Blower-Only Vs. Turbo-Only...
Blower-Only Vs. Turbo-Only (HP And TQ)If you're wondering why a turbo-only system would outperform a compound system, you need only look at the power differences between turbos and the Eaton supercharger. During the testing of our compound system, we couldn't help but compare the turbos versus the blower. With both systems run at 11 psi, the blower motor produced 397 rwhp, where the turbo motor produced 599 rwhp. At the same boost (air/fuel and timing), the turbo combination offered an additional 200 hp and 150 lb-ft.It's this differential between the turbo and blower combinations that all but determined the outcome of the test between the compound and turbo-only combinations. Where the crossover between the turbo and blower combinations was 3,300 rpm, the compound combination increased this to 3,700 rpm. Had we decreased the speed of the blower and increased the boost supplied by the turbos, this crossover point would have shifted even higher as the compound system became more and more efficient. Of course, this would also diminish the torque production down low.
One area that needs to be addressed in this test is the intake manifold. After running the HP twin-turbo kit on the supercharged combination, the blower was replaced with an intake manifold from an '01 (normally aspirated) Cobra. This is important, as the intake itself offered a performance gain over that employed by the supercharger. The factory '04 intake manifold used with the supercharger offered very short runners and a large open plenum. This design was chosen primarily for fitment, as packaging the blower, air-to-water intercooler, and intake manifold between the Four-Valve heads and under the factory hood clearance all but eliminated any chance of additional runner length. The short runners employed on the supercharger (and compound) combination actually reduced power production up through 6,000 rpm compared to the factory '01 NA Cobra intake. The runner length offered by the '01 intake was responsible for some of the additional power offered by the turbo-only combination compared to the compound and supercharged-only combos. Despite what you may have heard, runner length has a decided tuning effect on the power curve, even with the presence of boost. The additional runner length is no substitution for the instantaneous boost offered by the blower, but it certainly helped the turbo combination.
What this test boils down to is that when it comes to forced induction, the Roots-style blower offers plenty of low-speed boost response and torque production, but it's actually low man on the proverbial totem pole when it comes to power per pound of boost. Topping our power pole are the turbos, especially a system like the one employed from HP Performance. The turbos alone produced the highest peak power and torque numbers and even outpowered the compound system, despite a difference in boost of nearly 9 psi in favor of the compound system.
If the turbos are tops and the blowers are bottoms, where does this leave the compound system? By combining the less efficient Roots-style blower with the more efficient turbos, the compound system occupies the ever-popular middle ground. Just how close the compound system is to either side depends on how much boost is supplied by the blower and how much is supplied by the turbos. The slower the blower speed and the greater the turbo speed, the more the curves will resemble that of the turbos alone. Of course, the reverse is also true if we speed up the blower and slow down the turbos.
In the end, it all comes down to individual preference and how much emphasis you place on the cool factor of having a twin-charged motor.
 Installation of the '01 intake...  Installation of the '01 intake to run the turbo-only configuration required repositioning the accessories. Accessory position is different on the '03-'04 Cobra motor, thanks to the blower. |  Replacing the Eaton supercharger...  Replacing the Eaton supercharger was a factory '01 intake manifold. Unlike the lower intake used with the supercharger, the '01 Cobra intake offered sufficient runner length to enhance the power production of the turbo combination. The intake was one of the reasons the turbo-only motor produced more power than the compound system at the same boost level. |  Equipped with the turbos feeding...  Equipped with the turbos feeding the '01 intake manifold just under 20 psi of boost, the 4.6 motor produced 801 hp and 824 lb-ft of torque. This bettered the compound system by 175 hp and a similar amount of torque. |