All About Superchargers
By Marlan Davis
The “secret” to making power in an internal combustion engine really boils down to how much air can be stuffed into the cylinders on each power stroke. With more air comes more fuel. Add more air and fuel, and combustion produces more heat, energy, and pressure. Higher pressure exerts greater “push” on the piston and rod; the crank spins faster, resulting in higher power output at the flywheel. This is the idea behind big cams, ported heads, and stroker cranks.
Even with those improvements, a normally aspirated engine relies on atmospheric pressure (14.7 psi at sea level) and the vacuum created by internal piston suction to draw air and fuel into the engine. Ultimately, this limits how much mixture can be burned on each power stroke. Adding a compressor can force-feed a denser fuel/air charge into the combustion chamber for increased power.
Automotive engineers refer to such a compressor as a supercharger, regardless of the way it’s driven; however, gearheads commonly use the term “supercharger” or “blower” to refer to a compressor that’s driven by a belt, gears, or a chain. This distinguishes them from exhaust-driven superchargers, known to car crafters as “turbochargers” or “turbos.”
The "secret" to making power in an internal combustion engine really boils down to how much air can be stuffed into the cylinders on each power stroke. With more air comes more fuel. Add more air and fuel, and combustion produces more heat, energy, and pressure. Higher pressure exerts greater "push" on the piston and rod; the crank spins faster, resulting in higher power output at the flywheel. This is the idea behind big cams, ported heads, and stroker cranks.
Boost and Compression
Boost is the amount of air pressure over normal atmospheric pressure a given supercharger is capable of developing. Up to a point, the higher the boost, the higher the engine's power potential. Eventually the combination reaches its max practical boost level as constrained by blower size and design, overall blower and engine efficiency, available fuel octane rating versus engine compression ratio, and overall air/fuel charge temperature. As the blower compresses the air/fuel charge, the charge temperature increases and there is a point where heat buildup offsets the power generated by increasing boost. Also, the faster the blower spins, the more power it takes to make it spin. At some point, the faster-spinning blower's potential to make more power is offset by the added power required to spin it. Finally, the lower the compression ratio (CR), the more boost you can run on the same octane gas--but for street use, going too low on compression results in an engine that is lazy on the bottom end when not under boost.
Most sources consider a static CR of about 8:1 a good compromise for noncomputer-controlled cars running on premium street gas. More power is possible by dropping compression to 7:1, but the car won't be as responsive down low when not under boost. Static ratios over 9:1 limit boost and max power potential.
Generally you can run about 5 psi of boost without major engine, timing, or fuel system mods. Totally revamping the fuel and ignition system and building a strong short-block can support 8-10 psi (depending on blower design and efficiency), which at sea level equates to at least a 60 percent power increase. Modern computer-controlled ignition systems, automatic detonation retard, and EFI may permit running even higher compression ratios and boost pressures. Some race-oriented centrifugal systems run as much as 25-30 psi on race gas with an intercooler. At 300 psi, you're looking at a 200 percent power increase!
Experienced blower builders jockey CR and boost combos to meet their goals. The engine's final or effective compression ratio is equal to the static ("as-built") ratio plus boost, as shown by the equation Final CR equation.
The formula yields very high effective-compression ratios. This is possible without causing detonation because the total effective CR occurs at max boost, a level that's reached only at higher rpm where there's less engine loading. Nevertheless, without an intercooler or electronic engine management system, running 92 octane holds the max safe effective CR to about 12.5:1 on Roots systems and about 13.5:1 on centrifugal setups.
Actually achieving a given boost level requires selecting a blower that's properly sized to the engine it will be used on. With positive-displacement blowers you can use supercharger displacement as a baseline. The Boost equation shows how much boost the unit can generate--then it's merely a question of selecting the proper pulley ratio.
