A supercharger is basically a large pump that compresses and forces air into your vehicles engine to create extra horsepower. Because they can create reliable horsepower easily and affordably, they’re not only becoming popular choices for the aftermarket crowd, but also on OEM applications. We decided to find out where Superchargers came from, how they work, what types are available, and what type is right for you.
The beginning of Superchargers date back to 1860 when an Indiana man named Mr. Francis Roots created a twin-rotor industrial “air-mover”. This is where the roots style Superchargers we see today all started. Shortly after that a German engineer named Kriggr invented twin rotating shafts that compressed and pumped air. This is the same basic technology that can be found under the hood of many of today’s cars, known as the Twin-Screw Supercharger. But it wasn’t until 1900 when Gottlieb Daimler (sound familiar?) was issued a patent for a pump that would help move increased amounts of air and fuel into a vehicles cylinders. Although it didn’t go by it’s modern name at the time, this is what many consider to be the birth of the automotive Supercharger. Shortly after that, Superchargers started appearing on race cars. Lee Chadwick was one of the first American racers to use a centrifugal Supercharger in competitive racing with successful results. World War I military aircraft then used Superchargers to overcome the lack of horsepower at high altitudes. By 1921 Mercedes was manufacturing Supercharged cars and the Supercharger era had begun.
At a very high level, there are three types of Supercharger: Mechanically Driven, pressure-wave, and exhaust driven. The pressure-wave Supercharger is rarely ever used in the automotive world so we’re going to leave that out of this conversation. The ever popular exhaust driven Supercharger, also known as the TurboCharger, is becoming more commonly used in today’s high performance market place. Since most people today place this type of SuperCharger in it’s own category, we’re also going to leave that out for now and focus on the Mechanically driven versions for today. These are the blowers we think of when we hear the term Supercharger. They can also be broken down into different categories: roots, twin-screw, and centrifugal. They all have their advantages and disadvantages, and hopefully after you read this article, you can decide which one works best for you.
In contrast to TurboChargers which run of exiting exhaust gasses, Superchargers are mounted to the engine and are driven by a pulley that runs off the crank. Air comes into the Supercharger and is then compressed before being discharged into the engine’s intake. This raises the density of the air charge before it enters the cylinders. As the RPM’s rise and the crank begins to spin faster, so does the Supercharger’s impellers, forcing more air into the engine and creating Boost. Boost is created when air is being forced into the engine rather than being pulled into the intake and is measured by PSI (pounds per square inch). The more boost being created, the more dense the air charge in the engine’s combustion chamber, allowing the engine to burn more fuel, which results in more horsepower. If a car is producing 6 pounds of boost, it means it’s making 6 additional pounds of pressure over the atmospheric pressure at that elevation. Atmospheric pressure is 14.7 psi at sea level. As many of you are aware, cars perform their best around sea level in contrast to high elevations. This is because the air starts to thin out the higher you go and it becomes less densely packed with molecules. Superchargers provide power only under full throttle and therefor do not effect the engines reliability under normal driving conditions.
One problem with Superchargers is that because they spin at such a high rate of speed, they also produce a lot of heat. Some company’s overcame this obstacle by tapping into the vehicles oil pan to lubricate the gears inside the head unit of the Supercharger to minimize heat and friction. Others use internal belts or self contained head units where the oil never needs to be changed. The air itself also becomes hot because you are condensing it. Intercoolers are often used to cool the air and create a more densely packed air charge. An intercooler is much like a cars radiator. Two common types of intercoolers are Air-To-Air, which uses outside air to cool the air that just passed through the Supercharger, and Air-To-Water, which forces the air through a heat exchanger that is cooled by water. Intercooler are not always needed, but are usually found on applications that produce higher levels of boost.
Another term you’ll commonly hear among Supercharger conversations is the Bypass Valve. When a Supercharger is trying to force air into the engine, but the throttle shaft is closed, a situation called Compressor Surge is created. This can occur during deceleration or when the driver is between gears. When the Supercharger is trying to force the air into a closed throttle body, and the pressure inside the throttle body is greater than the pressure created by the Supercharger, the air tries to force itself backwards into the compressor. When this happens, the pressure inside the throttle body is released and the compressor forces the air back through the Supercharger and then back into the throttle body again, creating a loop. This is where a Bypass valve comes into play. It’s actuated by the vacuum from the intake, and then releases the excess pressure either into the atmosphere (blowoff valve) or back through the compressor.
Most cars today don’t come with a fuel system that will handle the power produced by a Supercharger. More air means more fuel – so you’ll need to make sure you have an adequate fuel system before bolting on one of these. Your ignition system is another area that should be taken into consideration. Ignition timing can be advanced or retarded causing the spark to fire earlier or later. Many Supercharger kits retard the timing to avoid detonation. This reduces maximum cylinder pressures and temperatures, and moves away from the detonation threshold. On some of the more recent computer controlled vehicles, a Dyno-Tune by a professional tuner may be necessary to have your car run correctly.
The majority of Superchargers sold today are centrifugal Superchargers. These are internal-compression Superchargers which means they create the boost (compress the air) inside the head unit before forcing it into the engine’s intake. Their impellers are spun by an external pulley that is typically driven from the accessory belt. This allows you to change the pulley size and control the speed at which the Superchargers impellers spin. By changing to a smaller pulley, you’re essentially adding more boost. One advantage of this style blower is that because it has a limited number of moving parts, they’re typically more reliable than other units. They also produce less heat than other style Superchargers and are capable of producing more boost. The one disadvantage is that it must spin at a high rate of speed to make a significant amount of boost, and therefore only makes power at higher RPM’s. It’s common to start seeing boost at 3000 RPM’s and to have it increase along with the RPM’s. These are great options for non-street cars who don’t typically care about low RPM’s. That’s not to say they aren’t a great option for street driven vehicles as well.
Both the Roots Style and Twin Screw blowers are External Compression Superchargers. Both are also positive displacement Superchargers meaning that it moves a fixed volume of air per rotation. The Roots Style has two counter-rotating lobes that trap incoming air and move it around the outside casing of the lobes before releasing it out the bottom through an outlet port. These blowers are very capable of making large amounts of boost at lower RPM’s and make incredible low to mid range horsepower. The major disadvantage of this style blower is that they create a lot of heat. An intercooler is almost always necessary on a roots setup.
The Twin Screw Supercharger is very similar in appearance to the Roots Style blower. One major difference in their design is that the Twin Screw style has two rotors (screws) that rotate towards each other. The two screws mesh together and draw in air to compress it and force it out into the engine. Due to the fact that the compression occurs inside the Supercharger, it produces far less heat than a roots blower (and not much less than a centrifugal). The tolerances between the two screws are very tight which results in an ability to create boost at a low RPM – much more so than the Roots or Centrifugal. Another advantage this has over the Roots style is that the two screws don’t actually touch each other resulting in virtually no wearing parts to replace. The main disadvantage of this style is that because of it’s internal compression ration, it’s compressing air even when it’s not sending boost into the engine. They do have internal bypass valves to release the pressure but due to the fact that it take energy to create that pressure in the first place, it’s drawing more power from the engine while not under boost in comparison to the other blowers.