Information On Superchargers - How It Works
Want to Know how to Smoke a Turbo Car with a Blower? Procharger Explains What Makes Today’s Centrifugal Superchargers Such Potent Performers.
From the April, 2012 issue of Chevy High Performance
By Stephen Kim
Have you heard the news? Turbos are in vogue. Everyone wants them. They make ridiculous heaps of torque. They have that “free energy” thing going for them, too. With such a glowing list of positive attributes, turbos must be the ultimate power adder, right? Not so fast. Whether it’s in NMCA, NMRA, ADRL, or PSCA competition, supercharged door slammers don’t just give their turbocharged counterparts a run for their money; they often beat them outright. Nothing puts this into perspective better than the impressively long list of national championships won by supercharged cars in high-profile heads-up racing classes over the years. How on earth can superchargers be putting turbos on the trailer? To get some answers on what makes these supercharged freaks run so fast, we went straight to the source and called up Ken Jones of ProCharger. The company’s F-series blowers consistently dominate heads-up drag racing classes, so we were eager to find out why. In addition to dissecting the anatomy of some of the most advanced superchargers on the market, we discussed a host of forced-induction theory, ranging from compressor maps to intercooler design to impeller design to the relationship between pressure and flow. So here’s the scoop.
ProCharger’s F-series superchargers have dominated many heads-up drag racing classes for years, with countless national championships. These blowers feature several design elements that make them outstanding performers. The biggest advantage of these units is our patented friction reduction drive system. That’s where the “F” in the F-series blowers comes from. It’s basically a bearing within a bearing. We’ve experimented with blower designs that very closely control bearing speed, and we found that our allowing the bearings to find their own “happy spot” in a bearing-within-a-bearing arrangement worked just as well. When combined with ProCharger’s self-contained oiling system and transmission, which dramatically improves load carrying capacity, the result is an extremely efficient supercharger assembly. Supercharger impellers can spin in excess of 70,000 rpm, and are therefore very high-speed devices. Naturally, the oiling needs of a supercharger are much different from that of an engine. Blowers need much lighter weight oil for better lubrication of the bearings and gearset. Furthermore, not only does a self-contained oiling system run cooler, it also prevents cross-contamination. These days we’re more concerned with the engine contaminating the supercharger rather than the other way around. There’s always a risk of fire whenever an oil line breaks, but you don’t have to worry about that with a self-contained supercharger.
Superchargers have mechanical and thermal properties that must be taken into account when measuring compressor efficiency. According to Boyle’s Gas Law, there is minimum amount of heat that’s going to be produced anytime air is compressed. Consequently, if a compressor is capable of achieving 100 percent adiabatic efficiency, it would still heat up the intake charge. The reality is that compressors can’t operate at 100 percent efficiency because of the additional heat that’s added to the intake charge due to mechanical components like the gear assembly. Most ProChargers operate at 70-80 percent efficiency, which is similar to a turbocharger. Efficiency matters, but where and how a supercharger is mounted matters too. Positive displacement blowers mount to the top of an engine, and since heat rises, they act as heat sinks. That heat gets transferred into the intake charge. Likewise, mounting a centrifugal supercharger in front of the motor, like in a race car, instead of off to the side nets a cooling effect as well. In the lab, we test superchargers by themselves for efficiency using SAE and industrial test standards, but that only tells you part of the story. To truly measure the efficiency of a supercharger as a system, you have to also look at underhood temps, because that heat will get absorbed into the intake charge and peak air temperature is what you have to tune to in order to prevent detonation.