Selecting the right centrifugal supercharger is more complex. Not only are they boost-limited by maximum shaft speed, but the overall compressor housing size and impeller "trim" configuration also influences how much and where maximum boost occurs (and this varies on different engines). Just like a turbocharger, each unit has a compressor "map" that defines it's maximum efficiency point in relation to engine size and boost level. Consult the tech support experts at your favorite blower company for help in selecting the optimum unit for your application.
Rotor or positive-displacement superchargers were first developed in the mid-1800s by the Roots brothers. "Positive displacement" means that with each full rotation of the compressor element, a specific fixed volume of air is pumped from the inlet side to the exhaust outlet. As its name implies, a rotor blower utilizes intermeshing rotors inside a case bolted atop a special intake manifold. A crank-mounted pulley spins a drive pulley via a belt (cogged on larger blowers); the drive pulley in turn connects to meshed gears that spin the rotors. As the rotors spin, they compress air and fuel supplied by carburetors or a fuel-injection system mounted on top of the blower case. In the classic Roots design, the mixture is pumped between the supercharger case and rotors.
Rotor superchargers build instantaneous boost pressure down low and maintain it as rpm increase, but efficiency tapers off on the top-end due to heat buildup inside the case and leakage past the rotor seals. Heat buildup makes the engine more detonation-prone--a real problem with today's low-octane fuel. According to Blower Drive Service, one solution is to run a slightly bigger blower, then underdrive it slightly. Bearing this in mind and assuming the blower is properly sized for the engine and application, a Roots supercharger would be a great choice for street cars were it not for its high system height. But for some hard-core gearheads, that's not necessarily a drawback--there's no better visual statement than a big Roots poking through a hole in the hood!
A variation on the positive-displacement configuration, externally a twin-screw supercharger appears much like the common Roots design, but inside the case it’s a different story. Air enters the twin-screw supercharger through the rear or top rear and is compressed internally between screw-like rotors. The twin screw’s shorter airflow path reduces the high turbulence, friction, heat, and pumping losses characteristic of classic Roots designs, but without sacrificing the positive-displacement design’s instantaneous boost virtues. In one comparison of similarly sized Roots and twin-screw blowers, the Roots’ intake temperature rise from boost was about double that of a twin screw (about 8.6 degrees F/psi for the twin screw compared to the Roots’ 15-18 degrees F/psi). That extra charge temperature robs power potential. High-end racing twin-screw blowers exist, but they are not widespread because many rule-makers discriminate against them. However, small-screw compressors sold by Kenne-Bell have gained some popularity among the 5.0L Mustang and import crowd.
Like a turbocharger, centrifugal superchargers use a spinning impeller to compress air. But unlike an exhaust-driven turbo, the centrifugal supercharger's impeller is spun off the crankshaft by pulleys and a drivebelt.
Depending on size and design, centrifugal blowers are capable of significant power increases, yet their relatively compact size in comparison to the massive positive-displacement design lets them fit under the hood of just about any vehicle. Centrifugal blowers are usually mounted somewhere on the front of the engine via brackets designed for each engine application. Because a special intake manifold isn't required, they are especially compatible with late-model fuel-injected cars where they're set up to blow through the air meter. Turnkey, smog-legal kits for late-model packages are available from companies like Vortec, Paxton, and Procharger and include the supercharger, ducting, oil lines, and all necessary adapter brackets. On the other hand, the latest race-oriented units are capable of feeding large-displacement, high-boost applications through 1,400 hp--and you can always use two!
A centrifugal supercharger optimized for max top-end boost and power usually won't build low-end boost as quickly as a rotor blower. While the rotor blower builds boost early and maintains it as rpm increases, a centrifugal blower builds boost exponentially--in other words, doubling the blower's rotational rpm causes the boost to quadruple. Because typical street engines are boost-limited due to emissions, engine durability, and fuel quality factors, the blower drive ratio must be set to provide a specified boost level at a specific rpm. This causes either a reduction in potential top-end power or less low-speed boost. On the other hand, lower boost downstairs can be an advantage--there are less traction problems and a reduced chance of detonation.
