Richard Holdener
June 1, 2008
When you add a pair of turbos to a supercharged Mustang motor, is the blower just in the way?

I Haven't been this excited about a story since the original "Boost Bash," where we compared Roots-style, twin-screw, and centrifugal superchargers to a pair of turbos. In this ultimate forced-fed Ford fiasco, we ran each of the different forms of forced induction at the same boost level, air/fuel ratio, and timing values, in an effort to demonstrate the power curves produced by each on an '03 Cobra crate motor supplied by Ford Racing Performance Parts.

We even went to the trouble of demonstrating the power curves at two different boost levels. In truth, we ran each combination at a variety of different boost and power levels, but space limitations prevented us from including every graph in the story. Run from 2,500 rpm to 6,500 rpm, the direct back-to-back test provided information not only on the ever popular peak power offered by each combination, but on the even more important overall power curve supplied by each. As with any engine combination, picking a winner is never as easy as simply choosing the one that produced the highest peak-power reading.

One critical element in this test was the datalogging provided by the AEM system. It allowed us to log things such as air/fuel ratio, boost pressure before and after the blower, and the all-important inlet charge temperatures.

After running this exhaustive test on modular forced induction, I figured enthusiasts had all of the information necessary to make an educated decision on selecting the forced induction that best fit their needs. Enthusiasts looking for instantaneous boost response and the attending torque production would do well with the (factory '03-'04 Cobra) Roots-style blower, while those looking to step up in performance could opt for the improved power potential of the Kenne Bell twin-screw.

Selecting the correct centrifugal supercharger offers even better peak power numbers but sacrifices low-speed power compared to the Roots-style and twin-screw superchargers. In terms of ultimate power production, turbos provide the most power per pound of boost, but again, the turbocharged combination doesn't provide the instant boost response offered by the pair of positive-displacement superchargers.Using an example from our "Boost Bash," with both the Eaton Roots-style blower and turbos pumping out a maximum of 11 psi, the supercharged combination offered an extra 120 lb-ft of torque at 2,500 rpm. At 5,200 rpm, the tables were turned in favor of the turbos to the tune of 156 lb-ft, and by 6,500 rpm, the turbos offered an extra 183 hp (remember, this was all run at the same boost level). The power gains offered by the turbos were even greater at higher boost levels.

For ultimate power production, turbocharging offers the most power per pound of boost-period. No amount of marketing hype, wishful thinking, or even out-and-out misrepresentation can change that fact. The efficiencies offered by the impeller design are one reason, but the single biggest advantage offered by the turbocharger is that there's no parasitic loss associated with driving the compressor.

Equipped with the blower only, the supercharged '04 Cobra motor produced 400 hp and 440 lb-ft of torque at a peak boost pressure of 11 psi.

Much like the air conditioning, the power steering, and the water pump, beltdriven superchargers require power to spin them. Generally speaking, the greater the airflow supplied by the supercharger, the more power required to spin them. By contrast, the compressor wheel on a turbo is spun using exhaust energy. Since this exhaust energy is usually wasted in the form of heat energy vented to the atmosphere, little power is required to produce the exceptional gains offered by the turbo. In the case of a typical supercharger, the parasitic losses may add up to 50 hp or more. The losses only increase with engine speed and airflow. The Roots-style blower employed on a Top Fuel motor absorbs nearly 500 hp at full song, but the losses are minimal compared to the eventual power output that's currently estimated to exceed 6,000 hp. (There are no dynos currently capable of testing of these fuel motors.)

The testing was run with the factory Eaton supercharger equipped with a 3-inch blower pulley.

Despite the obvious performance advantages offered by turbocharging, superchargers continue to be popular among Ford enthusiasts. The reasons are plentiful, especially in these days of ever-tightening emissions regulations. Since the catalytic converters are the last line of defense in the war on pollution, anything that modifies their placement or effectiveness is frowned upon by the feds. Superchargers all operate upstream of the exhaust and, as such, manufacturers have been able to produce a great many emissions-legal kits. The turbo crowd hasn't been quite as successful in their efforts, though it's certainly possible to build a system that will pass current emission standards. Superchargers also hold an advantage in terms of ease of installation, as there's no need to drop the exhaust system. This is especially true of supercharger systems that don't require drilling and tapping the oil pan for an oil return (something required with turbos).