Supercharger A-Z

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Par Jim Moody:

Superchargers A-Z
This article lays down the foundation of how superchargers came into being by taking a look at the fundamentals of creating more power, and looking back in history at where and how the technology originated.


Making More Power - Four Possibilities with One Common Thread
When it comes to extracting more power from an engine, the common goal, simply stated, is to burn more air and fuel per time. There are essentially four ways to achieve this end.

1.) The first way to make more power, is to make the engine more efficient by tuning the air and fuel delivery, reducing intake and exhaust restrictions, reducing rotating mass, enhancing spark energy, and tuning engine timing. This is the purpose of most aftermarket modifications, like air filters, ignition programmers, exhaust systems, etc. These modifications are very popular because they provide added power, they look good, and they sound good. Moreover, they can be done piece by piece, so your car can build with your budget. The problem with these kinds of modifications is that performance gains are small - often negligble and unnoticable. This is because most engines today are tuned fairly well from the factory, and are not equipped with highly restrictive intake or exhaust components, which would reduce fuel economy. In other words, if you're looking for more moderate power gains, you'll need to get to the heart of the engine where power is really made. Most of these modifications essentially have one goal in mind - make the engine more efficient so it can burn more air and fuel in a given amount of time.

2.) You can also make more power by speeding up the engine, i.e. spinning it at a higher RPM. This technique is very effective in producing more horsepower while keeping the engine lightweight and small. If you look at some of the fastest race cars in the world, you will find them spinning at incredibly high RPMs. The only drawback is that to spin at such high RPMs requires very high quality (and expensive) engine parts that can withstand the torture from the rapid rotation. Furthermore, the increased RPM substantially increases wear and tear on the engine resulting in decreased reliability and shorter engine life. Most street cars and trucks have a redline RPM of around 4000 to 7000 RPM. Spinning the engine faster than the redline RPM in street vehicles is risky without extensive engine modifications to support the higher rotational speeds. The goal with this option is also to burn more air and fuel per time.

3.) Another obvious way to make more power is to simply use a larger engine. Bigger engines burn more air and fuel, and hence, make more power per revolution. Of course, if it were that simple, we'd all be driving around with V-12s. You can fairly easily increase the size of the engine's displacement by boring the cylinders and running a larger piston, or by lengthing the stroke of the crank, but you can only go so far before you've bored the entire cylinder away or your piston is slamming into the cylinder head. To go really big requires a bigger engine, probably with more cylinders. The drawbacks of a bigger engine include their increased size (duh!?), increased weight, and reduced fuel efficiency. In addition, using a larger engine normally is not practical because it would require an entire engine replacement, which would be prohibitively expensive, and would require extensive modifications to mount it to the vehicle. Again, though, the goal of this technique is to (yep you guessed it) help the engine burn more air and fuel per time.

4.) The final way to make more power is to pack more air and fuel into the combustion chamber before igniting it. The end result is the same as using a larger engine. The problem with this technique is that it's not as simple as telling your engine to suck more air and fuel - it's restricted by atmospheric pressure. At sea level, atmospheric pressure is 14.7 psi, which is a measure of how densely packed our atmosphere is with air molecules. As elevation rises, air thins which, as you probably noticed on your last skiing / snowboarding trip, robs power from the engine. Now imagine if you could trick mother nature by making atmospheric pressure 21psi. You'd be packing around 50% more air, which means you could burn 50% more fuel, meaning you'd be making approximately 50% more power. You've probably already figured out that this is exactly what a supercharger does - it compresses air to pressures above atmospheric pressure (boost), thus packing more air into the engine. And you've probably also figured out that the goal of this technique is to burn more air and fuel per time. By utilizing this technique, a small engine can act like a big engine. It is more efficient because it has less weight and rotating mass. In addition, because you can control when the compressor (supercharger) is sending compressed air (boost) to the engine, and when it is not, you can enjoy stock fuel efficiency when the supercharger is not sending boost to the engine (normally at half throttle or less).

In reality there are more than four ways to make more power, but these are the four most conventional ways. You can also use a more potent fuel source that has more potential energy. This is the idea behind Nitrous Oxide and other high-energy fuels - a topic beyond the scope of this article.

