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.

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What do these improvements mean for a GT500 owner (and other Mustang applications as well)? Replacing the standard M122 supercharger on your GT500 with the more efficient TVS can be worth as much as 100 extra horsepower. Since the stock blower will already deliver 550 rwhp with bolt-ons and tuning (up from roughly 440 rwhp in stock trim), the TVS looks pretty enticing, and it's no wonder Shelby Automobiles chose to offer the TVS as on of the blower upgrades for its Super Snake program.

While the TVS is certainly a big step up from the standard M122 supercharger, Shelby knew that many Super Snake owners were going to want even more. To satisfy owners with a serious thirst for power, Shelby decided to make its Super Snake available with the Twin Screw design from Kenne Bell. Starting out where the TVS Roots blower left off, the 2.8-liter H-series blower from Kenne Bell is capable of supporting more than 1,000 flywheel horsepower and has already demonstrated that it's capable of producing 800 rwhp on an otherwise stock GT500 engine.

Officially, the Super Snake options from Shelby includes a 600hp version equipped with the TVS blower and a 725hp version using the Kenne Bell Twin Screw. These numbers are the rated flywheel power outputs just as the stock GT500 is rated by Ford at 500 flywheel horsepower. That the Kenne Bell Twin Screw is capable of producing 800 rwhp by simply cranking up the boost on a GT500 motor is truly impressive, and we'll likely see many Super Snake enthusiasts running the requisite race fuel and small blower pulley (2.5 inches) required to reach the 800hp mark.

Before beginning the official test, Kenne Bell tech tapped the 5.4-liter with all manner of data-logging equipment. The guys at Kenne Bell border on obsessive when it comes to information, and despite the time required, it's hard to complain about it. Our data logging included the air/fuel ratio, ambient air temperature, engine speed, boost pressure before and after the intercooler, inlet air temperature before and after the intercooler, water temperature through the intercooler core, mass air voltage, timing curve, and even inlet vacuum before the supercharger. The 5.4 looked like a cardiac patient with hordes of wires running out of every possible orifice.

In addition to the data-logging parameters, the Kenne Bell boys logged every ounce of data from the OBD-II port--coolant temps, ignition timing, throttle-position sensor values, and so on--to ensure accuracy between runs. In anticipation of the high-boost runs, the fuel tank was filled with 100-octane race unleaded fuel to eliminate any possibility of detonation. Naturally, backup runs were recorded to verify the power numbers generated by each pulley size. These runs help eliminate anomalies that can occur during testing. Knowing the intensity of GT500 owners (even more so, the Super Snake customers), every effort was made to ensure accurate, repeatable data.

Since it was already installed on the motor, the first series of tests was run with the stock Eaton M122. It should also be noted that the 5.4 was equipped with a standard 17V Boost-a-Pump, stock 52-lb/hr injectors, and a custom tune to maximize power production with an air/fuel ratio of 11.5:1 and 23 degrees of total timing (values used for all of the tests to ensure repeatability). The idea behind the fuel system upgrades was to ensure a stable air/fuel ratio at each boost level.

In stock configuration, the M122 blower provided a peak of 8.7 pounds of boost, but for our needs, we wanted to find out what was possible by running the smallest blower pulley possible. This meant the installation of a 2.59-inch blower pulley (using a smaller pulley requires machining of the blower snout).

To illustrate the power potential offered by the TVS, we installed a 2.59-inch blower pulley. The result was an impressive 609 hp at the wheels. The TVS has become a popular option as it fits under the stock strut-tower brace and doesn't require any fuel system upgrades to increase the power of a GT500.

Factory blower and turbo cars are great because additional power is just a pulley change away. Of course, these swaps require proper air/fuel and timing values that are commensurate with the available fuel octane. The combination of the performance tune and swapping on the 2.59 blower pulley brought the peak power output from 443 to 518 hp. It should be said that additional power is available from the stock M122 blower with a larger crank pulley and modifications to the intake tract (air filter and inlet tubing, mass air housing, and throttle body), but ultimately, it's the blower itself that limits power production.

With the testing completed on the M122, it was time for the Super Snake to receive the TVS blower upgrade. Rated at 600 flywheel horsepower by Shelby Autos, the TVS had considerably more to offer this GT500 motor. With an increase in both displacement (2.3 versus 2.1 liters) and efficiency (see previous description of changes to TVS), it's not surprising that the TVS blower increased the power output of the GT500 motor. By running the same 2.59-inch blower pulley on the TVS blower, the peak boost numbers jumped from 11.5 to 14.5 psi. The peak power jumped from 518 rwhp with the M122 to an impressive 609 with the TVS.

As with the M122, there was likely more power to be had with changes to the air intake and possibly a larger crank pulley. We've seen TVS GT500s, such as Justin Starkey's (of VMP Tuning), make nearly 700 rwhp so optimized. Not everyone can or will crank up the boost to the maximum setting and then make the concerted (and important) effort to minimize inlet restrictions. Positive-displacement blowers (Roots or Twin Screw) are sensitive to inlet restrictions. Check out the short sidebar on inlet system testing, but know that any restriction in the inlet system (basically anything in front of the entry to the rotors themselves) will result in a drop to flow, boost production, and ultimately, power production.

