even decided to toss in an intake runner length test along the way.

If you are interested in the theory behind the various forms, check out last month's issue. Basically, the positive displacement superchargers offer immediate boost response with the twin-screw bettering the Roots blower in terms of efficiency and power potential. The centrifugal supercharger offers even more power per pound of boost than the positive displacement superchargers, but lacks the response rate of either. In life, there is always a trade off and the turbo is no exception. Despite besting all the forms of supercharging in terms of peak power and torque, the turbo lagged behind the Roots and twin-screw blowers at the lower engine speeds.

As we indicated in part one of our "Boost Bash," we were not trying to declare a winner, but simply to illustrate the differences so the reader can make an informed decision. In most cases, the decision will not be based solely on the maximum (or peak) power potential. There are obviously other issues that come into play when choosing a supercharger upgrade or to simply retain the existing Roots unit. These variables might include cost, complexity, ease of installation and reliability, to name just a few.

Our test motor was augmented with a quartet of Comp Xtreme Energy (XE2672AH) cams, an Accufab throttle body and inlet and Flowtech 1 5/8-inch, long-tube headers. The DUB pulley system from South Florida Pulley Headquarters allowed easy pulley changes to adjust the boost pressure on both the Eaton and Kenne Bell, though Kenne Bell also supplied a variety of different blower pulleys for the twin-screw. The engine was tuned with the F.A.S.T. management system and equipped with a set of 65-psi injectors to provide sufficient fuel flow from the Aeromotive A1000 fuel pump. A Kenne Bell Boost-a-Pump and Boost-a-Spark were also employed on the test motor to ensure adequate fuel flow and spark energy.

Pulley swaps are equally easy on the Kenne Bell blower, requiring removal of only a single retaining bolt. Combining the 3.75-inch blower pulley with an 8.5-inch crank pulley resulted in a peak boost pressure of 14.5 psi.

The factory Eaton M112 was used as the baseline against which the three other forms would be judged. Equipped with a 2.93 DUB blower pulley and an 8.5-inch DUB crank pulley, the Eaton supercharger pumped out a maximum boost pressure of 14.2 psi. The peak power checked in at 583 hp while the torque stood at 574 lb-ft. To really appreciate the torque production offered by the Roots blower, check out the supplied power graph. It is important to point out that the peak power only increased by 11 hp when we upped the peak boost pressure from 11.7 to 14.2 psi, a sure indication that the M112 had very little flow left in reserve.

After our baseline, we replaced the Eaton with the Kenne Bell 2.2L twin-screw supercharger. As before, it was necessary to dramatically reduce the speed of the twin-screw compared to the Eaton to limit the peak boost production to near (as possible) 14 psi. In this case, the twin-screw was run over 4,000 rpm slower than the Eaton, yet managed to up the power peak from 583 to 704 hp (a gain of 121 hp). The peak torque was up slightly as well, from 574 lb-ft (with the Eaton) to 597.

Where the horsepower curve fell off with the Eaton, it continued to climb with the Kenne Bell (to a peak of 14.5 psi). In fact, we know that the power would continue to climb had we elected to run the motor higher than 6,600 rpm. The Kenne Bell blower was run using the factory air-to-water intercooler and lower intake manifold. Like the Eaton, we installed a free-flowing Accufab throttle body to minimize any airflow restrictions at this elevated power level. Of all the systems, the Kenne Bell was probably the easiest to install, requiring nothing more elaborate than an intake manifold (and intercooler core) swap.

Equipped with the twin-screw from Kenne Bell, the 4.6 pumped out 704 hp and 597 lb-ft of torque.

Next up was the Vortech centrifugal supercharger. As before in the previous test at 11 psi the Vortech easily out-paced the Eaton in terms of peak power. Upping the boost pressure to a maximum of 14.0 psi came courtesy of a 3.12-inch blower pulley (up from the 3.48-inch blower pulley used at the 11 psi level). Like the Kenne Bell, the Vortech utilized the factory air-to-water intercooler thanks to a custom upper intake manifold. The fabricated intake also positioned the throttle body in the stock location. Breathing through the custom intake and factory air-to-water intercooler, the Vortech upped the power peak to 725 hp. The peak torque checked in at 575 lb-ft.

Looking just at the peak numbers, the Vortech handily out horsepowered the Eaton, by a solid 142 hp and even managed to produce one additional lb-ft of torque. While the peak numbers looked impressive, the graph clearly shows that the centrifugal supercharger lagged behind the positive displacement supercharger up to 5,250 rpm. From 5,250 to 6,600 rpm (and beyond) it was all Vortech, but below that point, the immediate boost response of the Eaton offered as much as 200 additional lb-ft (at 2,500 rpm). Were this a 5,000 to 7,000 rpm motor, the Vortech would be the clear winner (in terms of acceleration), but it takes one heck of a top-end charge for the centrifugal to overcome all that low-speed response on the street.