A Brief History of the Supercharger
You may be wondering, "Who first thought of compressing air before sending it to the combusion chamber?" Don't run to the library just yet. We'll tell you!

It seems that just before the turn of the century (1900 that is), a German engineer named Gottlieb Daimler (yes, of Daimler Benz, Daimler Chrysler...) obtained a patent for a pump to aid in the delivery of increased amounts of air and fuel to the cylinder, and to aid in the removal of exhast gasses. He didn't call it a "supercharger" in his patent application, but that's what he was describing - this was the birth of the automotive supercharger. But in order to get to the true beginnings, we have to look evern further back in history.

Gottlieb's automotive supercharger design was modeled after a twin-rotor industrial "air-mover" invented and patented nearly 40 years earlier by Mr. Francis Roots (from Indiana) back in 1860. This technology is the foundation of the roots type "blowers" still used today! Soon after the roots air movers (they were not called "compressors because they did not compress air - they only moved it) were used in industrial applications, a German engineer named Krigar invented an air pump that itlizied twin rotating shafts that compressed. This technology would go on years later to become the foundation of the Lysholm twin-screw compressor used in today's automotive applications.

Apparently our old friend Gottlieb didn't have much luck in the early stages with his new invention, but the idea inspired French engineer Lois Renault, who patented his own type of supercharger soon after the turn of the century. It wasn't long before superchargers started to show up on American race cars. Lee Chadwick is credited with being one of the first American racers to successfully use a centrifugal supercharger in competitive racing, starting in the Vanderbilt Cup in Long Island, New York in 1908.

Soon thereafter superchargers took to the air as World War I military engineers looked for new ways to make more powerful airplanes. Because airplanes fly at such high altitudes, the internal combustion engines that worked great on the ground, suffered at altitude in the thinner air. Although the technology wasn't successfully used in combat before the war ended, it was clear that sueprchargers were well on their way to becoming a mainstream power adding device.

Meanwhile, back in Germany, Mercedes was hard at work trying to make old Gottlieb's supercharger work. By 1921 they found success and released a glimpse of the first production supercharged vehicle utilizing a roots-type supercharger. Mercedes went on to manufacture several supercharged models with great success in the following years.

In the racing scene, supercharged cars were finding more and more success. By 1924, superchargers made their way to the Indy 500. Around the world, racers in Mercedes, Fiats, Bugattis, Alfa Romeos, Buicks, and MGs began using superchargers to help them to the victory circle. Mercedes found great success with their supercharged Grand Prix cars, while Harry Miller's supercharged Indy cars dominated at the Brickyard.

In the mid 1930's Robert Paxton McCulloch started McCulloch Engineering Company and began manufacturing superchargers as the first large American commercial supercharger manufacturer. They began developing superchargers for use on American passenger cars and hydroplane boats. This was the start of the supercharger industry in America as we know it today.

Then came World War II in 1939, and the Allied forces had an ace up their sleeve in the form of the supercharged Spitfire fighter planes and B-29 SuperFortress bomber. These supercharged planes seemed almost unaffected by the altitude to the delight of Allied pilots and soldiers.

After the war, superchargers took on a new life in the world of racing. Alfa Romeo and British Racing Motors used superchargers on their Grand Prix cars to the horror of the competition before they were eventually outlawed. At Indy, there was no such rule, and centrifugal superchargers howled their way to many vicories.

By 1950, McCulloch had formed Paxton Engineering as its own entity, which took over the supercharger development and took on the task of creating an inexpensive supercharger fit for use by the general public. After $700,000 in research, and two years of testing, the VS57 supercharger was ready for the public in 1953. Initially it worked only on 1950 - 1953 Fords, but by 1954 kits were made for nearly every commercially available 6 and 8 cylinder engine.

The rest is history, as Paxton developed newer and better superchargers until they became a part of life, not only in the world of racing, but also in the street-legal aftermarket world. Today it's hard to keep track of all the supercharger brands and models, but that's the way we like it!