Constantly pushing the envelope of supercharger design is a combination of several different elements. In 2011, we introduced four new race superchargers, which is unheard of in this industry. In addition to our staff of more than a dozen mechanical engineers and many high-skilled machinists, our in-house manufacturing capability is a big part of what enables us to bring superior products to the market. Five-axis CNC machines, Zeiss coordinate measuring machines, computer modeling software, and engine dynos are just some of the tools that we rely on. Using our CNC equipment, we machine impellers out of 7075-T6 billet aluminum instead of casting them. This allows us to quickly build prototypes without having to spend lots of time and money creating a casting. When produced in bulk, CNC-machining impellers out of billet is more expensive, but it enables us to adapt and modify designs more quickly based on feedback from racers or our own R&D testing. Furthermore, billet impellers are less likely to crack than a casting and are easier to balance. This is very important because an unbalanced impeller will significantly reduce supercharger life. Additionally, ProCharger’s industrial division, Inovair, has benefitted our automotive division greatly. Inovair supplies centrifugal compressors that are used for aircraft deicing for clients such as the U.S. Air Force, FedEx, and UPS. Inovair also supplies centrifugal compressors and blowers for use in pneumatic conveying and other continuous duty applications. Since these machines run 24/7, it gives us a tremendous amount of feedback, which is used to further improve our products.
A centrifugal supercharger and the compressor side of a turbocharger are very similar, but there are pros and cons of each. There are parasitic horsepower losses associated with a supercharger, since it is driven off of the crankshaft. On the other hand, the exhaust side of a turbo transfers heat into the intake charge, which isn’t an issue with a supercharger. Some people think that since turbos are exhaust driven, that it’s free energy, but that’s not entirely accurate. Turbos create backpressure that, when combined with the heat added to the intake charge from the hot side, increases the potential for detonation. Interestingly, if you have two identical motors, but put a turbo on one and a centrifugal supercharger on the other, you might only run 8 psi of boost with the turbo versus 11 psi with blower on 91-octane pump gas. Consequently, blowers will beat turbos on the street on pump gas in net power gain. However, it’s a different story at the track in motors that run on racing fuel or methanol. Since those fuels eliminate the detonation issue with turbos, they make big power. Even so, supercharged cars run more consistently because it’s much easier to manage the boost in them compared to a turbo car. Ultimately, turbo cars qualify well on race day, but supercharged cars win championships.
All superchargers have an optimum operating range in which they are the most efficient. The key issue is understanding what flow and pressure range you are designing the supercharger for. You want to be at the peak island on the compressor map. In industrial applications, you’re targeting a specific pressure and flow range, say 600 cfm at 12 psi, but things are much more complex in a car because pressure and flow are constantly changing with engine rpm. Centrifugals have two key advantages over positive-displacement blowers in this regard. First off, centrifugals produce more power and less heat than any positive-displacement blower. Secondly, centrifugals have a broader operating range. A positive-displacement blower will work well near its peak operating range, but above or below peak operating range, its average efficiency will be much lower. That means there’s less capacity to upgrade the blower by spinning it faster because it has a narrower operating range than a centrifugal blower. Across an engine’s rpm band, a centrifugal supercharger will achieve an average efficiency that’s closer to its peak efficiency. That’s what makes it so easy to pulley a centrifugal blower for more boost as your engine combination grows.
Boost and airflow are two different animals, but the terms are often used interchangeably. What really matters at the end of the day is how much air and fuel you move through a motor. Boost is simply a measure of the backpressure inside the intake manifold. If you reduce any restrictions in the induction path of an engine, such as increasing cylinder head flow, boost pressure will decrease but the engine will make more power. We get calls on our tech line all the time with people concerned because they made modifications to their engine and their boost went down. When we ask them if they’re making more power, the answer is always yes, and the reason is because they reduced the restriction in their motor. That said, for any given displacement engine, after you’ve optimized the other variables—like the cylinder heads, camshaft, and exhaust—then boost pressure becomes the primary lever used to increase airflow and power. If these other variables are fixed, then you can increase pressure to increase flow, but that’s not the only tool available. You can actually increase power and have boost go down. The trick is to maintain the same boost pressure while reducing the restrictions inside an engine to dramatically increase power. People often get a certain boost figure stuck in their head, and think that’s what they need to run, but 20 pounds of boost on a big-block represents much more airflow than 20 pounds of boost on a small-block.
