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Kenne Bell 3.6L Twin Screw - Bigger, Better Blower
Testing Kenne Bell's 3.6L Twin Screw
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.