The Roots Supercharger (aka "blower")
The roots supercharger was originally designed as an air moving device for industrial buildings. The roots supercharger features two counter-rotating lobes that trap air from the intake side of the supercharger (normally at the back of the supercharger), move it around the outside casing of the lobes, and out the bottom of the supercharger through an outlet / discharge port. Like the twin screw supercharger, the roots is a "positive displacement" aka "fixed displacement" supercharger, meaning that it moves a fixed volume of air per rotation. Notwithstanding minor amounts of air-leak at low rpms, the roots supercharger cannot flow backwards like a centrifugal supercharger, and is thus nearly as efficient in its ability to pump air at low rpms as it is at high rpms. What this means is that roots superchargers are very capable of making large amounts of boost even when engine rpms are very low. This makes for great low-end and midrange power, and also makes them great for trucks and towing vehicles. The roots is also self lubricated, and is the simplest of the supercharger designs, meaning it is reasonably priced and very reliable. This is why roots superchargers have been the choice of GM, Ford, Mercedes, and Toyota for OE applications.

The only real disadvantage to the roots supercharger is that it creates a lot of heat. There are two reasons for this. First, the roots supercharger does not compress air - it only moves from the intake port to the discharge port (i.e. it is the only supercharger design with no internal compression ratio). All of the compression is done in the intake manifold. Laws of thermodynamics kick in in favor of supercharger designs with an internal compression ratio (centrifugal and twin screw) because they do less work on the incoming air charge. We will leave the mathematics of this phenomenon to a later (much more boring) discussion. Another reason roots superchargers create higher amounts of heat is because they tend to carry some of the compressed air in the intake back into the supercharger because it gets trapped by the rotating lobes that are exposed to the hotter air in the intake manifold.

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The Twin Screw Supercharger
The twin screw supercharger at first glance appears to look similar to a roots supercharger both inside and out. The two technologies are indeed similar, however there are significant differences. At the heart of the twin-screw supercharger are two rotors, or "screws" that rotate towards each other. The rotors mesh together and draw air from the back of the supercharger. The twisting rotors move the air to the front of the supercharger, while compressing the air before discharging through a port at or near the front of the supercharger.

Because the compression is done inside the supercharger, this design produces less heat than a roots supercharger - in fact, it is almost as thermally efficient as a centrifugal design. Like the roots design, the twin-screw is a fixed displacement supercharger (meaning that it pumps a fixed volume of air per revolution), and because the tolerances between the rotating screws are very tight, its ability to create boost at low rpms is unparalleled. These characteristics make it ideal for trucks and towing vehicles, where low to mid range power is primary in importance. Another important advantage of the twin screw compressor is its reliability. Unlike a roots supercharger, the rotors in a twin screw supercharger do not actually touch, so there are virtually no wearing parts. For this reason, twin screw compressors are commonly used to pressurize cabins in passenger aircraft. Like roots superchargers, twin screw superchargers are self lubricated and do not tap into the engine's oil supply.

One disadvantage of the twin screw design is that, because it has an internal compression ratio, the twin screw is compressing air even when it is not sending boost to the engine (i.e. under cruising or deceleration). An internal bypass valve releases the pressurized air, but because it takes work to pressurize the air in the first place, the twin screw supercharger draws more power from the engine than while not under boost. Like the roots, the throttle body must be placed before the compressor because it is a fixed displacement supercharger.

The Centrifugal Supercharger
Although the centrifugal supercharger is founded on a technology much newer than either the roots or the twin screw, it was the first supercharger to be successfully applied to automotive applications. Unlike the roots, the centrifugal supercharger is NOT a positive displacement / fixed displacement supercharger because it does not move a fixed volume of air per revolution. The centrifugal supercharger essentially operates like a high speed fan propeller / impeller, sucking air into the center of the supercharger and pushing it to the outside of the rapidly spinning (40,000 + rpm) impeller blades. The air naturally travels to the outside of the blades because of its centrifugal force created by its rotating inertia. At the outside of the blades, a "scroll" is waiting to catch the air molecules. Just before entering the scroll, the air molecules are forced to travel through a venturi, which creates the internal compression. As the air travels around the scroll, the diameter of the scroll increases, which slows the velocity of the air, but further increases its pressure.

