After our NA versus boost...
After our NA versus boost comparison, we reinstalled the Eaton supercharger and installed a 3.20-inch DUB blower pulley.
Now that we had the N/A power figures, we could compare them to the supercharged output at a given boost level. Using the baseline runs in "Mod for 4V Mods, Part 1," we know that the Cobra motor produced 501 hp with 23 degrees of timing, an optimized 11.8:1 air/fuel ratio and long-tube headers at a peak boost pressure of 8.8 psi. Here is where things get tricky with the calculations. The Eaton supercharger produced a peak boost reading of 8.8 psi at 4,700 rpm, but only 8.0 psi at the peak of 6,500 rpm. Using the maximum boost reading of 8.8 psi, we see that the gain of 131 hp offered by the supercharger equates to roughly 35 percent (of the original 370 hp).
By comparison, 8.8 psi equates to almost 60 percent of 14.7 psi, thus we increased the boost pressure by 60 percent and only improved the peak power by 35 percent. Even using the lower boost pressure reading of 8.0 psi, we still see that the pressure increased by 54 percent, yet the power was up only 35 percent. This is an indication that the Eaton (Roots) supercharger is not terribly effective at improving the power output compared to other forms of forced induction. We will take a much more in-depth look at all the forms next month as we subject the '03 Cobra mule to no less than four different types of forced induction in a no-holds barred shootout.
With our normally aspirated comparison completed, we turned our attention to finding more power. The Eaton supercharger was reinstalled and equipped with a 3.2-inch blower pulley. We installed a 7.75-inch crank pulley and tuned the air/fuel and timing curves. Running 23 degrees of timing and a steady air/fuel ratio of 11.8:1, the supercharged motor produced a peak boost pressure of 12.1 psi. Despite a dropping boost curve (to 10.9 psi) the 4.6 managed to produce 537 hp at 6,500 rpm and 530 lb-ft at 3,500 rpm. A back-up run showed the power curve to be repeatable, as did a comparison with the previous testing from Part 1, where we ran an identical combination.
We also installed a 7.75-inch...
We also installed a 7.75-inch DUB crank pulley.
After we were satisfied with the power output of the Eaton, we swapped on the twin-screw Autorotor supercharger upgrade from Kenne Bell. Wanting a direct comparison, we decided to install the same 3.20-inch blower pulley on the Kenne Bell supercharger. Unfortunately, we only had a slightly larger 3.25-inch blower pulley that would slightly reduce the blower speed (and boost) supplied by the twin-screw Autorotor. Undaunted, we swapped over the factory Cobra intercooler and installed the 3.25-inch blower pulley. Like the Eaton supercharger, the Kenne Bell was equipped with an Accufab throttle body and run with dyno water running through the air-to-water intercooler.
To say that we were impressed by the results of the blower upgrade would be an understatement. Installing the Kenne Bell supercharger (with the larger blower pulley no less) increased the power output to an amazing 682 hp. Think about it: the Kenne Bell added more power (145 hp) to the Eaton supercharged Cobra motor than the Eaton supercharger did to the normally aspirated motor (just 130 hp). Where the power curve leveled off with the Eaton, the horsepower kept climbing with the twin screw.
The Kenne Bell blower upped the torque peak as well, from 530 to 595 lb-ft. As further testament to the efficiency of the twin-screw, the blower not only pumped out more pressure at 3,000 rpm (14.2 vs. 11.5 psi), but the boost curve kept climbing with the twin-screw to a maximum reading of 16.5 psi (vs. 10.9 psi for the Eaton) at 6,500 rpm. The Eaton is simply not large or efficient enough to compete with the twin-screw supercharger at the elevated power levels.
The Eaton supercharger was...
The Eaton supercharger was equipped with the Accufab inlet and throttle body.
The same can be said (to an even greater extent) when comparing the Eaton to a centrifugal supercharger, the only difference being that the centrifugal supercharger will lose out dramatically at lower engine speeds. Not so for the twin-screw supercharger, as the positive displacement Autorotor makes more power everywhere than the roots blower. Too bad SVT didn't equip the Cobra with a twin-screw in the first place.
After running the Kenne Bell blower, we began to think like any good Cobra owner. Hey, let's crank up the boost! While the peak boost pressure was already over 16 psi, we knew the Autorotor had a bunch more to offer. Knowing this, we replaced the DUB 7.75-inch crank pulley with a larger 8.5-inch version. The DUB system made pulley swaps something to look forward to rather than dread. The pulley change took all of two minutes, including changing the drivebelt to suit the larger crank pulley. Remember that our Cobra motor was previously upgraded with a Metco idler system to help minimize belt slippage.
We also installed these Flowtech...
We also installed these Flowtech long-tube headers to improve the exhaust flow.
On the dyno, the pulley combination...
On the dyno, the pulley combination allowed the Cobra motor to produce a peak boost reading of 12.1 psi along the way to producing 537 hp and 530 lb-ft of torque.
After running the Eaton supercharger...
After running the Eaton supercharger at the higher boost level, we once again pulled the blower to make room for the Kenne Bell twin-screw.
The factory air-to-water intercooler...
The factory air-to-water intercooler was removed from the bottom of the Eaton supercharger to be installed on the Kenne Bell blower.
Here is a shot of the bottom...
Here is a shot of the bottom of the Kenne Bell twin-screw supercharger prior to the installation of the intercooler. Note the aluminum adapter plate designed to accept the factory intercooler.
We installed a 3.25-inch blower...
We installed a 3.25-inch blower pulley on the Kenne Bell in an effort to match the 3.20-inch blower pulley previously on the Eaton.