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Kenne Bell 3.6L Twin Screw - Bigger, Better Blower
Testing Kenne Bell's 3.6L Twin Screw
Life is good if you're a mod motor maniac looking for boost, as there are over a dozen different blowers to choose from. If immediate boost is high on your list, the factory Roots blowers offered on the '03-'04 Cobra and GT500 offer instantaneous response combined with plenty of mid-range and top-end power. The average power production of a force-fed Ford has to be experienced to be believed. Those who stand by their decision to run "all-motor" modular combinations have obviously never experienced one with boost!
While the factory (and aftermarket) blowers are impressive, there are always those looking to step things up. These overachievers all but mandated blowers like the Kenne Bell 2.8L H-series. Capable of supporting over 1,000 hp (we recently produced 1,036 hp on a modified 5.4L Four-Valve), the 2.8L H-series has proven to be the class of the field for high-boost, high-horsepower applications. Credit the billet casing for minimizing deflection (allowing higher blower speeds and boost levels), along with the a unique rotor pack and discharge design (port timing) for the impressive packaging and performance of the 2.8L.
Not content to rest on its laurels, Kenne Bell has recognized the need for even higher power levels and recently introduced the new 3.6L. From a size standpoint alone, the 3.6L is capable of supporting over 25 percent more power compared to the 2.8L (yet it still accepts the factory fuel rail and fits under the hood). Not satisfied with a simple increase in displacement, the new 3.6L also offers a number of desirable features including a proprietary rotor configuration, seal pressure equalization, and a revolutionary new liquid cooling system.
The result of these changes is a larger, more powerful supercharger that also offers a reduction in parasitic losses. All superchargers suffer from parasitic losses associated with driving the unit. Different superchargers operating at different boost and power levels require different amounts of energy to drive. When the new design offers more power and boost potential with a reduction in parasitic losses, you know you've done something right. Throw in a decrease in inlet air charge temperatures and increased component life from the liquid cooling and seal pressure equalization, and you have one serious deal.
Naturally, we needed to test the new blower, but before getting to the impressive results (don't sneak ahead and look at the dyno results), we need to understand why the new blower offers so much more power.
First and foremost, the 3.6L is bigger than the old 2.8L. What this means is that for every revolution, the bigger blower will process more airflow. While this bigger-is-better philosophy seems like a no-brainer, there are limitations, or more accurately, proper applications for the larger blower. The ideal application for the larger 3.6L blower is not a stock 4.6L Two-Valve, running 7 psi of boost, but rather a 5.4L GT500 running 15-plus psi of boost. The larger 3.6L was designed for high-horsepower and/or high-boost applications. Given its ability to support 1,200-plus horsepower, it is less than ideal for someone looking to produce just 400 hp. The big blower will obviously do the job at lower boost and power levels, there are just better blower choices for the lower-horsepower applications.
Not just bigger, the 3.6L blower is also better. Better in this case means that with extensive research and development, Kenne Bell has managed to significantly reduce parasitic loses associated with driving the supercharger. Since the parasitic losses are essentially power absorbed by the motor to drive the blower, the power is not applied to propel the vehicle.
According to KB, the new design has reduced the parasitic losses by as much as 76 hp! To put this into perspective, the 3.6L will produce the same amount of power as the smaller 2.8L blower at roughly 3 psi less boost. Put another way, the improvement in parasitic losses is equivalent to running an extra 3 psi of boost, but without the associated increase in charge temperature. Some of the drop in parasitic losses can be attributed to the fact that the larger blower simply spins slower to produce the same amount of boost as the smaller 2.8L. Additional gains come from a revised 4x6 rotor pack, meaning the male and female rotors sport four and six lobes respectively, differing from the less efficient 3x5 combination used by the competition. Changes to the rotor pack were combined with a unique discharge shape to optimize port timing, maximize airflow, and minimize charge temperature.
Additional improvements include liquid cooling and seal pressure equalization. One of the obstacles to overcome with a twin-screw supercharger is the temperature differential between the inlet and outlet sides of the blower. Since internal compression takes place inside the twin-screw, the air entering the blower is much cooler than the air exiting the blower. This temperature differential increases with elevated boost levels.
Running very high boost means one end of the blower can become much hotter than the other. The problem is the expansion rate of the case and rotors is also a function of the temperature. Given the dramatic temperature differential between the two ends of the blower at high boost levels, the growth rate of the case and rotors will be dramatically different. Obviously the clearances must be designed to facilitate this differential, but Kenne Bell has found a way to minimize the temperature differential by cooling the hot end of the blower. This is accomplished by routing the intercooler water through a cooling cavity on the hot side of the supercharger. The result is a reduction in rotor spread, which causes gear lash and the attending changes in rotor timing.
