Superchargers are driven continuously while the motor runs, like a fan or air conditioner unit, and they run at a fixed ratio to the engine rpm, delivering the same curve of boost regardless of the engine's natural aspiration, so they normally impose their own characteristics on the vehicle's breathability. But some of them can deliver instant boost practically from idle, which is held to be the great differentiator between the turbo and the supercharger..Superchargers give a much more Linear power curve, but is hte potential for big hp the same?...well, No in a word!
Here's the lowdown on the different types of supercharger:
There are essentially two ddifferent types of supercharger and the first, a centrifugal supercharger, is an air pump driven directly from the crank, injecting air it has ALREADY compressed. It delivers exponentially increasing boost as rpm increases.
Some like to think of the centrifugal supercharger and the turbocharger as the same thing except with the compressor wheel being powered by different means. The centrifugal does experience lag at the lower end of the rev range as with a turbo
Centrifugal superchargers aim for max boost at the high end of the rev range. A centrifugal supercharger is basically the same type of pump as the compressor side of a turbocharger, but it is mechanically driven off the crankshaft instead of by exhaust gas as with a turbocharger. Internal gearing then steps up the spin speed of the compressor wheel, and gives a distinctive whistling sound, that most drivers enjoy hearing, but the centri SC is basicallyt pretty damn noisy for a daily driver.
The centrifugal's type of compressor - like the turbo's - is efficient only at high rpm. Because all superchargers by definition are under continual drive from the crank (i.e. with no comparable turbocharger wastegate system to cut the drive), design decisions set the limits to how much internally to gear-up (or overdrive) the spooling:
The centrifugal aims for high-end boost and in consequence gives almost no boost at low rpm.
Although centrifugal superchargers build boost relative to engine rpm: boost builds exponentially in the upper half of the powerband - generally they start boosting around 2000 to 3000 rpm, and attain full boost at max rpm - i.e. the quoted maximum boost of a centrifugal happens at redline.
Enthusiasts say that in situations where they demand performance from their vehicle, they spend most time in the mid to high rpm ranges.
Best applications of centrifugals
The centrifugal is good for lightweight, manual transmission vehicles (you can downshift for a higher rpm) , and is popular with muscle cars that have plenty of low-end torque already in NA form (naturally aspirated), but they're not so good for heavier vehicles under load or towing, or automatic transmissions - in these cases a positive displacement supercharger is much more suitable.
This positive displacement aspect brigns us on to the......
Roots Blower - essentially a positive displacement air mover
The roots-type supercharger is a positive displacement system referred to as a blower since it doesn't compress air directly, but simply moves it quickly. It employs 2 or 3 counter-rotating lobes to scoop a fixed amount of air from the outside and deliver it into the intake manifold. Unlike the compressors of the turbocharger and the centrifugal supercharger, which deliver exponentially greater boost with increased rpm and which are not designed to perform at lower rpm, a positive displacement supercharger pumps a fixed amount of air for every revolution, and delivers instant boost even at low rpm.
Since positive displacement superchargers are both continually under drive and also delivering boost at all rpm an intake bypass system is used in street applications to divert airflow and prevent overboosting.
The roots type supercharger is based on the very efficient air moving principles of positive displacement, but loses supercharging efficiency for several reasons, and primarily because it doesn't compress the air directly.
High heat from turbulence
The blower delivers more and more air rapidly into the intake manifold, where it compresses from the addition of increasingly more air. The turbulence of uncompressed air entering the compressed environment - with associated backflows - creates heat, the great negative factor of supercharging.
The adiabatic efficiency of a roots type supercharger is only in the 40%-60% range. Furthermore, while the other compressors tend to gain efficiencies at higher rpm, the roots tends to experience decreased efficiencies as boost levels rise or for sustained maximum output periods.
A positive displacement supercharger has great advantages in lower end delivery - more power at lower rpm equals less strain on the engine.
The roots type blower is inherently noisy, largely from the gearing required to run the rotors. It is a very simple supercharger, and moves a great volume of air very easily. The roots blower has always been very popular, but is now yielding some ground to the twin screw.
The history of the Roots blower extends from 1859, predating supercharger history - positive displacement is an air pumping invention