Richard Holdener
December 1, 2004
Eaton vs. HP Performance Twin Turbo (11 psi)
Compared to the Eaton, the twin-turbo kit from HP Performance offered both more and less power. Down at 2,500 rpm, the immediate boost response of the Roots blower was the winner hands down. With an additional 100 lb-ft, the Roots blower would provide an immediate shove in the back, but, unfortunately, that shove would be short-lived-at least compared to the one supplied by the turbo combination. From 2,500 rpm to 3,500 rpm, the Roots blower out-boosted the turbos, but at 3,600 rpm, the power curves shifted in favor of the turbos. The improved efficiency combined with the torque producing nature of the long-runner '01 intake allowed the turbo system to produce an extra 178 hp and 154 lb-ft of torque. Once you past 3,600 rpm, replacing the Eaton with the turbos made as much power as adding some form of forced induction on a normally aspirated motor. Why such a dramatic difference in power at the same boost level? The power differences between the Roots blower and the turbos can be attributed to several factors. Much of the difference can be attributed to the power required to mechanically drive the supercharger. The difference in intake runner length of the intake had a noticeable effect on the power curve as well. The final factor is that the turbos are much more efficient at processing air (especially at pressure) than the supercharger. Combining all these factors will give you the difference between the blower and the turbos.

The final contestant was supplied by the alien-friendly folks from Roswell, New Mexico. The gang from HP Performance has exceeded 730 wheel hp on a modified two-valve GT motor at 20 psi using its twin-turbo GT kit. Like the impressive two-valve GT kit, this four-valve Cobra setup featured a pair of powerful 57mm turbos. The standard kit comes with 46mm turbos, while huge 67mm units are available for those building an all-out race motor. The 57mm turbos tested on this Cobra motor were reported to be capable of exceeding 1,000 hp, so the kit has plenty of potential. Unlike the trio of blowers, the HP kit came equipped with a front-mounted air-to-air intercooler. The Eaton supercharger, intercooler, and even lower intake manifold were removed in favor of a factory '01 intake. The benefit of replacing the '03 Cobra assembly with the '01 intake was that the turbo combination was the beneficiary of the torque production offered by the additional runner length. Contrary to what some would have you believe, short-runner intakes do not make good blower or turbo manifolds. Runner length determines the effective operating speed of the motor, regardless of whether pressure is present or not.

Incidentally, we also ran the Vortech with the '01 intake (minus the intercooler), so check out the sidebar on the results of the effect of runner length in Part 2 of our "Ballistic Boost Bash."

Both the Vortech and HP turbo system required provisions for an oil drain back into the stock oil pan. We removed the pan and welded a pair of drain fittings.

There seems to be come confusion about comparing a twin-turbo system to each of the single blowers. That there were two turbos present compared to one of each of the superchargers was in no way an advantage to turbo kit. It is possible to produce the same power with a larger single turbo. The twin system was designed primarily for packaging, though commonly held theory suggests that two smaller turbos will spool up faster than one big one. In reality, the exhaust energy is divided in half, as each bank feeds one of the two turbos.

So is feeding one small turbo with half the exhaust energy that much better than feeding a single larger turbo with twice the exhaust energy? The wheel (compressor and turbine) sizes and weight have much less to do with spool-up than the exhaust energy supplied in relation to the flow orifice. Regardless of the theory, the twin system featured a pair of wastegates, an air-to-air intercooler, and custom tubular exhaust manifolds. The complete system featured Jet-Hot coating to keep the exhaust energy inside the tubing where it will do the most good. The boost pressure was controlled using a Turbo XS manual wastegate controller (essentially an adjustable bleed orifice). By bleeding the boost signal to the wastegate, they were tricked into staying closed (not bleeding off any exhaust energy), thus increasing the boost pressure. Of all the forced induction systems, adjusting the boost was easiest on the turbo kit.

I would like to go on record here and thank HP Performance, Kenne Bell, and Vortech Engineering for allowing us to put their systems to the test. It is a testament to the confidence in their products that such a banzai boost comparison was even possible.

The wildest-looking system on the dyno was hands down the twin-turbo system from HP Performance.

Our '03 Cobra test mule was internally stock with the exception of a quartet of XE262AH Comp Cams cams recently installed. Externally, the four-valve motor was sporting a set of 1 5/8-inch Flowtech headers, a Meziere electric water pump (no accessories), and an Accufab throttle body and inlet system. The timing and fuel curves were controlled using a F.A.S.T. programmable engine management system. We kept the air/fuel ratio safe at 11.8:1 and the total timing stationary at 23 degrees under boost.

To produce the desired starting boost level of 11 psi, the stock Cobra (Eaton) supercharger pulley was combined with an 8.5-inch crank pulley. Thanks goes out to South Florida Pulley Headquarters for the Dial-Ur-Boost (DUB) hub and bolt-on pulley system that allowed quick pulley changes to dial in the boost pressure. Equipped with the stock blower and 8.5-inch crank pulley, the supercharged Cobra motor produced a peak boost pressure of 11.6 psi on its way to 572 hp and 533 lb-ft of torque. Please don't make the mistake of looking only at the peak power numbers. The supplied graphs tell a much better story that the peak numbers.