Compound Vs. Turbo Only (Boost)The...
Compound Vs. Turbo Only (Boost)The two curves above illustrate the boost pressure supplied by the compound and turbo-only systems. The upper line is the boost supplied in the manifold by combining the turbos and blower boost. The lower line is the boost supplied by just the turbos on the turbo-only configuration. Note that the boost curve on the compound system is higher than that supplied by the turbos alone throughout the rev range, and the two meet only at the top of the rev range. Between 2,000 and 2,500 rpm, the compound system provided an additional 10 psi of boost-no wonder the compound system offered so much more low-speed torque.Unfortunately, that situation ended at 3,700 rpm, as the power output of the turbo-only combination surpassed that of the compound system (despite a boost differential of 9 psi in favor of the compound system). Not shown is the boost graph offered by the turbos feeding the blower in the compound set up, but they produced right at 7 psi from 2,000 rpm to redline.
Understanding this basic formula is important, as it can now be applied to the compound system. For this example, if we apply 7 psi of boost from the turbos to the already supercharged motor, we can think of the supercharged motor as nothing more than a high-horsepower, normally aspirated motor. Suppose our theoretical supercharged motor running 11 psi of blower boost (making 437 hp) was supplied an additional 7 psi of boost from a pair of turbos. Rather than deal with the compound portion of the equation just yet, it's easier to think of the 437hp supercharged motor as a 437hp normally aspirated combination. To this 437hp motor we will add 7 psi of boost pressure, which should improve the power output by roughly 48 percent, or 208 hp, bringing the total to 645 hp (437 hp plus 208 hp). If we were to add the same 7 psi of boost from turbos to the original normal 250hp normally aspirated motor (without the supercharger), we would wind up with only 369 hp. This means that adding 7 psi of boost from a pair of turbos to a normally aspirated Cobra motor would result in less power than adding the same boost from turbos to a supercharged '03-'04 Cobra motor. No major revelations here just yet, but there's more to the story than simply adding 7 psi of boost to both combinations.
From our example, although it seems that adding 7 psi of turbo boost to the supercharged combination would provide a great deal more power than adding the same amount of boost to a normally aspirated motor, the reality is otherwise. While we treated our supercharged combination as a normally aspirated motor in the formula, the reality is that the supercharged motor doesn't run with just 7 psi of boost.
You'll remember that the 437hp supercharged motor was originally supplied 11 psi from the blower. When we add the additional 7 psi of boost from the turbos, we wind up with a total boost pressure of 18 psi, right? Wrong. While it would seem logical that adding 7 psi of turbo boost to 11 psi of blower boost would combine to produce a total of 18 psi, the reality is that the combination actually winds up with a peak boost pressure of 23 psi. What this means is that we have a compound turbo/supercharged motor that produces 645 hp at 23 psi of boost instead of the 7 psi supplied by the turbos. The question you should be asking yourself now is, what would the original 250hp normally aspirated motor produce with the same 23 psi of boost? More importantly, what basis do we use to properly compare the compound system to the turbo-only system? In the end, we decided the basis for comparison would be the boost pressure supplied in the manifold.
To prepare for the test, the '04 Cobra was run first with the stock Eaton supercharger. The '04 Cobra was typical of most street versions, having been modified with a slightly smaller blower pulley, a MagnaFlow after-cat exhaust and x pipe system, and a BBK intake. Equipped with these mods, the Four-Valve Cobra motor produced 400 hp and 440 lb-ft of torque. Not that it matters, since all testing was performed on the same dyno, but these numbers were generated on a Mustang chassis dyno. The peak boost produced by the supercharged combination was 11 psi. Now it was time for more boost.
The guys at HP Performance were more than happy to provide said boost in the form of its twin-turbo kit. The kit supplied featured a pair of 57mm turbos, a front-mounted air-to-air intercooler, and all the necessary lines and fittings to provide 20-plus psi to the '04 Cobra motor. The motor already featured a set of 65-pound injectors and a dual-GT fuel pump setup. This combination provided the necessary fuel supply to allow HP Performance to safely crank up the boost. After all, this compound turbo/blower system was ready to dish out some serious boost.
 Boost from the turbos was...  Boost from the turbos was supplied to the front-mounted intercooler. |  HP performance modified the...  HP performance modified the inlet tube for its twin-turbo Cobra kit to work with the revised throttle-body position on the compound system. |  With the turbos providing...  With the turbos providing 7 psi and the blower 11 psi, the compound system resulted in a peak boost pressure of 23 psi (falling to 20 at the power peak). Running this boost level, this compound system produced 625 hp and 651 lb-ft of torque. Once we had all the data from the compound system, off came the factory Eaton supercharger. |