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).

All of the back-to-back testing was run on a Mustang chassis dyno.

For some, the visual appeal of a supercharger is important. There's definitely something to be said for lifting the hood to reveal a positive-displacement supercharger nestled between a pair of Four-Valve cylinder heads. Likewise for a polished centrifugal supercharger packaged with an efficient air-to-water intercooler. To some, this show is every bit as important as the go. Unfortunately, most turbo systems are hidden from view.

One thing the positive-displacement supercharger offers that the turbo would have extreme difficulty duplicating is the instantaneous boost response. As mentioned previously, the Roots-style blower tested on the '03 Cobra motor in our original "Boost Bash" offered an extra 120 lb-ft of torque over the turbo combination at 2,500 rpm. That the turbo eventually caught and passed the supercharged combination (at around 3,700 rpm) means nothing to the blower brigade-to them, torque is king. It can probably be argued that having an extra 120 lb-ft of torque at just 2,500 rpm is nothing more than a recipe for tire spin, but having all that wonderful boost and the attending torque is certainly enticing. The ideal combination would obviously be to have all the low-speed torque offered by the Roots-style (or positive displacement) supercharger combined with the midrange and top-end power of the turbo. The obvious answer is to join the two to produce a compound forced-induction system that offers such an impressive combination. What could be better than having a pair of turbos feeding a Roots-style supercharger?

The turbo kit from HP Performance included a set of dedicated tubular headers.

While this compound forced-induction system sounds great on paper, and testing has shown it's capable of producing exceptional power, naturally there are trade-offs associated with such a system.

Despite the recent test run in MM&FF on the combination from Hellion, compound forced induction is far from new. It has been in use almost since the introduction of the internal combustion engine and employed extensively in military and civilian aviation applications. Lancia employed a system on one of its Gruppe B rally cars, and Volkswagen now offers such a system on one of its production cars. Of course, this history doesn't mean we aren't terribly excited about tuners applying it to our wonderful Ford products. This excitement over the compound forced induction has to be (excuse the pun) combined with proper testing to ensure understanding (beyond the usual Internet level) and safe operation. The main reason for this is the tremendous potential for elevated boost pressures associated with a compound system. As we shall see from the test results, compound systems do not simply combine, or add, the boost from the blower to that supplied by the turbo. There's a multiplier effect that quickly elevates the boost pressure and the associated potential for detonation.

Installation of the turbo kit required a tubular K-member. This provided the necessary room to run the tubing to and from the turbos. All of the testing was run through a 3-inch MagnaFlow dual exhaust equipped with an x pipe system.

To best illustrate the benefits and limitations of compound forced induction, we decided to run a series of dyno tests using an '04 Cobra. The supercharged 4.6L motor was run first with the stock Eaton supercharger, then again with turbos feeding the supercharger (compound induction), and finally using just the turbos. Before getting to the results, it's important to understand a few basic elements of the process. As always, we start with the basic, somewhat simplified, premise that any forced-induction motor is simply a normally aspirated motor with additional atmospheric pressure. That is to say, the supercharged '04 Cobra motor used for this test is nothing more than a normally aspirated Cobra motor with additional pressure supplied. Using this premise, we can estimate the power gains offered by the boost supplied by the supercharger. If we have a 250hp normally aspirated motor and add 11 psi of boost, we can increase the power output by nearly 75 percent, since 11 psi is 75 percent of the normal atmospheric pressure of 14.7 psi. Adding 11 psi to the 250 hp motor should theoretically give us 437 hp. This formula doesn't take into account the effect of changes to timing and air/fuel, changes in the induction system, or losses associated with driving the blower, thus the actual output will likely be lower with a supercharger.

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.

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 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.

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.

Blower-Only Vs. Turbo-Only (HP And TQ)If you're wondering why a turbo-only system would outperform a compound system, you need only look at the power differences between turbos and the Eaton supercharger. During the testing of our compound system, we couldn't help but compare the turbos versus the blower. With both systems run at 11 psi, the blower motor produced 397 rwhp, where the turbo motor produced 599 rwhp. At the same boost (air/fuel and timing), the turbo combination offered an additional 200 hp and 150 lb-ft.It's this differential between the turbo and blower combinations that all but determined the outcome of the test between the compound and turbo-only combinations. Where the crossover between the turbo and blower combinations was 3,300 rpm, the compound combination increased this to 3,700 rpm. Had we decreased the speed of the blower and increased the boost supplied by the turbos, this crossover point would have shifted even higher as the compound system became more and more efficient. Of course, this would also diminish the torque production down low.

One area that needs to be addressed in this test is the intake manifold. After running the HP twin-turbo kit on the supercharged combination, the blower was replaced with an intake manifold from an '01 (normally aspirated) Cobra. This is important, as the intake itself offered a performance gain over that employed by the supercharger. The factory '04 intake manifold used with the supercharger offered very short runners and a large open plenum. This design was chosen primarily for fitment, as packaging the blower, air-to-water intercooler, and intake manifold between the Four-Valve heads and under the factory hood clearance all but eliminated any chance of additional runner length. The short runners employed on the supercharger (and compound) combination actually reduced power production up through 6,000 rpm compared to the factory '01 NA Cobra intake. The runner length offered by the '01 intake was responsible for some of the additional power offered by the turbo-only combination compared to the compound and supercharged-only combos. Despite what you may have heard, runner length has a decided tuning effect on the power curve, even with the presence of boost. The additional runner length is no substitution for the instantaneous boost offered by the blower, but it certainly helped the turbo combination.

What this test boils down to is that when it comes to forced induction, the Roots-style blower offers plenty of low-speed boost response and torque production, but it's actually low man on the proverbial totem pole when it comes to power per pound of boost. Topping our power pole are the turbos, especially a system like the one employed from HP Performance. The turbos alone produced the highest peak power and torque numbers and even outpowered the compound system, despite a difference in boost of nearly 9 psi in favor of the compound system.

If the turbos are tops and the blowers are bottoms, where does this leave the compound system? By combining the less efficient Roots-style blower with the more efficient turbos, the compound system occupies the ever-popular middle ground. Just how close the compound system is to either side depends on how much boost is supplied by the blower and how much is supplied by the turbos. The slower the blower speed and the greater the turbo speed, the more the curves will resemble that of the turbos alone. Of course, the reverse is also true if we speed up the blower and slow down the turbos.

In the end, it all comes down to individual preference and how much emphasis you place on the cool factor of having a twin-charged motor.