A charge-air cooler or intercooler is an air-to-air or air-to-water heat exchanger. Installed between the compressor and engine, it dramatically reduces the elevated air temperatures created by superchargers, thereby increasing air density to make more power. At low boost with an efficient compressor, a charge-cooler is an unneeded luxury, because the air does not get very hot. But with higher boost pressures, higher compression ratios, or an inefficient supercharger, it can really improve performance. The crossover point is somewhere around 8-psi boost—but of course packaging constraints also come into play.
A supercharger, by definition in Webster's Dictionary, is "an apparatus consisting of a pump, compressor, or blower used to increase the volume of air over and above that which would normally be drawn into an internal combustion engine due to the action of its pistons."
When the engine is naturally aspirated it relies on the pumping action of the pistons to draw air into the cylinders. The negative pressure in the cylinders combined with the ram effect of the intake manifold, camshaft overlap and exhaust scavenging, allows each cylinder to draw in the air/fuel mixture every time the intake valve opens. With supercharging, air is constantly being packed into the intake manifold under pressure (called boost), so the air is forced into the cylinders when the intake valve opens, rather than being drawn in solely based on the pressure differential between that of the cylinders and the atmosphere in the manifold. This additional air provided by the supercharger simply permits the engine to burn more fuel, thus creating more cylinder pressure and, with any luck, more power. Supercharging is really that simple--on the surface.
Since a supercharger is a compressor driven by the crankshaft, the output of the blower changes with rpm (speed). Generally, the output will increase until the point of peak efficiency, and then output falls off. Eventually the blower will reach the point where it makes no more boost, just extra heat.
Supercharging has been around for quite some time and has been used on all types of engines, including piston-driven aircraft and generators in big industry. In auto racing (and in high-performance street applications), we've learned to apply supercharger technology to help our internal combustion engines achieve incredible power levels. With a blower, volumetric efficiency can easily exceed 100 percent, while most naturally aspirated engines struggle to achieve 60-80 percent VE.
Another benefit is that owners can retain the stock cam, heads and, in some cases, the induction system, therefore, retaining much of the OE driveability, yet still realize a huge increase in performance.
The most extreme supercharged engines can be found in drag racing, namely in the Top Fuel and Funny Car ranks. These extreme machines utilize 500-cubic-inch engines with hemispherical combustion chambers and they burn a specialized fuel called nitromethane. Using large 14-71 Roots-style blowers, these "fuel burners" produce 45-50 psi of boost and generate upwards of 8,000 hp. Today's Top Fuel cars run quarter-mile times in 4.40s at speeds over 330 mph.
A good portion of the massive power comes directly from the fuel and its explosive force. Unlike leaded racing gasoline, nitromethane carries its own oxygen and requires a nearly equal 1.7:1 air/fuel ratio. Compare that to a gasoline-burning supercharged engine that will require a 11.5-12.0:1 a/f ratio.
At wide open throttle, the nitro burner's fuel flow is equivalent to a garden hose with the nozzle held wide open--and that's per cylinder. If you've watched drag racing on TV you've seen the header flames, but at times there is raw fuel pouring out of the headers during a run. The engine is said to have "dropped a cylinder" and it happens when the spark plugs can no longer light the massive quantity of fuel. This can lead to all kinds of problems. One is engine hydraulicing and that's often followed by a massive explosion and ensuing fireball. But Top Fuel engines aren't the only supercharged beasts that go boom in the night when things go wrong.
We've seen many Mustangs and Lightnings have failures as a result of boost, however, this isn't a fair way to explain all the blow-ups. Read on and I'll explain. Remember one of the questions that I said we often get, "Will a blower hurt my engine?" The answer here is yes and no. (And I'm not trying to be a wise guy, either.)
First I'll cover the "yes." A supercharger can cause engine damage that otherwise wouldn't occur, but the common failures (blown head gaskets, cracked blocks and burned pistons) are often the result of a poor engine tune-up, gasoline with insufficient octane rating, abuse from the driver or any combination of the three.