ProCharger offers both air-to-air and air-to-water intercoolers. In a drag race–only application, air-to-water intercoolers are very popular since you can pack them with ice and since the car is only run for a very short period of time. For a street car, however, an air-to-air intercooler is a much more practical solution. The problem with an air-to-water intercooler system in street cars is that it requires a secondary heat exchanger to remove heat out of the liquid. Essentially, the water that cools the intercooler gets heated up by the intake charge, and that heat must be removed from the intercooler liquid with a separate heat exchanger. In reality, an air-to-water street intercooler is really an air-to-water-to-air system. If you only need to cool the intake charge for short periods of time, packing an air-to-water intercooler works great and you can eliminate the need for a separate heat exchanger. On the other hand, the inherent inefficiency of the air-to-water-to-air process makes air-to-water intercoolers a poor choice for the street. We do use air-to-water intercoolers in marine applications because when you’re on a boat, you’re sitting on the world’s largest heat exchanger. In marine applications, you pump cold water into one side of the intercooler, and dump it back into the ocean out the other side.
A big part of intercooler selection is fitment. The highest capacity air-to-air intercooler we offer supports 1,550 hp, but it measures 27 by 12 by 6 inches. Realistically, it’s difficult to fit an intercooler that’s any larger than that into a car. For motors that produce more power than that, we recommend an air-to-water intercooler. Moreover, cars with small front ends, like fourth-gen Camaros, make it tough to package intercoolers. Fortunately, the retro styling of newer cars have bigger front ends, and more room for intercoolers.
Pressure Drop and Efficiency
Two key metrics used to gauge the performance of intercoolers are effectiveness and pressure drop. Effectiveness measures how much heat is being removed from the intake charge, and the flow capability of the intercooler determines pressure drop. The nature of intercooling means that there will be less pressure at the intercooler outlet than at the inlet. Realistically, it’s not practical to design an intercooler that has no pressure drop because there needs to be at least some pressure drop in order to scrub heat from the intake charge. Nevertheless, it’s not just the intercooler that must be considered, but also the tubing. You can’t just flow the intercooler alone. You have to take into account the intercooler tubing diameter and layout. Generally, it’s best to limit the number and severity of the bends. In other words, intercooling works as a complete system, not just the intercooler itself. Generally, at 8 psi of boost our supercharger kits are designed to keep the inlet air temperature within 20 to 30 degrees of ambient air temperature. Not only is this good for power, but for engine longevity as well. The key goal is repeatable performance, not something that requires a lot of cooldown time, and our superchargers and intercoolers quickly recover enough to maintain performance in back-to-back runs. A lot of testing is done on the dyno, and sticking a fan in front of the bumper can’t replicate the dynamics of driving on the street. Likewise, sometimes there are interesting aerodynamic effects over the front of the car, which is another reason why you have to go to the track to see how well an intercooler really works in the real world. Through these efforts, we have been able to design air-to-air intercoolers that are over 70 percent efficient.
Three of the primary components in supercharger compressor design are the inducer, exducer, and volute. The inducer is the inlet where air enters the supercharger. Increasing the inducer diameter affects airflow potential, which is why sanctioning bodies place limits on inducer size. The exducer is the outside diameter of impeller wheel, and changing its dimensions affects pressure at a given rpm. Exducer design is very important because the number of blades, shape of blades, tip height, and stagger all affect flow. The exducer diameter together combined with shaft rpm gives you tip speed, which gives you pressure. The volute is the compressor housing, which merely acts as a collector for the compressed air. An undersized volute will negatively impact performance, so the goal is to design it so it’s not a constraint. Generally, you just want the volute to be neutral in design. Interestingly, you can’t just scale a small blower up to make it work on a bigger motor. You have to completely redesign a supercharger for the pressure and flow range of each application. CHP