Compound vs. Turbo-Only (HP)For...
Compound vs. Turbo-Only (HP)For this test, the compound system was set up to produce a peak boost pressure (at the power peak) of 20 psi. Due to the nature of a Roots-style supercharger, the boost pressure was not consistent throughout the rev range, peaking at 23 psi around 4,700 rpm and dropping off to just 20 psi at 6,200 rpm. Running a peak of 23 psi and 20 psi at the power peak, the compound system managed to produce 625 hp. Running the same 20 psi of boost at the power peak, the turbo-only combination produced 801 hp.Naturally, the compound system offered more low-speed power, but from 3,700 rpm all the way to redline, the turbo-only system outperformed the compound system. With the exception of the launch, the engine speed will always exceed 3,700 rpm after each shift, meaning the extra 175 hp offered by the turbos will be in full effect. Run heads-up, the turbo-only combination would easily walk away from the compound system in a drag race.
For this comparison, the turbo kit was installed and fed through the Eaton supercharger. No changes were made from the runs with just the supercharger. The boost pressure supplied by the twin-turbo kit from HP was adjusted to supply roughly 7 psi to the supercharged motor. Common sense will tell you that if we supply 7 psi from a pair of turbos to a motor that's already being fed 11 psi from a supercharger, the resulting combination should provide 18 psi, right? As we have already indicated, the answer is a resounding no, as the compound turbo/blower combination produced a peak boost pressure of 23 psi, with the pressure falling off to just 20 psi at the power peak.
It's important to note that the boost pressure supplied by the turbos never varied by less than 0.5 psi, but the boost pressure from the blower dropped in both the supercharged and compound combinations. The result of adding 7 psi from the turbos to the already supercharged motor was a peak power reading of 625 hp and 651 lb-ft of torque. We'd certainly welcome any motor that produced 625 hp at just 7 psi from the turbos, but the reality is that this motor was making 625 wheel horsepower at roughly 20 psi (at the power peak). While 625 wheel horsepower is nothing to sneeze at, testing has shown that it shouldn't require 20 psi to achieve it on this '04 Cobra motor.
This point was further illustrated once we removed the supercharger and replaced it with an intake manifold from the '01 Cobra. Once we had a proper intake manifold, we were free to feed the boost to the '04 Cobra motor. Naturally, we snuck up on the eventual peak boost reading of 20 psi, but once we had the correct boost pressure, this motor started making power.
Before getting to the big numbers, know that the turbo motor produced a solid 200 hp more than the blower-only combination at the same 11 psi-turbos are that much more efficient than Roots-style blowers. Running 20 psi (actually a peak of 19.7 psi), the turbocharged '04 Cobra motor produced just over 800 wheel horsepower and 824 lb-ft of torque. This compares to the 625 wheel horsepower and 651 lb-ft of torque produced by the compound system at the same boost level. Naturally, the compound system improved the boost response and low-speed power thanks to the pressure supplied by the supercharger. One need only look at the boost curves (see graph) supplied by the compound system versus the turbo-only to explain the difference in low-speed power. At 2,500 rpm, the compound system outpowered the turbo-only system to the tune of 10 psi of boost and more than 300 lb-ft of torque. Of course, no one ever stomps on the throttle at 2,500 rpm in Fourth gear to perform some sort of ludicrous roll-one comparison, but having all that extra torque means tire spin is just a flick of the throttle away. The choice (as always) comes down to where in the rev range you want to concentrate your power production.
Here's a shot of the 4.6L...
Here's a shot of the 4.6L Four-Valve motor with the blower removed. It was replaced with a stock intake from an '01 Cobra motor.
As in the "Boost Bash" run previously, this test on the compound turbo/supercharged system illustrated that positive-displacement blowers excel at low-speed torque production, while turbos promote midrange and top-end power. Here are a couple of additional points that surfaced during all this testing. If you compare the boost curves offered by the compound and turbo-only systems, you'll see that the boost curve supplied by the compound system always exceeded that supplied by the turbo-only system from HP Performance. This was by design, as we didn't want to crank up the boost any more from the turbos (though more was available for the 57mm turbos) simply to match the boost pressure supplied by the compound system.