"First off you have to consider the condition of the engine," stated Ricky Best, the Race/Media Relations Manager for Vortech Engineering. "Most bolt-on systems making about 8 pounds of boost are designed and tuned to be able to be applied to completely stock, as well as modified engines, without having any adverse effects on the motor itself. If the engine is ailing or high in mileage and is using oil, then the supercharger is only going to help your engine find an early grave."
With today's kits, you can easily add 100-150 hp to a stock engine, and anytime you take a component and push it that far past the factory-designed capabilities there is a chance of failure. When it comes to building and tuning your own supercharged car, you must carefully select each piece of the puzzle. This includes the short-block, the heads, the fuel system, and don't forget the fuel management system, the type of gaskets, the intake manifold, ignition and, of course, the type of fuel.
Still, what people fail to realize is that despite the power increase, most problems come from a poor tune-up. In addition, remember that as horsepower increases there is additional heat as a result. It takes extra air and fuel to make extra power and burning extra fuel means more heat. Compressing the air also creates heat, and the stock 5.0 or 4.6 engine was not originally designed to dissipate all that extra heat, so you have to manage it properly. Mismanaging it leads to problems and one example of mismanaging is beating the snot out of your supercharged engine with it cooking hot. Another is turning up the boost without re-tuning.
"Tuning is a big thing," adds Best. "Assuming the engine is healthy and the kit has been installed correctly, the number one killer of most supercharged applications is bad tuning. Bad tuning can be anything from improper air/fuel ratio, too much or not enough ignition timing, too much or not enough fuel pressure, improper FMU, and incorrect spark plugs. Lastly is boost pressure. It's really easy to get your first supercharger and get it installed and running and have a lot of fun, only to find out that you can swap a pulley and get more boost. What you need to keep in mind is that adding boost means more airflow and that usually means more heat. And more heat requires more fuel and less timing. The bottom line is that all of the variables need to be addressed."
With this you can also see how the answer to the original question could also be no. A blower alone can't hurt the engine, because all it's doing is feeding in more air. In other words, it's the tune and the ability of the driver to pay attention and use the equipment in a mature fashion that will help longevity.
When it comes to the tune, care must be taken to ensure that the compressed air (read: boost) is matched with the correct ratio of fuel and the proper timing advance curve. Force-feeding the engine presents us with challenges that must be overcome in order to maintain good power, driveability and reliability. The most notable demon is detonation. By its nature, compressing air creates heat and this potentially results in engine-damaging detonation.
"We've worked with flame-front experts and we know that auto-ignition or detonation creates cylinder pressures that are up to 10 times that of normal combustion," stated Dan Jones of ATI/ProCharger. "Pre-ignition or unscheduled spark ignition occurs when the piston is traveling up the bore and is not yet at the point for the scheduled ignition but ignition occurs without the spark plug firing."
When this occurs, the mixture begins to burn prematurely and cylinder pressure skyrockets. The piston is still on the way up, but it's fighting the cylinder pressure that's not supposed to be present. Then, the spark plug fires and the scheduled flamefront begins. Ultimately the two flamefronts collide and the sound is heard as a knocking or pinging, i.e. detonation. This not only creates tremendous pressure in the cylinder, but also creates unwanted harmonics throughout the engine. And over time (sometimes a really, really short time) these harmonics cause major engine parts to fail. But even when pistons or head gaskets do survive detonation, the related effect places severe load on the crankshaft and the bearings, as well as the rings and the block.
According to Jones, the problems associated with detonation far exceed the alleged problem of extra load on the engine imposed from simply making extra horsepower and torque.
"When the engine is running properly there is a very short period in degrees of crankshaft rotation where big force is applied to the pistons. This occurs during the power stroke and for a short time just after the mixture is combusted as the piston crosses TDC," said Jones. "It's important to note that with forced-induction engines there is less total peak rod/piston/crank load, but the duration of the power application is longer than with naturally aspirated engines. This means the force is being applied closer to the point at which the crank nears the 90-degree angle and this means more torque can be applied."
