While we have yet to exhaust the available normally aspirated modifications for our early 4.6L Four-Valve motor, we have decided it is high time for boost. Previously, our high-mileage '96 Cobra mill was subjected to IMRC eliminator plates, a modified (shortened) stock intake, and a larger throttle body. While we had it on the dyno, we also engaged a Zex wet-nitrous system to the tune of 100 hp. After those mods, we installed a set of Comp XE262AH cams with excellent results.
In addition to properly degreeing the cams, we also demonstrated just how far off they can be when installed using the factory keyways. With 3 degrees of slop in each cam/sprocket interface, you're looking at a wide variance using the factory-keyed sprockets (to say nothing of using the early non-keyed sprockets). Though not specifically featured in the "Mods For Early Four-Valve Mods" series, we also subjected the motor to further testing with the Comp cams by retarding the intake cam from 110 degrees to 116 degrees. The results were interesting in that the power was up with the stock manifold, but adjusting the cams actually reduced the power with the VRI intake designed by the author. Equipped with the VRI intake and XE262AH cams (adjusted at 110 degrees), the motor eventually produced 411 hp and 342 lb-ft of torque (35 hp over the stock intake).
For this episode, we removed the VRI intake and replaced it with the stock manifold and throttle body. Previous testing has shown the stock dual-blade throttle body (in fact, the entire lid and plenum volume) had little effect on power at this level. The motor still featured the Comp XE262AH cams installed with the intakes set at 116 degrees and the exhaust cams at 112 degrees. In this slightly retarded position, the motor produced its highest peak power with the stock intake.
The normally aspirated motor was set up with the FAST management system, 36-pound injectors, and Hooker long-tube headers. Meziere sent us a new electric water pump that featured an idler pulley, which allowed us to run the electric pump with the Kenne Bell blower. Run at an air/fuel ratio of 13.0:1 and 29 degrees of total timing, the normally aspirated 4.6 produced peak numbers of 374 hp at 6,500 rpm and 342 lb-ft of torque at 5,200 rpm. Torque production exceeded 300 lb-ft, from 3,600 rpm to 6,500 rpm. While 300 lb-ft of torque at 3,600 rpm is fairly impressive from just 281 ci, that number was about to change by more than 100 lb-ft with the installation of the Kenne Bell blower.
While the '03-'04 Cobras grab a lot of attention when talk turns to supercharging, the '96-'98 Cobras are the cars that actually got things started. True, there is not much comparison between the power of an early normally aspirated Cobra motor and the later supercharged version, but like its big brother, there is plenty of power in the N/A version that's just waiting to be unleashed. All it takes is a little forced induction.
With that in mind, we decided to step up the power production of the early motor with the installation of a Kenne Bell supercharger. Unlike the later-production Cobras, which rely on an Eaton Roots-style blower, the Kenne Bell utilized a twin-screw design. Compared to a Roots-style, the twin-screw offers much more power potential (per liter of displacement), reduced inlet charge temp, and it requires less power to drive. The combination of these attributes means the Kenne Bell twin-screw blower will provide more power per pound of boost than a Roots-style. That it is a positive displacement design means efficient boost is provided the minute you stomp on the throttle.