The 2.8-liter H-series Kenne Bell supercharger offered increased displacement and efficiency and the ability to produce huge gains on an otherwise stock 5.4.

The final test was to remove the TVS and install the Kenne Bell Twin Screw supercharger. Like the comparison between the M122 and the TVS, the Kenne Bell Twin Screw supercharger was both larger in displacement (meaning more output per revolution) and more efficient (meaning less charge temperature per pound of boost). While individuals and companies alike argue the different efficiency rating of a supercharger, the real test is running them on the dyno. Of course, we also had charge temperature measurements and even test results from Kenne Bell's own blower dyno.

To say that a Twin Screw supercharger is more efficient than a Roots supercharger is hardly news. Every SAE paper ever written confirms this fact, and the real reason OEMs use the Roots blower is that it provides an excellent combination of power, reliability, and cost. Basically, it does the job of reaching the desired power level (usually not terribly high on a production vehicle) at an attainable cost and with quiet execution. Were maximum power the design goal, you'd quickly see a change to the Twin Screw design. Nowhere was this more evident than the use of the Twin Screw on the (cost-is-no-object and higher horsepower) Ford GT. Further illustrating the efficiency and size advantage held by the Kenne Bell supercharger was the fact that removing the Eaton TVS and replacing it with the Twin Screw resulted in a jump in power to 788 hp using the same 2.59 pulley. Boost production jumped from just 14.5 psi with the TVS to 23 with the Kenne Bell.


While the power numbers alone confirm the blower hierarchy, here's additional information provided by another form of dyno testing: namely, a blower dyno. What is a blower dyno, you ask? Just as it sounds, the blower dyno is used to evaluate superchargers without regards to the engine. On the blower dyno, the superchargers are mounted to a test fixture and spun using (in this case) a 400hp V-8 LS-series GM engine. The blower dyno allows Kenne Bell to regulate all of the variables to properly evaluate the supercharger, namely blower speed, air temperature and pressure into and out of the blower, and the parasitic losses associated with driving the blower. That's why such a powerful drive motor is required.

While it doesn't compare to the 500 hp required to drive a Top Fuel Roots blower, some of these larger street/strip superchargers required as much as 150 hp to spin them at 18,000 rpm and 20-plus pounds of boost. Not surprisingly, results from the blower dyno correlate perfectly with the chassis dyno. Configured to produce 20 psi of boost (a level not attainable with the TVS on the 5.4), the TVS checked at 18,000 rpm, 340 degrees of discharge temperature, and a whopping 147 hp worth of parasitic drive losses.

This means it took 147 hp to drive the TVS supercharger at this speed and flow level. By comparison, the Kenne Bell required only 12,700 rpm to produce the same 20 psi. Running the same boost level, the discharge temperature registered only 282 degrees, and the Twin Screw blower absorbed only 111 hp in the process. Thus the Twin Screw reduced the inlet charge temp, blower speed, and parasitic losses required to reach the desired boost level.

Certainly it would be interesting to compare the Roots and the Twin Screw in the exact same displacement and with equivalent aero-restriction inlet systems to see the similarity of the numbers, but for now, Kenne Bell's test provides an interesting look at two of the popular upgrade options for GT500 owners as available. Either one has the capability to provide more power than your average driver is really equipped to handle, but for those who find too much is just right, now you're better educated as to how these two options compare.

Popping The Cork
One of the often-overlooked aspects of any supercharged engine is its induction system. Unfortunately for enthusiasts, positive-displacement superchargers are ultra-sensitive to inlet restrictions. Lucky for us, it's actually easy to determine whether the inlet system (basically anything in front of the spinning rotors) is restrictive. In the case of the 5.4-liter GT500 motor, a simple vacuum gauge hooked up between the throttle body and supercharger will indicate whether vacuum is present. If any vacuum is present when running at or near the peak engine speed at wide-open throttle, there's a restriction in the inlet system somewhere.

Taking these same readings between the throttle body and mass air or between the mass air and filter will help you know exactly where the restriction is most prominent. Is the restriction in the intake manifold between the rotors and the throttle body, the throttle body, the mass air, the inlet tube, or the filter? Luckily for us enthusiasts, the gang at Kenne Bell has spent long hours on the dyno replete with data logging at each point to determine where the restrictions exist, and then to produce the necessary products to cure the situation.

How much do these restrictions hurt the power output? Replacing the throttle body, inlet air tube, mass-air housing, and filter assembly improved the power output of the supercharged 5.4 from 664 hp to 745 hp. These power gains came with no change in air/fuel, timing or blower pulley size.