Roots and Screws
There are three common types of superchargers (centrifugal, Roots and screw-type or twin-screw) and each one of them is designed to do the same thing--make boost. But while they all force air into the engine, they possess different characteristics.
Hot rodders are familiar with Roots-style blowers seen perched atop Pro Street or Top Fuel engines. Roots blowers have been around since the 1800s and they fall in the "positive displacement" or "fixed displacement" family. Twin-screw blowers are also positive-displacement blowers.
Positive-displacement superchargers are labeled as such because they move a fixed displacement of air per each revolution of the blower. As the rotors or lobes spin, a fixed amount of air is trapped and that air can not reverse in flow. An increase or decrease in airflow through the blower will be noticed based on the position of the throttle, but once the air enters the blower and is sealed between the rotors and the case (or the screw lobes) the amount of air per revolution can't change.
On the other hand, a centrifugal blower can allow a backflow of air because the air is not sealed within the compressor at any point. It's also important, but not critical, to note that screw and centrifugal superchargers compress the air within their housings, whereas Roots blowers force air through the blower and the compressing is done in the manifold.
Each blower has a case, usually cast from aluminum, with a machined inside, which houses two (or three) rotors. The rotors will have either two or three lobes, and some units, like the Eaton blower found on the Lightning and Cobra, have the lobes twisted. Air is drawn in at one point and guided towards the rotors. The rotors accelerate the air towards the outlet, where it is carried and fed into the intake manifold.
The blower or compressor is attached to a plenum that serves as the intake manifold. There's usually a machined surface where the blower sits, a plenum and runners to supply the ports. In most cases, the ports are short, due to space limitations and because a long runner is just not necessary when you have boost pressure. Roots blowers do a tremendous job of making instant boost, thus filling the cylinders at low rpm and this helps to generate great throttle response and torque.
Disadvantages to this style of supercharger are that (in most applications) they sit atop the engine, which can create packaging or hood clearance problems and that they generate lots of heat. There's not much you can do to solve the clearance problem, but with the use of intercoolers, aftercoolers and heat exchangers, the issue of heat is not as prevalent.
"Roots blowers are good reliable units, but the twin-screw is a much more efficient design. And that's why Ford has gone to a twin-screw on the new GT," stated Jim Bell of Kenne Bell Inc.
A screw blower looks similar from the outside, but the internals are completely different. With a twin-screw there are male lobes that intermesh with female lobes. Both sets rotate inward and as air is drawn in it is compressed and "screwed" forward towards the front of the case. According to Bell, rotor speed can approach 24,000 rpm.
Bell also said that by design the Roots is about 30 percent less efficient. Therefore, it must be 30 percent larger to pump the same amount of air. And larger blowers take more energy to turn so there are greater parasitic losses and more heat.
Because of the way each of the three blowers arrives at making boost, there are great debates as to which system is more efficient. The generation and dissipation of heat within a blower system has to do with thermodynamics (the physics of relationships between heat and other forms of energy) and this is quite the complicated subject.
Any time you compress air its temperature rises. You can't avoid this--it's one of the laws of physics. You also have heat generated by the blower itself due to internal friction, or more technically, by the work necessary to get the air from its natural pressure up to the desired boost pressure.
For instance, heat is generated at the bearings, within the blower's internal drive system, by the drivebelt and even by the friction of the air flowing through the blower. Centrifugal blower manufacturers tell us these units tend to be the more efficient because they are not bolted directly to the intake, so less heat is transferred to the engine. They are also easier to intercool, and intercoolers are an important tool. Intercooling (or aftercooling) reduces the temperature of the intake air charge and allows tuners to dial in more boost pressure and more ignition timing without as much fear of detonation.