The twin-turbo system from HP offered only slightly more boost pressure at 2,500 rpm than the centrifugal (3.7 vs 1.9 psi), but was well down on the 12.3 psi supplied by the Eaton. That difference in boost pressure translated into a torque deficiency of 160 lb-ft. The turbo lagged behind the Roots blower until 3,900 rpm at which point it took off with a vengeance. Running a maximum boost pressure of 13.6 psi, the HP turbo system eventually produced an amazing 830 hp and 756 lb-ft of torque. Imagine, the turbo system bested the peak torque production of the twin-screw and the peak horsepower production of the centrifugal by a country mile. Anytime you can produce an extra 100 hp and 150 lb-ft of torque at a similar boost reading, you know you've down something right.

Of course, all that extra efficiency comes with a penalty of low-speed torque (at least compared to the positive displacement blowers). Were we running the turbos at just 11 or 14 psi, we probably would have selected the smaller 46mm units (or better yet, an electronic waste gate controller) to further improve turbo response and the attending low-speed torque production. Of course, if you are looking to top 900 hp (something we did at just 17 psi with the HP system), these 57mm turbos are the hot setup.

This cast-aluminum discharge tube assembly was designed to connect the T-Trim Vortech to the throttle body. Note the compressor bypass valve designed to eliminate pressure surges in the intake during cruise and lift-throttle conditions.

There you have it, the ultimate boost bash. Now all you have to do is look over the attending graphs and choose the system that best meets your needs. For some, the factory Eaton will forever power their four-valve Cobra. It's hard to argue with the immediate boost (and torque) response of the Roots design. With plenty of aftermarket support for the '03-04 Cobras, extra power is just a phone call and Visa number away. Of course, there is a limit to the power available with the Eaton and once you reach it, it's time to start thinking about a blower upgrade.

In terms of bolt-ons, the Kenne Bell twin-screw is tough to beat. Offering a considerable chunk of additional boost and power potential combined with the immediate boost response you've come to love, the twin-screw can easily push your Cobra into the 10s. The Vortech centrifugal supercharger will easily produce more peak power than the Eaton (by a solid 200 hp or more), but that explosive top end charge is going to come with a penalty in low speed torque.

Fans of the centrifugal will love the never ending power surge, but whether Cobra owners will be willing to give up all that glorious torque remains to be seen. The turbo system easily offered the best horsepower and torque values, but like the centrifugal, the cost was low-speed torque production. At least in the case of the turbos, the explosion will come at just 3,900 rpm and not past 5,500 rpm like the centrifugal. In the end, what works best is all up to the individual Cobra owner.

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Eaton Supercharged 4.6 (14 psi)
Roots blowers are great for producing a ton of low-speed and mid-range torque and the M112 Eaton was no exception. This 4.6 thumped out over 550 lb-ft from 3,000 to 5,200 rpm. The downfall of the Roots blower was that the peak power gains were not as impressive after upping the boost pressure from 11.7 to 14.2 psi. That the M112 was nearing its maximum flow limit was evident by the fact that the peak power was only up by 11 hp (583 vs. 572 hp) compared to the runs made at 11 psi.

Eaton Roots Vs. Kenne Bell Twin-Screw (14 psi)
The Kenne Bell twin-screw supercharger out powered the less-efficient Eaton Roots design, this time to the tune of 121 hp. Despite being spun more than 4,000 rpm slower than the Eaton, the twin-screw blower produced 704 hp at 14.5 psi. In terms of peak torque, the twin-screw produced 595 lb-ft and bested the Eaton from 3,500 to 6,600 rpm. Note how the power curve on the Kenne Bell continued to climb with engine speed.

Eaton Roots vs. Vortech Centrifugal (14 psi)
We suspect that minor belt slippage hindered the Vortech from producing maximum peak power, but the 725 hp number at 14.0 psi was still nothing to sneeze at. From 5,300 to 6,600 rpm the Vortech easily showed heels to the Eaton, but things were a different story below that point. As with any centrifugal supercharger, the boost (and power) curve increased with engine speed. Offering slightly less than 2 psi at 2,500 rpm compared to 12.3 psi for the Eaton; is it any wonder the centrifugal was down over 200 lb-ft? Were we to rev this motor to 7,000 rpm or beyond, the power gains offered by the centrifugal would be even more impressive. In the end, you have to decide if a storming top end is worth the trade off in low-speed torque.

Eaton vs. HP Performance Twin Turbo (14 psi)
Given the fact that the major gains offered by increasing the boost pressure on the Eaton were realized at lower engine speeds and that increasing the boost pressure on the turbo system had no effect until the turbos were at full song, the power difference at 2,500 rpm actually increased when we upped the boost pressure from 11 to 14 psi. At 2,500 rpm, the Eaton realized a torque advantage of nearly 180 lb-ft and carried a torque advantage until 3,900 rpm. If you don't have last month's issue handy, know that the crossover point for the Eaton and HP turbo system occurred at just 3,600 rpm at 11 psi. As before, once the turbos came up, they pulled away from the Eaton with a vengeance. Running 13.6 psi, the turbocharged Cobra motor produced 830 hp and 756 lb-ft of torque. Despite a slight boost deficit, the turbos out-powered even the most powerful supercharger by more than 100 hp. The turbos produced an extra 150 lb-ft of torque despite being down by almost one full pound of boost to the blowers.

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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.

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.