Winds of Change
Different Types of Forced Induction Explained
Different types of forced
There was a time when adding forced induction to your engine meant a supercharger and nothing else. Even then, it wasn’t a purposebuilt, high-performance unit, but a re-purposed scavenging pump borrowed from a GMC two-stroke diesel. Those days are now a half-century in the rearview mirror. Today, when it comes to boost, you have options. Below are the most popular methods to add unnatural aspiration to your engine of choice as well as some of their unique benefits.
The oldest form of forced induction, superchargers (also known as blowers) have come a long way in terms of design, efficiency, and ease of installation. Supercharger is a fairly generic term that encompasses many different sub-genres of boost builders. The unifying characteristic between all variations of superchargers–or at least what the word has evolved to mean–is a mechanically driven compressor.
This is accomplished via a drive-belt and system of pulleys 90 percent of the time, but gears and chain drives have been used historically. Below are the most common styles of superchargers in production.
• Roots Type
Easily the oldest and most traditional style of supercharger, roots-type setups are light-years ahead of their ancestors. Their basic architecture consists of an aluminum housing, a case, internal drive gears, and two aluminum rotors. This style is a positive-displacement supercharger, meaning they move a fixed amount of air per revolution, regardless of how fast they are spun.
Think of them like a wheelbarrow that can carry a specific number of bricks per trip. It doesn’t matter how fast you push it, each trip it carries the same number of bricks. Keeping with the masonry analogy, as the supercharger spins, it grabs bricks of air from the atmosphere (the size of which is dictated by the supercharger’s displacement) and stacks them in the intake manifold. Eventually, a point is reached where there is a higher pressure of air in the intake manifold than atmospheric conditions. This is known as boost. The amount of boost can then be influenced by spinning the blower faster or slower (more or less revolutions), which equals more bricks of air in the intake.
One of the biggest benefits of roots blowers is that they tend to have comparably large displacements and can make instant boost at very low rpm. This makes them excellent
at providing immediate throttle response, exceptional torque, and a wide powerband. They do, however, produce a considerable amount of heat in the incoming air charge, making higher-octane fuel and lowered engine compression ratio nearly mandatory.
• Twin Screw
Twin-screw superchargers, which date back to the ’30s, use two rotors spun by the engine to move a set volume of air per revolution. If that sounds a lot like a roots blower, that’s because it is. However, unlike a roots blower, which has mirror image rotors, a twin-screw design (also called a Lysholm supercharger) utilizes one male rotor and one female rotor.
Much like a catch-and-release fisherman, a roots blower snags air, reels it into the blower housing, and releases it into the intake manifold. In comparison, a twin-screw blower pulls in air, compresses it internally, and expels the already-compressed air into the intake manifold. This is possible because the lobes of a twin-screw blower are slightly tapered and the volume between them shrinks from back-to-front as they rotate.
This “internal pressure ratio” helps the twin-screw blower more efficiently pressurize the intake, improves thermal efficiency, and makes for a very power-dense supercharger for a given displacement.
Modern twin-screw blowers also contain an internal bypass valve (as
do some modern roots blowers), which can open under low-load conditions, allowing air to bypass the supercharger assembly and keep the engine out of boost when it’s not desired. This is extremely beneficial for daily driven vehicles that still need acceptable fuel economy.
Often confused with turbochargers, centrifugal superchargers are anything but. The basic anatomy of a centrifugal supercharger includes housing, an impeller, and a system of internal drive gears. This style of blower is not a positive displacement style, meaning it requires rpm, and a lot of it, to produce boost. A common misconception is that an engine needs to be revved extremely high to make power with this type of blower. While that might have been true in very early setups, today’s centrifugal superchargers contain internal transmissions, which enable
them to spin at significant multiples of engine rpm.
It’s not uncommon for a centrifugal supercharger to have an internal transmission ratio of 4.00:1 or larger. For example: if the engine were spinning at 4,000 rpm, the supercharger would be clocking at 16,000 rpm. And, while that may sound excessive, modern centrifugal superchargers are made with the correct materials and engineering finesse to tolerate such speeds.