In direct contrast to the Roots or screw blower is the centrifugal supercharger. There is a huge difference between the designs of the two. Where the Roots and screw units are positive-displacement blowers, the centrifugal blower's displacement is not fixed.
Like their Roots cousins, centrifugal superchargers are also driven by the crankshaft, however, they are generally much smaller and are usually mounted at the front of the engine rather than on top (although there are some Roots blowers that are front-mounted and driven directly by the crank). This allows them to adapt easily to EFI engines because the owner can retain his or her complete throttle body and intake manifold system. In most cases, just the inlet tube (or system) needs to be modified.
The centrifugal housing is shaped similarly to a turbocharger and in place of rotors or screws, it uses an impeller (also similar to a turbo) to draw in air and direct it to the housing. "Centrifugal blowers are true compressors," stated Best. "As the supercharger draws in air it accelerates and compresses the air internally. The scroll collects the compressed air and forces it into the discharged tube and then into the intake manifold. A well designed compressor stage exhibits much higher efficiency than the Roots design, resulting in much greater net gains due to lower charge air temperature and parasitic loss," he added.
Centrifugal blowers accelerate the air due to centrifugal force, hence the name. The impeller wheel is driven by an internal transmission with a "step-up ratio" and a drive pulley system, therefore it can drive the impeller much faster than the actual engine rpm. Impeller speeds are generally in the range of 50,000-65,000 rpm.
"Centrifugal blowers take in air and the impeller carries or directs the air and accelerates it. It whips it up to speed dramatically, but the impeller doesn't compress the air or generate boost. The flow of air exits the impeller and enters a vaneless diffuser where it is straightened out and sent into the scroll. Air then slows down and pressure is created," explained Jones.
And since there are a virtually unlimited number of applications, blower manufacturers have developed a variety of housings and impeller types to suit the needs of everything from a stock 3.8- or 5-liter engine to a 6-second, 200-mph Pro racer. The size of the housing and the shape of the impeller blades (or fins) have a great affect on the boost curve and changes to the impeller can be made to fine tune this curve to maximize airflow and boost for a specific application.
If there is a downside to the centrifugal superchargers, it's that they rely on rpm to make boost and they give up low-rpm performance to the Roots and screw units in this department. Nevertheless, they are generally more efficient at making boost in the higher rpm ranges.
Today there is a huge number of blower kits available to Mustang and Lightning owners. The choices can be overwhelming, but we've found that picking the right blower requires nothing more than a little research on your part. The key to finding the best one for your combination is to select a unit that can supply the most efficient level of boost in the rpm range that you're building your engine for. In addition, consider the combination as a whole. For instance, heavier vehicles need more torque than lighter ones do and that's why the Eaton, Magnum Powers or Kenne Bell is the best choice for a 4,500-pound Lightning. But a Paxton, Powerdyne, ProCharger, Vortech may be the way to go for your 3,000-pound LX.
And lastly, remember that boost is awesome for a street-driven car and peak power numbers are important, but reliability and driveability should outweigh maximum power. We've seen too many people shoot for the moon, you know, for that last ounce of power and end up with a worthless pile of pistons, rods and crankshafts.
People often ask how long their stock short-block will last with a blower. To them, we point out our "Ice Box" '01 Mustang GT project car. It's had a Vortech SQ on it virtually since it was new. Thanks to a ported set of stock heads, Comp cams and a ported Bullitt intake, it now makes 542 rwhp and runs 11.2 at 126 on pump gas. It is driven daily year round on 93- or 94-octane fuel and has eclipsed 40,000 trouble-free miles on the untouched factory short-block.
Much of the credit has to go to its conservative JDM Engineering tune up. When tuning the car, JDM proprietor Jim D'Amore told us he could make more horsepower, but didn't feel comfortable going higher with the factory rods.
In other words, common sense should prevail. Always run good gas, keep a check on fuel pressure and timing and remember there's a time and a place to hold the gas to the floor.