A positive displacement supercharger is a type of forced-induction device that pushes a fixed volume of air into the engine with every revolution. Unlike centrifugal superchargers, which spin air faster and faster to build pressure, a positive displacement design physically traps and moves air in discrete gulps. This fundamental difference gives it a distinctive power delivery: strong, nearly instant boost from low RPM rather than a gradual build as engine speed climbs.
How It Works
Every internal combustion engine is essentially an air pump. More air (combined with more fuel) means a bigger combustion event and more power. A positive displacement supercharger forces extra air into the engine by mechanically capturing a set amount of air in its housing and pushing it into the intake manifold on each rotation.
The supercharger is belt-driven off the engine’s crankshaft, so it spins in direct proportion to engine speed. Because it displaces a fixed volume of air per revolution, it always moves more air than the engine can naturally consume. This creates positive pressure (boost) in the intake manifold starting almost immediately, even at low RPM. The result is a boost curve that stays relatively flat across the entire rev range rather than climbing exponentially with speed.
That flat boost curve is the defining trait. A centrifugal supercharger produces pressure proportional to the square of its RPM, so boost falls off rapidly at lower speeds. A positive displacement unit keeps delivering extra air even when you’re lugging the engine at low revs, which translates to strong pulling power in everyday driving, towing, or launching from a stop.
Roots vs. Twin-Screw Designs
There are two main types of positive displacement superchargers, and while they look similar from the outside, they compress air in fundamentally different ways.
Roots Superchargers
The Roots design is the oldest supercharger concept, recognizable as the bulky rectangular housing that sits on top of a V8’s intake manifold. Inside, two lobed rotors spin in opposite directions, catching air between themselves and the housing walls and pushing it downward into the manifold. The key detail: Roots blowers don’t actually compress the air inside the supercharger itself. They work as air pumps, shoving air into the manifold where it gets compressed by the backpressure of more air being pushed in behind it. This is less thermally efficient, because the compression happens through turbulence and pressure buildup rather than a controlled squeeze. Some modern Roots blowers use lobe seals to reduce air leakage and improve efficiency, but those seals add friction and further increase intake air temperatures.
Twin-Screw Superchargers
Twin-screw superchargers use two helical, grooved rotors that mesh together like worm gears. As they rotate, air gets trapped between the rotor threads and progressively squeezed into a smaller space as it moves from one end of the housing to the other. The air is compressed internally, before it ever reaches the intake manifold. This internal compression makes twin-screw units more thermally efficient than Roots blowers, meaning the air they deliver is cooler and denser for a given boost level. Cooler air is denser air, which means more oxygen per cylinder fill and more power potential.
Power Delivery and Driving Feel
The practical payoff of a positive displacement supercharger is throttle response. Because boost is available almost from idle, you get a surge of torque the moment you press the accelerator. There’s no waiting for the supercharger to spool up or reach a certain RPM threshold. This makes the engine feel larger than it is, with strong low-end and midrange pull that’s especially noticeable in stop-and-go traffic, highway passing, and situations where you need power right now.
The trade-off is at the top of the rev range. Positive displacement superchargers are physically limited by their displacement volume, so while they provide excellent low-end and midrange performance, they can run out of breath at very high RPM. A centrifugal supercharger, by contrast, keeps building boost as RPM climbs, so it suits engines designed to make peak power at high revs. In short: a positive displacement supercharger builds a low-RPM torque monster, while a centrifugal unit produces a more linear power curve that keeps pulling harder the further you rev.
Heat and Parasitic Loss
Because a positive displacement supercharger is bolted directly to the engine and driven by the crankshaft, the engine has to spend some of its own power just to spin it. This is called parasitic loss, and it’s a real cost. Across various supercharger types, parasitic losses can range from roughly 50 horsepower at moderate boost levels to well over 100 horsepower at higher boost. The net power gain is still substantial (the extra air allows the engine to produce far more power than the supercharger consumes), but the engine is always working harder than it would naturally aspirated, even at partial throttle when you don’t need the extra boost.
Heat is the other concern. Compressing air raises its temperature, and positive displacement superchargers, particularly Roots types, can generate significant heat because they sit directly on top of the engine. Extended hard driving can cause heat soak, where the supercharger and intake charge temperatures climb high enough to reduce power output. Intercoolers help, and many modern kits include them as standard. Running high-ethanol fuels also helps because ethanol absorbs more heat as it evaporates in the combustion chamber, keeping intake temperatures in check.
Installation and Packaging
One practical advantage of positive displacement superchargers is how they fit under the hood. On V6 and V8 engines, the supercharger typically nestles into the valley between the cylinder banks, replacing or sitting atop the stock intake manifold. This compact packaging means it often fits within the original hood line without major modifications. Centrifugal superchargers mount to the front of the engine like an alternator or power steering pump, which can be easier in some setups but tighter in others.
The belt-drive system is mechanically simple: a pulley on the crankshaft, a belt, and a pulley on the supercharger. Changing the pulley ratio changes how fast the supercharger spins relative to the engine, which is the primary way to adjust boost levels. Smaller supercharger pulleys spin the unit faster and produce more boost, while larger ones reduce it. This simplicity makes positive displacement setups relatively straightforward to tune compared to variable-geometry turbocharger systems.
Common Applications
Positive displacement superchargers dominate in applications where low-RPM torque matters more than peak horsepower. Muscle cars, trucks, off-road vehicles, and tow rigs all benefit from the immediate throttle response and flat torque curve. Factory examples include the supercharged Dodge Hellcat and Demon engines (twin-screw type) and several GM and Ford performance variants that use Roots-style Eaton blowers from the factory.
In motorsport, they’re most popular in drag racing and street-performance builds where launch traction and mid-range acceleration are priorities. Road-course and high-RPM racing applications tend to favor centrifugal superchargers or turbochargers, which breathe better at sustained high engine speeds and generate less heat over long stints.