Compound vs. Turbo-Only (TQ)While...
Compound vs. Turbo-Only (TQ)While the top-end power favors the turbo system, the bottom half of the curve is all compound. Compared to the turbo-only setup, the compound system offered an extra 300 lb-ft of torque at 2,500 rpm. That's as much as a normally aspirated Cobra motor! With more than 600 lb-ft of torque at less than 2,500 rpm, the compound system is one serious torque monster. It's doubtful that any street Cobra can hook up over 600 lb-ft in First or even Second gears, but having that tremendous torque production will certainly come in handy on the streets.As indicated in this article, the choice comes down to where you want to optimize your power production-at the bottom or top of the rev range.
What's important is that, despite running less boost pressure, power production from the turbo system exceeded that offered by the compound system starting at 3,700 rpm. That the power curves crossed here in favor of the turbo system-despite the fact that the compound system offered an additional 9 psi of boost-is telling, especially since both systems were run with the same turbos. How do two forced-induction systems produce the same power when one is running 13 psi and the other 22 psi? The answer comes down to the fact that the power came strictly from the more efficient turbos in the turbo-only combination, while the compound system made due with the combination of a less efficient blower and the turbos. Combine this with the fact that the turbos were regulated by boost pressure, which the compound system artificially exaggerates, and you have the dramatic power difference between the two systems at any given boost level.
While we've clearly demonstrated that a well-designed turbo system will make more power than a compound (or supercharged) system, that doesn't mean the compound system doesn't have merits. One thing that initially surprised us during this testing was the relatively low inlet air temps. Given the previous testing, we expected elevated charge temps exiting the supercharger, but this wasn't the case. In fact, the charge temps matched that of most street supercharged systems running the Eaton supercharger alone, despite the 20-plus psi of boost present in the intake manifold.
The reduced charge temps (relative to a supercharger running 23 psi) can be explained by a combination of the dual intercoolers employed in the compound system along with a sizable increase in efficiency of the supercharger itself. The air exiting the turbos was cooled by an efficient air-to-air intercooler (part of the twin kit from HP Performance), while the air exiting the supercharger was further cooled by the factory air-to-water system. Given that the turbos only supplied 7 psi of boost and this minimal increase in charge temp was sent through the air-to-air cooler, the inlet air was darn near ambient as it entered the blower. Since the turbos were feeding the blower, the fact that it produced 23 psi of boost with 7 psi of inlet pressure meant the blower was operating at a much higher efficiency level. Higher efficiency means lower charge temps. The inlet air was then cooled using the air-to-water cooler, which resulted in just 88 degrees in the intake manifold.
The benefit of this lowered inlet air temperature is that despite the unstreetable (on pump gas) boost level of 23 psi, the compound turbo/supercharged motor is less likely to experience temperature-induced detonation than the turbo system run at the same boost pressure. The main reason for this isn't the intake temperature itself, but the tremendous cylinder pressure associated with the additional 175 hp offered by the turbo combination. The inlet air temps were similar between the compound and turbo-only system at the 20-psi level, but having all that extra torque and power offered by the turbo system would certainly require more care in tuning. This also applies to the tremendous low-speed torque offered by the compound system. Running a maximum 23 psi of boost, the compound system produced more than 600 lb-ft of torque at just 2,500 rpm. That much torque produced that early in the rev range is detonation just waiting to happen. Of course, when you'd be running at wide-open throttle at 2,500 rpm remains to be seen, but the tremendous torque production offered by the compound system definitely requires great care when tuning (especially the spark tables). The instantaneous boost supplied by the supercharger did improve spool-up of the turbos by more than 1,000 rpm, but care must be taken when harnessing all that torque.