TEST HP VAC SC MANIFOLD TB INLET MASS AIR FILTER
Stage 1 664 5.3 2.8 Mammoth Stock Stock Stock Stock
Stage 2 727 3.3 2.8 Mammoth Stock Stock Kenne Bell Kenne Bell
Stage 3 745 0 2.8 Mammoth Kenne Bell Kenne Bell Kenne Bell Kenne Bell
Stage 3+ 801 0 2.8H Mammoth Kenne Bell Kenne Bell Kenne Bell Kenne Bell
Next in line in the induction system is the throttle body. To improve flow and minimize inlet restrictions, Kenne Bell ditched the factory throttle body for a dual 75mm.

It should be evident from the chart that eliminating inlet restriction on a supercharged application can yield big dividends. Running the stock ind-uction system from the blower manifold forward resulted in a peak power number of 664 hp. Replacing the mass air housing and filter (Stage 2 upgrade) dropped the vacuum present in the inlet tract from 5.3 inches down to 3.3 inches and increased the peak power to 727 hp. Stepping up to the Stage 3 induction system replaced the stock throttle body (with a dual 75mm unit) and intake tube (between the throttle body and mass air).

This further reduced the vacuum in the induction system to 0 and increased the power output to 745 hp. Replacing the standard 2.8 blower with the 2.8 H-series took the 5.4 to 800 rwhp (using the same 2.5-inch blower pulley and Stage 3 induction). Just think, you could be stuck with just 664 hp running the same pulley configuration or making as much as 774 wheel hp (800 with H-series blower). It's all in the induction system.

On The Dyno
It's hard not to be impressed by the power potential of the GT500's 5.4-liter engine. Equipped with the factory M122, 2.59-inch blower pulley and optimized tune (11.5 A/F and 23 degrees timing), the 5.4 produced 518 hp (up from 443 hp in stock configuration). Running the TVS blower in the same state of tune and with the same pulley size, the peak power numbers jumped to 614 hp.

This test reveals that the TVS supercharger upgrade offered by Shelby on its Super Snake is a good bit more powerful than the factory offering. The last blower upgrade featured the Kenne Bell 2.8-liter H-series blower. Shelby offers the Kenne Bell blower package as its most powerful Super Snake option. Run with the same blower pulley, timing, and air/fuel, the Super Snake pumped out nearly 790 rwhp, a gain of 175 hp more than the already powerful TVS and nearly 300 hp more than the stock M122. Thats what we call a blower upgrade.

How serious are the folks at Kenne Bell about superchargers? Check out this blower dyno used specifically to test superchargers

As expected of a positive-displacement supercharger, the torque curves were pretty flat. Equipped with the M122 supercharger, torque production exceeded 500 lb-ft from 3,000 to 4,700 rpm, with a peak of 514 lb-ft. Equipped with the same blower pulley, the TVS supercharger increased the peak boost pressure from 11.5 psi with the M122 to 14.5 psi. The increase in boost pressure has a positive effect on the torque curve, elevating the peak torque output to nearly 600 lb-ft. Torque production exceeded 500 lb-ft from 2,200 rpm all the way to 6,300 rpm (our redline).

The largest blower tested, the Kenne Bell 2.8-liter H-series, produced the highest boost pressure. Despite the same blower pulley size, the Kenne Bell offered 23 psi of boost and 736 lb-ft of torque. Torque production exceeded 700 lb-ft from 3400 to 5700 rpm. Harnessing more than 600 lb-ft of torque at just 2,100 rpm would be difficult, but it's always better to have too much than not enough.


Kenne Bell even offers a larger mass air housing and filter assembly. Every component in the induction system must be up to the task of supporting the desired power level. Any vacuum present in the inlet systems costs you power.





  STOCK BLOWER TVS DIFFERENCE KENNE BELL DIFFERENCE
RPM TORQUE POWER TORQUE POWER TORQUE POWER TORQUE POWER TORQUE POWER
2,000 442.48 168.50 473.73 180.40 31.25 11.{{{90}}} 566.69 215.{{{80}}} 92.96 35.40
2,500 472.68 225.00 524.35 249.60 51.67 24.60 652.51 310.60 128.16 61.00
3,000 501.39 286.40 565.46 {{{323}}}.90 64.07 37.50 672.43 384.10 106.97 60.20
3,500 513.35 342.10 585.25 390.02 71.90 47.92 701.07 467.20 115.82 77.18
4,000 512.72 390.50 587.30 447.30 74.58 56.80 718.99 547.60 131.69 {{{100}}}.30
4,500 504.30 432.10 587.17 503.10 82.87 71.00 725.24 621.40 138.07 118.30
5,000 497.{{{57}}} 473.70 579.50 551.70 81.93 78.00 734.43 699.20 154.93 147.50
5,500 481.46 504.20 559.76 586.20 78.30 82.00 708.92 742.40 149.16 156.20
6,000 448.95 512.90 530.88 606.50 81.93 93.60 685.21 782.80 154.33 176.30
6,{{{300}}} 416.90 500.10 471.59 565.70 54.69 65.60 658.91 790.40 187.32 224.70