Double Trouble Mods: Twin-Turbo Cobra Motor

Words And Photos: Richard Holdener

There is a rule that performance enthusiasts live by that goes something like this, if some is good, then more must better. While it can be argued that there are exceptions to this rule, it certainly applied to our 4.6L Modular Ford test motor. In the hierarchy of factory 4.6L Ford motors, one particular model stands above all others, the supercharged 4V Cobra motors. The factory Eaton supercharger provided an underrated power level unmatched by any previous 4.6L and the short, two-year production run from 2003-04 further enhanced desirability by ensuring limited quantities. The great thing about any factory forced induction application is not only the stock power output (underrated by Ford no less), but how much power the factory left hiding in the combination. Shortly after the introduction, Cobra owners were hard at work dialing in the tune, cranking up the boost and taking the supercharged motors well beyond the power output originally designed by the factory.

As good as the factory supercharged Cobra motors were (and continue to be), the limiting factor in terms of power output is actually the supercharger itself. Superchargers are sized specifically for the intended application and power level. There was obviously some wiggle room with the factory Eaton supercharger used on the 2003-04 Cobra motors, but ultimately, there was a limit to blower speed and maximum power potential. Since we were looking to double the power output, we needed something decidedly more powerful than the factory supercharger. In keeping with our more is better theme, what we really needed was two somethings! In our case, thing 1 and thing 2 turned out to be identical twins in the form of 57mm Garrett turbos. Turbos alone would not do us much good, but these turbos were supplied as part of a complete twin-turbo kit from HP Performance. No longer in business, the twin kit offered not just turbos but dedicated turbo manifolds, wastegates and an air-to-air intercooler. In fact, the twin kit offered everything we needed to crank up the boost on our Cobra motor.

Twin turbos are all well and good but there was much more to our Double Trouble experiment than more boost. Since our test motor was already sporting a supercharger, the first order of business was to replace the blower. It is possible to feed the supercharger with the turbos in a compound application, but that is another test for another day. For now, we simply replaced the supercharger with an intake manifold from a 2001 (normally aspirated) 4V Cobra. The supercharger was only a small part of the supercharged 2003-04 motor, as the entire package was fortified for forced induction. This made it the ideal combination for our turbo system. Even without the blower, the 4V motor offered plenty of power in stock trim, but we wanted more! To get more, we replaced the factory Cobra cams with a set of XE262AH grinds from COMP Cams. The Xtreme RPM series cams offered .425 lift, a 226/222-degree duration split and 114-degree lsa. Tested on the dyno, these cams were worth 50 horsepower over the stock cams with no other changes.

To illustrate just how effective the turbos were at increasing the power output, we ran the 2004 Cobra motor normally aspirated with the COMP Cams and Cobra intake. The 4V motor was run using an Accufab throttle body, FAST XFI management system and Hooker long-tube headers. Tuned to perfection with 65-pound FAST injectors, the normally aspirated Cobra motor produced 427 hp at 6,400 rpm and 388 lb-ft of torque at 5,300 rpm. Adding the turbo kit required drilling and welding oil drain fittings in the oil pan. The wastegates were set up with 7-psi springs and additional boost was dialed in using a manual wastegate controller from Turbo XS. Using the FAST management system to dial in the air/fuel and timing, we dialed up the boost to the eventual maximum of 14.8 psi. Thanks to race fuel and the air-to-air intercooler, the twin-turbo combination was plenty safe. Running 14.8 psi of boost, the twin-turbo Cobra just missed doubling the power output of the normally aspirated motor with peaks of 843 hp and 788 lb-ft of torque. Nearly doubling the power output with twin turbos at 2 bar surely earned this 842 horsepower Cobra motor the nick name Double Trouble.

We started out with a supercharged 2004 Cobra motor, but instead of running the factory Eaton supercharger, we replaced it with turbos. The supercharger assembly was replaced with a factory 2001 Cobra intake, while the stock 03-04 Cobra cams were ditched in favor of a set of COMP XE262AH grinds. Run in normally aspirated trim, the modified 4-valve Ford produced 427 hp and 388 lb-ft of torque. After adding the twin turbo kit, the Power numbers jumped to 843 hp at 6,500 rpm and 788 lb-ft of torque at 5,200 rpm. We were just a few horsepower short of doubling the normally aspirated power output at a peak boost pressure of 14.8 psi-Double Trouble Indeed!