In addition to the reduction in the expansion rate, the cooling system also increases component longevity and decreases the charge temperature. The increased component life comes from a reduction in oil temperature. The supercharger gears are spun in a sealed oil bath. The elevated temperatures produced by the higher boost levels obviously have a negative effect on oil temperature at the hot side of the blower (the discharge is right next to the oil reservoir). Elevated boost and rpm levels significantly increase the temperature of the gear oil. The revolutionary new cooling system helps reduce the case and rotor temperatures, which in turn help reduce the gear oil temperature. The reduction in gear oil temperature increases the service life of the drive gears, rotors and associated bearings. The byproduct of the decrease in case, rotor, and oil temperature is a reduction in charge temperature.
According to in-house testing by KB, the liquid cooling (LC) system reduced the charge air temperature exiting the supercharger (before the intercooler) by over 20 degrees (from 367 degrees to 344 degrees on a Shelby running 27 psi of boost). Lower temperatures equal more oxygen molecules, which in turn equals more power. Let's not forget the associated reduction in the potential for harmful detonation.
Another trick applied to the new 3.6L Twin Screw is something called seal pressure equalization (SPE). The twin-screw design features a front seal to separate the gearbox from the rotor pack. The problem is that this front seal must resist the internal pressure supplied by the supercharger. The greater the boost levels, the more pressure applied to the seal. Recognizing the potential of the situation, Kenne Bell designed the seal pressure equalization (SPE) system, which effectively equalizes the pressure on both sides of the seal to eliminate the differential.
Using a pressure relief system, the pressure is vented from the back of the seal to the inlet tube or air filter. Relieving the pressure eliminates the chance of seal failure thereby preventing rotor pressure from entering the gear box. According to Kenne Bell, the standard seal system works very well up to 20 psi of boost. It is in the 21- to 30-psi range where the SPE system comes into play, though the SPE does offer considerably less seal wear and friction under high-vacuum conditions. This helps eliminate the tendency to pull oil from the reservoir. The combination of the temperature reductions from the liquid cooling and the extended seal from the SPE has practically eliminated seal wear at high boost and vacuum conditions (not to mention adding a few extra hp).
All these ideas sound good on paper, but would all of the improvements equate to more power? To find out, we ran a series of back-to-back tests using a Shelby GT500 Super Snake comparing the 2.8L H and the new 3.6L. The first test was actually performed by Kenne Bell on its blower dyno. Running both blowers at identical boost (24 psi) levels demonstrated that the 3.6L require just 177 hp to spin compared to 217 hp for the 2.8L.
The 3.6L also reduced the all-important charge temperature by 31 degrees (at the same boost level). All of the results on the blower dyno were achieved by stabilizing the discharge temperature with a sustained run of 20 seconds. The differences would be slightly less in the real world unless you were running flat out for more than 20 seconds (think Silver State Classic open road race, Maxton Mile, or in a marine application).
The blower dyno results obviously indicated the power potential of the 3.6L over the smaller 2.8L, but how would these results translate on the chassis dyno? To illustrate the gains offered by the 3.6L, we ran the Shelby with both the 2.8L H and the new 3.6L. The first test was to turn the two blowers back to back at 15 psi. Naturally, both blowers were run with the same air/fuel and timing as well as the same air and coolant temperatures, the only difference being the blower swap.
Running both blowers at 15 psi, the 2.8L produced 643 hp while the 3.6L upped the ante to a peak of 674 hp, a difference of 31 hp. Remember this test was run at identical boost levels of 15 psi. Stepping things up to 23 psi resulted in 747 hp for the 2.8 and 832 hp for the 3.6L. The 3.6L offered an additional 76 hp over the 2.8L at 23 psi.
The final test was to illustrate what happens when you replace the 2.8L with the 3.6L using the same pulley size. Running both blowers with a 3.25-inch blower pulley resulted in 619 hp for the 2.8L at 13.5 psi and a whopping 774 hp for the 3.6L at 22.4 psi. Replacing the smaller blower with the larger one (run at the same blower speed) was worth an additional 155 hp.
With the capacity to support over 1,200 hp, these tests indicate that in the case of the 3.6L Kenne Bell, bigger is certainly better.