Richard Holdener
June 1, 2009
Could your Shelby GT500 use an extra 100, 200, or 300 hp? Those kinds of numbers are just a blower upgrade away.

Horse Sense: If you have a GT500 and want it to undergo a Super Snake conversion, the base package with the TVS blower and all the other mods that make it a 600hp Super Snake will set you back $27,995. If you want to lose the warranty and add the Kenne Bell, that program will squeeze your wallet for $31,995.

The more things change, the more they stay the same. Not so many moons ago, I was responsible for one of many supercharger comparison tests (in another magazine), putting an Eaton Roots supercharger against its positive-displacement cousin, the Twin Screw, and here I am again. The previous test, like most comparisons, was not without its difficulties. Case in point, that positive-displacement supercharger slugfest pitted the like-displacement 3150 Twin Screw Autorotor versus the standard Roots-style M90 Eaton. From a reader's standpoint, I'm sure you were mostly concerned with how much power each system made, but there was--and still is--much more to the equation than sheer power numbers.

One of the problems associated with comparing any two performance components, let alone superchargers, is how to accurately test them. Oftentimes it's difficult enough to get a normally aspirated motor to cooperate and produce repeatable results on a dyno, let alone some supercharged mod-motor monstrosity. As expected, there are any number of variables such as timing, air/fuel ratio, and even the various temperatures (air, water, and oil) that can alter the results. All of these things must be accounted for in order to produce reliable results. Lucky for us, the good folks at Kenne Bell have taken data logging to the extreme to ensure every variable is present and accounted for.

While the previous test involved the smaller M90 Eaton and the 3150 Autorotor, this one involved much-improved versions of the original combatants. Even the test vehicle (originally an '00 Saleen equipped with a Two-Valve 4.6) has been upgraded to none other than the mighty Shelby GT500. In fact, both blowers in this comparison are offered by Shelby as upgrades to the original M112 used on the GT500--the new 2.3-liter TVS Roots blower from Eaton and 2.8-liter H-series Twin Screw from Kenne Bell--and as power options on the Super Snake versions.

With both systems available for the Super Snake and as upgrades for the standard GT500, it was only natural that we take a long, hard look at the performance offered by each. Is it really necessary to upgrade a supercharged motor with another supercharger? If you have to ask that question, maybe you're reading the wrong magazine, as these blower upgrades offer at least 100 hp more than the factory M122 Eaton supercharger. The most powerful Twin Screw version from Kenne Bell is good for a solid 800 rwhp with no other changes to the stock 5.4 engine. Who (you might ask) really needs 800 rwhp? The answer is easy: anyone who pulls up next to a car sporting 790 rwhp!

Before getting to the results, it's worth recounting a bit of forced-induction theory. Though enthusiasts tend to lump the positive-displacement superchargers into one group, there's a significant difference between the Twin Screw Kenne Bell and the Roots-type Eaton. Unlike the traditional Roots, the Twin Screw is (to a minor extent) a true compressor, meaning that internal compression takes place inside the blower. The Eaton, on the other hand, simply moves air from one side of the blower to the other. The lack of internal compression actually lowers the efficiency of the Roots blowers, especially as the boost pressure (pressure ratio) and rotor speed increase.

The new Twin Vortices Series is a solid step up from the Gen 5 M122 supercharger factory installed on the GT500's 5.4. Compared to the more traditional M122 Roots supercharger, the new TVS offers a greater lobe count (four versus three), increased rotor twist (160 degrees versus 60 degrees), and revised inlet and discharge architecture, to say nothing of the increase in displacement (2.1 versus 2.3 liters). These revisions have greatly improved not only the ultimate power and boost potential of the Roots blower, but the all-important average power production.