Words By Richard Holdener
There is an old saying in the performance automotive world, once you go boost, you never go back. Okay, so maybe that’s not exactly the phrase, but it should be. Not to take anything away from normally aspirated (all-motor) combos, but things just go better with boost. I’ve yet to find a normally aspirated combo that didn’t improve with the installation of a good supercharger.
Think about this. How often do you hear a performance enthusiast wish for less power? Yes, fellow force-fed fans, the extra airflow supplied by a supercharger can transform an already good motor into a great one. Even an additional 6-7 psi from a decent supercharger can improve the power output by 30 to 40 percent, depending on the combination and blower. If you are starting with a typical normally aspirated combination making 400 to 450 hp, an extra 40 percent equates to an additional 160 to180 hp. Things definitely go better with boost, but the question now is, what blower do you choose?
For this discussion, we will eliminate turbocharging from the mix, though it is very popular and potentially powerful. When it comes to supercharging, there are two basic (popular and readily available) types: positive displacement and centrifugal. The positive displacement superchargers can be further divided into roots and twin-screw designs. The two differ in terms of efficiency and rpm potential, but for most of this test, we sampled the positive displacement roots design.
Positive displacement superchargers provide a fixed amount of airflow to the motor for each revolution. With a fixed displacement and somewhat limited rpm capability, it is important to size the positive displacement supercharger for the engine size and intended power output. By contrast, centrifugal superchargers are required to spin much faster to supply the necessary boost to the motor. Both systems offer benefits and limitations, so let’s take a look at some of the major differences.
The Boost Curve
Any comparison between positive displacement and centrifugal superchargers has to include what we call boost response. Both types are mechanically coupled to the engine, so any increase in engine speed will automatically increase the blower speed, and therefore, airflow to the motor. Though they share this coupling, the boost curves provided by each couldn’t be more different.
Positive displacement (PD) superchargers provide near instantaneous boost response at nearly any engine speed. Larger rotors in the PD blowers provide a massive amount of airflow with very little rotor speed (relative to the centrifugal). Mashing the gas on a positive displacement supercharger provides immediate gratification. The boost curve offered by the PD blower is generally flat and consistent. It is also possible to produce a slightly rising boost curve depending on the blower sizing, speed, and engine combination.
By comparison, a centrifugal supercharger will provide a rising boost curve, with peak boost occurring at the top of the rev range. Properly sized, the boost will continue to increase right to the engine’s redline (or beyond). Compared to the positive displacement supercharger, a centrifugal supercharger will offer less boost down low, then catch up (and possibly pass) at the top of the rev range.
Check out the supplied graph to see the difference in boost response between the positive displacement and centrifugal superchargers (on a Modular Ford), but know that the boost curve doesn’t tell the whole story. It stands to reason that if one makes more boost, then it should also make more power, right? As we shall see, more boost doesn’t always equate to more power. There are a number of components that also factor into power production, not the least of which include efficiency, the parasitic losses associated with driving the supercharger, and even intake design.
The Other Stuff
If we take a look at the second graph, we can see that, true enough, the positive displacement made more power than the centrifugal through most of the curve (where it made more boost). However, we need to examine it a little closer to see the real story.
Looking at the boost curve, we see the positive displacement supercharger offered more boost through the entire rev range, up to 6,350 rpm, then fell off slightly to the centrifugal. Given the extra boost, we would expect the power to follow suit, but comparing the boost curves to the power curves, we see that the centrifugal starting making more power than the positive displacement some 800 rpm earlier, at 5,550 rpm. How can a combination with less boost make more power (assuming identical air/fuel and timing values)?
One of the answers is the blower itself, as the positive displacement supercharger required considerably more power to create the available boost. The parasitic losses associated with driving the supercharger were higher on the PD blower, so the resulting power output was less. Combine that with the difference in efficiency between the two superchargers and you have another chunk of power.
Basically, this boils down to the fact the centrifugal made more power per pound of boost (and a lower charge temperature) than the roots blower did. This becomes all too evident when we compare the two blowers at identical boost levels (which occurred at 6,350 rpm). The centrifugal offered an additional 80 hp at that engine speed. Out at the peak rpm (6,600 rpm) where the boost level of the centrifugal reached 11.5 psi, the power difference was more than 120 hp (though the roots blower had lost 1 psi there).
The Intake Manifold
There is one final element that bears mentioning when it comes to a comparison between different forms of supercharging. Typically, a positive displacement supercharger will reside where the intake manifold sits. This location, combined with limited hood clearance, usually requires a compromise in intake design, especially if the blower is run in conjunction with an intercooler (as was all of the previous testing). By contrast, a centrifugal supercharger is generally mounted either off to one side, or directly in front of the motor on race applications. This positioning allows more freedom for intake design, or more specifically additional runner length.
By necessity, most PD blower configurations include very short intake runners. The loss of torque from the shorter runners is more than offset by the instant boost offered by the PD blower, but the gains would be even greater had the PD blower been able to run with longer runners.
The previous two graphs used to illustrate the difference in boost and power between the PD and centrifugal blowers actually ran identical intake manifolds. In this case, the factory 2003 Cobra lower intake that was used included short runners and an intercooler. The centrifugal supercharger was run with a custom plenum designed to mate to the factory lower intake (see photo). However, in most cases, the position of the centrifugal supercharger allows for both other forms of intercooling and a long-runner intake.
To illustrate just how much power was possible with a change in intake design, we replaced the plenum and lower factory short-runner intake with a factory (normally aspirated) Cobra manifold. The resulting increase in runner length improved the power output of the combination by more than 60 hp, with consistent gains from 3,500 rpm through 6,500 rpm. Had we run this intake on the previous comparison, the centrifugal would show even greater peak gains and offer more power slightly lower in the rev range (than the 5,550 rpm cross over point).
The two different forms of supercharging both have plenty to offer, but which is the better choice? The answer here is an easy one, because they both win. Were one form of supercharging clearly better in every aspect than another, the lesser of the two would simply cease to exist. The PD blower offers immediate gratification, something we all crave. Sure, it might fall off in efficiency for a maximum effort, but the vast majority of us aren’t looking to shave every last 1/10th off the old ET. The centrifugal offers a top-end charge that can’t be beat, but is the (relative) lack of boost down low a concern? These are things only the buyer can answer, but know that whatever you choose, adding boost to any motor, regardless of the curve, is nothing but good!
Sources: ARP, Arp-bolts.com; COMP Cams, compcams.com; FAST, fuelairspark.com; Holley/Hooker/NOS, holley.com; MSD, Msdignition.com; Procharger, Procharger.com; Speedmaster, Speedmaster79.com; Vortech Superchargers, vortechsuperchargers.com