The centrifugal supercharger enjoys several advantageous characteristics that make it the most popular supercharger design in the aftermarket world. First, it is simple and reliable because it has very few moving parts - just a few gears and the impeller. Second, the centrifugal supercharger produces very little heat because of its internal compression ratio. It is also small in size and very versatile because it can "free-wheel" and allow the engine to suck air through it or even flow air backwards. For this reason it can be placed anywhere in the intake tract - it can even "blow through" the throttle body, meaning it can be mounted nearly anywhere. It is also the most thermally efficient supercharger, meaning that it produces the lowest discharge temperature.

The only significant disadvantage of the centrifugal supercharger is that it must be spinning at a relatively high speed before it begins to make a significant amount of boost. For this reason, it is not helpful in creating boost (and power) at low engine rpms. Normally the supercharger only begins to create boost at around 3000 rpm, and the boost curve gradually and increasingly rises with engine RPM. Many centrifugal superchargers do not have a self-lubricating oil system, and draw oil from the engine's oil supply. The disadvantage to this is that you must tap the oil pan for the oil return line. However, in doing so, the supercharger becomes virtually maintenance free. Some manufacturers make a "self-contained" centrifugal supercharger that is self-lubricated like roots and twin screw superchargers.

The Turbocharger
You may be wondering where the turbocharger fits in to this equation. Technically, a turbocharger IS a type of supercharger - one that is driven by exhaust gasses rather than from a pulley that draws power from the engine's crank. Because the turbocharger relies on a technology substantially different from the three traditional supercharger technologies discussed above, it is beyond the scope of this article.

That's it for part 2 of the series - next time we'll pull everything together and discuss what goes into making a complete supercharger system, and how the supercharger works in conjunction with the engine.

The Air Intake System
Because a supercharged engine draws substantially more air than a normally aspirated engine, it is important to minimize intake restrictions. To ensure a smooth delivery of air to the supercharger, most supercharger systems include a high-flow air filter as well as low-restriction tubing or ducting to deliver air from the atmosphere to the supercharger. It is important to maintain a clean air filter to minimize the particles that come into contact with the supercharger's impeller, rotors, or screws. Most supercharger systems will draw air from behind the fender wall, where there is an abundance of cool air that has not been heated by the engine. Because superchargers heat air as it is compressed, a cool air supply helps to keep the charge temperatures at a reasonable level. On a non-intercooled application, the cold air pickup can lower the charge temperature by up to 60 degrees!

On most vehicles the incoming air charge passes through a Mass Air Flow sensor (aka MAF) on its way to the supercharger, although on centrifugal superchargers, the Mass Air Flow sensor can be mounted after the supercharger ("blow-through" setup). The Mass Air Flow sensor measures, you guessed it, the mass of air that the engine is drawing. This reading allows your engine's ECU (Electronic Control Unit) to calibrate and deliver the appropriate amount of fuel for the incoming air charge.

Once the supercharger has worked its magic, the air must be delivered from the supercharger to the engine intake. Although many roots and twin screw superchargers bolt directly to the manifold, most centrifugal superchargers require an extra tube called a Discharge Tube to carry the air to the intake through the throttle body. This tube will normally be mandrel bent to minimize restrictions.

The Bypass Valve
Compressor surge is a problem that affects most superchargers and develops when the supercharger is creating boost, but the throttle shaft is closed. Although not a problem on some low-boost (5psi or less) applications This condition can occur under deceleration or while shifting between gears, and can cause the car to sputter and chirp. Under surge, the compressor forces air into the closed throttle body until the pressure inside the throttle body is higher than the amount of pressure being created by the supercharger, and the air tries to pop backward through the supercharger. At that point, pressure is released inside the throttle body and the compressor forces air back through the supercharger and into the throttle body, which again has nowhere to go, and the process repeats. While surge normally is not highly damaging to the engine it is certainly annoying and can cause damage with time. To eliminate these problems under surge conditions, a bypass valve (sometimes called an anti-surge valve) is used to release the excess pressure. The bypass valve is actuated using intake manifold vaccuum, which opens the vent valve and releases pressure in the air-intake. Air is either released into the atmoshpere (blowoff valve) or recirculated back through the supercharger compressor (bypass valve).

