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...
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.
 As expected, the Vortech easily...  As expected, the Vortech easily out-powered the Eaton supercharger. The peak numbers were impressive, with the 4.6 now producing 725 hp and 575 lb-ft of torque, though the centrifugal could not match the low-speed torque output of the positive displacement Eaton. |  After seeing the huge power...  After seeing the huge power produced by the HP twin turbo system at 11 psi, we were anxious to see how much power was available at 14 psi. |  The twin-turbo kit supplied...  The twin-turbo kit supplied by HP Performance included a pair of 57mm turbos capable of supporting roughly 500 hp each. |
 Changing the boost pressure...  Changing the boost pressure on the HP kit was a snap thanks to the manual waste gate controller from Turbo XS. A simple twist of the power screw was all it took to up the boost from 11 to 14 psi (actually 13.6 psi). |  We replaced the C&L (blow...  We replaced the C&L (blow through) mass-air meter with a simple section of silicone hose. The MAF was not used with our F.A.S.T. fuel injection. |  Running through the (front-mounted)...  Running through the (front-mounted) air-to-air intercooler, the HP twin turbo kit bested every other form of forced induction by at least 100 hp by producing 830 hp and 756 lb-ft of torque. Mind you this was at a peak boost pressure of just 13.6 psi-down by as much as one full psi to the others. |