Is all boost created equal? Obviously not or we wouldn't have a story.
We ran out of room for the Eaton supercharger, but we figured twin turbos feeding a Vortech feeding a Kenne Bell twin-screw was impressive enough.
Imagine installing nearly every available form of forced induction available for the four-valve 4.6 on the motor in a boost-bashing, charge-cooling, head-to-head test. Running all four of the different forms of forced induction on the same engine at the same boost level certainly demonstrated the advantages and disadvantages offered by each form.
In reality, the reason behind this particular adventure was exactly that, to illustrate inherent differences in the boost and power curves-not to crown an absolute winner. We will discuss shortly why it would be impossible to determine an absolute winner, but know this very basic fact: If one form of forced induction was vastly superior to all others on all levels, no other forms of forced induction would continue to exist. That we have so many forms to choose from only provides alternatives that might be best for our particular application. In the end, it will be the user and specific application that will determine the appropriate form of forced induction that best applies, but before you can choose, you need to see how they all stack up.
As luck would have it, we had the perfect test bed on which to run this boost bonanza. Our '03 Cobra crate engine (from the Ford Racing catalog) was not only factory-equipped with one of the methods being tested (Roots-style Eaton M112), but was also configured to easily accept the rigors of forced induction. The forged crank, rods, and pistons provided an ultra-sturdy reciprocating assembly on which to pump up the pressure, while the low-compression made such an exercise a safe and welcome proposition. The '03 Cobra mill had already proven itself plenty capable of supporting prodigious power levels, exceeding the 700hp mark at elevated boost levels (naturally with a blower upgrade) without ever so much as lifting a valve cover. Who knows exactly how much power these motors are capable of, but we may attempt to find out after running all four of our boost-meisters.
Baseline '03 Cobra-Eaton M112 (11 psi)What's not to love about the supercharged '03 Cobra 4.6, especially when the boost is cranked up to 11.7 psi (thanks to an 8.5-inch crank pulley)? Equipped with the Accufab throttle body and inlet, XE264AH Comp Cams cam and Flowtech long-tube headers, the Eaton-supercharged 4.6 produced 572 hp and 533 lb-ft of torque. While the 533 lb-ft of torque is an impressive number, it is nowhere near as important as the fact that the torque curve exceeded 500 lb-ft from 3,100 rpm to 5,800 rpm. Even down at 2,500 rpm, the blown Cobra motor pumped out 480 lb-ft.
In addition to the reinforced reciprocating assembly, the '03 Cobra 4.6 also sported free-flowing four-valve heads. We were plenty successful in producing power from the two-valve GT motors in our previous "Mods for Two-Valve Mods" series, but even the best-flowing two-valve heads are no match for the stock four-valve Cobra heads. We will eventually take a look at ported versions of the Cobra heads, but for now, the stock heads seemed to offer plenty of performance.
Before getting to the test, we need to take a quick look at some of the problems inherent in such a comparison. The first obstacle to overcome was deciding on how to properly evaluate them. While enthusiasts might like the idea of running all four forms up to their absolute maximum power potential, this is both difficult and probably unrealistic. Sure, we could produce some impressive power results, but the reality is that not many (street) Cobras out there are running around with the blowers set on kill.
The vast majority are more likely dialed down to stun-meaning minor mods, additional boost pressure, and a few with blower or turbo upgrades. Short of the crazies that populate the Cobra classes at the dragstrip, how many have you ever run across sporting 25-30 psi? If we were running a comparison between two identical-sized superchargers, we would simply pulley them the same and see what happened, but since even the Kenne Bell twin-screw and Eaton M112 were sized differently (to say nothing of the Vortech and twin turbos), we decided to select boost pressure as our regulating device.
Since most Cobra owners are quick to crank up the boost well beyond the factory 8-9 psi level, we decided to start the comparison at 11 psi. To further illustrate the limitations of the factory blower versus the aftermarket upgrades, we also chose to run a high-boost 14-psi test. Running the two different pressures would (hopefully) serve to differentiate the boost and power potential offered by each form. When searching for a particular type of forced induction, more information is always better than less.
The baseline for our boosted blowout was an Eaton M112 Roots blower, which was original equipment on the '03-04 Cobras. The supercharged four-valve motor has earned a well-deserved reputation out on the mean streets. | 
To eliminate any inlet restrictions (especially important once we cranked up the boost), we installed an Accufab throttle body and inlet on the Eaton supercharger. | 
On the dyno, the Eaton-supercharged 4.6 produced 572 hp and 533 lb-ft of torque at a peak boost reading of 11.7 psi. |