The Intercooler / Aftercooler
Some supercharger systems include an aftercooler (more commonly called an "intercooler"). The purpose of the intercooler is to remove heat from the air to create a cooler, more densely packed air charge Vortech Style. Although the intercooler is not necessary on most street applications, its performance becomes increasingly important on higher-output systems (with correspondingly higher charge temperatures). The intercooler can be compared to a automotive radiator, only instead of cooling water or coolant, the intercooler cools the air. Air-to-air intercoolers force the air through a large air-cooled finned and fluted core, normally mounted in front of the car's radiator. Air-to-water intercoolers force the incoming air charge through a much smaller finned and fluted heat exchanger that is cooled by water. The water is, in turn, cooled by a compact radiator that mounts next to the stock radiator.

The two main purposes of the intercooler are 1. to allow more boost on a given octane level of fuel without detonation, and 2. to help create more power by condensing the air charge. Thus, intercoolers are very common on high boost applications (10+ psi) and on roots-style superchargers, where discharge temperatures are higher than normal. Most street supercharger systems (5-8psi) do not come standard with intercoolers.

The Fuel System
As increased amounts of air are pumped into the engine with the supercharger, so too must increased amounts of fuel be delivered. This is where the power gains come from. Most stock fuel systems are not up to the task of delivering the increased volumes of fuel demanded by a supercharged engine. Without a proper fuel system, your engine may run lean, detonate, and obviously perform below its potential. Because every engine is different, the fuel system requirements vary greatly with different vehicles and with different supecharger systems. Sometimes larger fuel injectors and a larger fuel pump is required. On some applications, a fuel management unit (FMU) will do the job by restricting the fuel return line to build up fuel pressure. On other applications, additional fuel injectors are mounted to the intake manifold, while on some applciations the stock fuel system works like a charm. Fortunately most supercharger systems include all of the fuel system components necessary to tune the engine to perfection. On some race kits, tuner kits, custom installations, and high output systems, it is up to the engine tuner to determine the engine's fuel requirements and tune the fuel system accordingly.

The Ignition System
The engine's ignition system serves the important role of telling the spark plugs when to fire so the compressed air and fuel is ignited at the exact right time to produce maximum power. Ignition timing can be advanced, causing the spark to fire earlier, or retarded, causing the spark to fire later. Ignition timing is critical not only for performance reasons, but also for engine longevity as it used to eliminate detonation (aka spark knock). With the added air and fuel that is compressed in a supercharged engine, the engine is closer to its detonation threshold. To avoid detonation, many supercharger systems retard the ignition timing, thus reducing maximum cylinder pressures and temperatures, and moving away from the detonation threshold. Because retarding the ignition timing causes a slight loss in power, a higher octane fuel or an intercooler are recommended for optimal performance, both of which allow for more timing without detonation. To ensure a complete and cool burn, high quality, cool heat range irridium spark plugs are also recommended for use on supercharged engines.

The Pulley
All superchargers are driven by a pulley that sits inline with the accessory belt or crank pulley. The size of the supercharger pulley is what regulates the speed at which the supercharger spins. Obviously, a smaller pulley turns the supercharger faster, and vice versa. The pulley is easy to change on all superchargers and is often used to increase (or decrease) the ouput of the supercharger. A simple pulley-swap can equate to huge power gains if the rest of the system is up to the task (in particular the fuel and ignition system).

The Rest
Other components serve self explanatory roles. Mounting brackets obviosly are used to attach the supercharger to the engine in a position such that the pulley can be spun from the accessory belt or an additional supercharger belt. The belt tensioner keeps the belt tight around the supercharger pulley, which is important to avoid slippage, especially on centrifugal superchargers which spin at high RPMs. Hardware, hoses, and fittings are of course necessary to attach the supercharger to the engine, connect the oil and fuel lines, and to install the ignition components.

That rounds out the complete supercharger system. Remember that every supercharger system is designed to meet the specific needs of the engine, given the desired level of output from the supercharger. For this reason, some supercharger systems come with only a few of the components mentioned in this article, while others come with it all. Generally speaking, higher output supercharger systems come with more components to meet the increased volume of air, which is why they cost more than entry level systems. Congratulations if you made it through all three parts of this series - you deserve a gold star and are now a supercharger expert!