While this style of blower cannot deliver the immediate boost of a much larger, positive displacement setup, its strong suit is extreme efficiency. Centrifugal blowers require very minimal engine power to drive them and also introduce minimal heat into the air charge. They are also more modular and adaptable and can be designed
to snake around obstacles in an engine bay. Furthermore, they can be plumbed into an air-to-air-intercooler, a high-flow finned heat exchanger, which further cools the intake charge; an impossibility with a roots blower.
Centrifugal blowers have enjoyed tremendous endorsement in the modern drag-radial racing scene and with late-model muscle cars.
With mainstream using dating back to World War II, turbochargers are certainly not new to the party. However, today’s declassified units are more compact, durable, and affordable than ever before. The main difference between a turbo and supercharger is where they derive rotational energy from. Unlike the aforementioned superchargers, which all rely on a mechanical link to the engine to spin them, a turbocharger recycles lost energy from the engine’s exhaust stream. As exhaust exits the engine it’s funneled into the turbocharger’s turbine wheel, spinning it at revolutions that exceed 100,000 rpm. The turbine wheel, via a common shaft spins a compressor wheel, which compresses air and directs it into the engine.
Because it uses exhaust energy that is otherwise wasted, turbochargers are exceptionally efficient at making horsepower. Per unit of boost pressure, they can usually outperform a supercharger but at the trade-off of a far more complex installation.
A turbocharged setup requires significantly more piping and requires a sub-system to control boost pressure. This consists of a wastegate and blow-off valve. The wastegate is plumbed inline with the exhaust that feeds the turbo. In essence, it’s
a diverter valve that, via an internal plunger and spring, can vent exhaust around the turbo at specific boost levels. When the wastegate opens, the pressure spinning the turbo lessens and the turbo begins to spin slower, regulating boost.
On the outlet of the turbo lives the blow-off valve. Because the turbo is driven off exhaust pressure and not directly connected to the engine, it is less directly reactive to throttle inputs. For example, when the driver lets off the throttle, there’s a momentary period where the turbo produces unwanted boost. As the throttle shuts, this boost has nowhere to go and poses a risk of damage to the fragile compressor wheel. To counteract this, the blow-off valve opens and vents this extra boost to the atmosphere.
The last, must-mention aspect of turbochargers is the concept of “lag.” Because turbos are driven off of exhaust system pressure, they require a sufficient volume of exhaust flow to spool/spin them to the point that they begin to produce boost. The lull between no boost and boost is dubbed lag. While a turbocharger will never be as responsive as a mechanically driven supercharger, it’s important to note that a properly sized turbo can still be very responsive.
■ A supercharger can make all the statement in the world or nearly none at all.This TorqStorm supercharger on Classic Trucks Editor Ryan Manson’s 265ci Chevy is hiding in plain sight. Beneath the stealthy air cleaner beats a modern 383ci heart with fuel injection and, of course, boost.
■ The most traditional blower is the roots style. This Weiand 8-71 requires a dedicated intake manifold but otherwise is an easy installation into most hot rods with a few brackets and external belts.
■ Roots blowers are often the most costeffective solutions, especially for carbureted engines. This Weiand 142 blower has been around for decades and is still an effective boost maker–especially on a traditional smallblock Chevy.
■ Modern roots blowers (left) utilize lobes with a slight twist (helix) twin-screw blowers have one female rotor and one male rotor. They also taper so that the air is compressed as it travels, front to back, along the rotor face.
Centrifugal superchargers, such as these from Vortech and ProCharger use a geardriven impeller to rapidly accelerate air into the intake manifold creating boost. They are very thermally efficient and extremely universal, with only a set of unique brackets and piping needed for each engine application.
■ Turbochargers can typically make the most power per unit of boost because they do not require any drive energy from the engine. Instead, they scavenge lost energy from the exhaust system.
■ This ProCharger cutaway shows the internal transmission, which is needed to multiply engine rpm to the high-rpm centrifugal superchargers required to make boost.
■ An air-to-air intercooler such as this helps pressurized air “cool down” on its way into the engine. Cooler air is denser and contains more oxygen per unit, meaning more fuel can be added to maximize horsepower.
■ A turbocharger uses exhaust pressure to spin a turbine, which, via a common shaft, spins a compressor wheel. The compressor wheel pipes pressurized air into the intake manifold creating boost.