There they are, the stock...
There they are, the stock cats. With 20 pounds of boost available from the twin-screw supercharger, getting to some combination of headers, high-flow cats, and mufflers is imperative to experiencing all the supercharger has to offer. At the 600hp level, a set of long-tube headers, a high-flow X-pipe, and an after-cat exhaust is likely worth 40 hp.
The Kenne Bell Blower
Fitting the Kenne Bell blower to a Cobra holds no mechanical surprises, and we're not going to detail its relatively simple installation here. Suffice it to say the stock Eaton blower is removed from atop the engine, and the KB blower assembly is fitted in its place. This isn't to discount the tight working room around the blower, especially in the rear, but overall, the installation is much simpler than a centrifugal thanks to a self-contained oil supply and stock-replacement, bolt-on nature. The excellent stock intercooler remains intact, with the KB supercharger using a short adapter plate to mate with the Ford bolt pattern. The Ford throttle body remains in its stock location, and with more-than-adequate fuel pump and injector capacity on board, those items need no attention--at least not at the 500hp level.
Brent made his first pass with the KB supercharger using the stock computer (no chip) and stock air filter. Wham, the air/fuel ratio hit 16:1 while the power advanced a nearly meaningless 3 hp to 454 hp. The lean A/F ratio obviously wasn't going to work, so the same chip Ken had come up with for the Eaton's 450hp level was installed and another pull made. This gained 100 hp at 3,500 rpm (!), but the top end stayed the same. Ultimately, Ken calculated the stock 90mm mass air meter was still not pegged, but the computer's load calcu-lation tables had run out. That meant writing a new chip, but in a much trickier portion of the code.
Note: Temp In and Temp Out...
Note: Temp In and Temp Out are charge air temperatures into and out of the intercooler. In other words, they are blower discharge air temperature and intercooler discharge air temperature. The Water temperature is the intercooler water temperature, not engine-coolant temperature. All temperatures were taken at 6,000 rpm; cool post-intercooler temps on the final run are attributed to cooler ambient temps/slightly longer cooldown. Note how KB discharge temps at 17 pounds of boost are similar to Eaton at 10.5 pounds. Boost is given at both 4,000 and 6,000 rpm to illustrate any blower drop-off with rpm. Such drops indicate inefficiency, typically from overspeeding. The Pulley diameter is in inches, and the Blower rpm is true blower rpm after all gear and belt ratios are accounted for. Eaton and Autorotor typically quote around 12,500 rpm as a maximum, but that is in continuous operation. Both blowers are commonly turned to 15,000-20,000 rpm in hot-rod applications, but you don't want to go higher due to seal heating from friction.
After working overnight on the problem, Ken had a new chip ready the next day. Bingo! With adequate fuel available, the power shot to 489 hp. As this is the first run with the standard pulley, stock air filter, and where the air/fuel ratio was properly sorted, this 489hp figure serves as the Kenne Bell baseline run.
A major concern of ours was that we wanted to see the Kenne Bell supercharger run at the same boost and configuration as the Eaton, so we could reasonably compare the two from a heat-efficiency and power-production standpoint. Because of the differences in displacements between the blowers, this meant running them at different speeds (different pulleys), as seen in the charts. Note that both tests were run using the stock air filter.
While you have to extrapolate the rising and falling boost curves somewhat, the Kenne Bell made about 70 more horsepower in the same boost range as the Eaton. To be more precise, we had run the Eaton to the end of its useful range, so while it had made 13 pounds of boost at 4,000 rpm, that had fallen to just 10.5 pounds of boost at 6,000 rpm. The KB blower, however, was just getting started, so it happily chuffed out 12-13 pounds of boost right through 6,000 rpm. The extra 2.5 pounds of boost at the peak probably accounts for 25 hp of the 72hp power gain enjoyed by the Kenne Bell. The rest comes from reduced power-drive loss and more efficient airflow through the blower. Impressive.
To more closely compare peak boost power figures, but with an extra variable, you could compare the 451 hp the Eaton made while breathing through the bazooka to the 489 hp the twin screw made using the stock air filter.