Supercharger technology has progressed in leaps and bounds over the past decade. Depending on what your application is and what sort of driving characteristics you're chasing, there's almost certainly a blower to suit your car's engine, whether as a ready-to-go bolt-on engineered kit, or as a one-off installation.
Positive Displacement vs Centrifugal Blowers
Positive displacement blowers have been around for many years. They work on the principle of trapping a specific volume of air between their rotor and housing, compressing it (through lobe rotation) and forcing it through the engine. The supercharger displaces the same volume of air per rotation, regardless of rpm (give or take a bit). It helps to think of a positive displacement blower similar to a bicycle pump; for each identical pump stroke - or blower rotation - you get a fixed volume of air from the outlet.
The main advantage of a positive displacement blower is its ability to force a heap of air through the engine even at low rpm, giving good boost straight off idle. This helps give small engines a big engine feel and good throttle response. On the other hand, positive displacement blowers create hotter charge air temperatures and, through their boost-from-everywhere delivery characteristics, place the remainder of the car's driveline under considerable stress.
Centrifugal blowers are something of 'the new kid on the block', despite the basic technology being around for eons. Centrifugal blowers vary from their traditional counterparts by using an impeller wheel (within a housing) to draw air in from a central inducer and force it out of an axial discharge nozzle. The volume of air pushed out the nozzle is not fixed per rotation - it's related to speed and the technical design of the impeller blades. Think of a centrifugal blower like a household fan - spin one slowly and not much air comes out. Spin it fast a helluva lot of air flows.
The attraction of centrifugal blowers is they're relatively easy to fit in most engine bays, they're relatively easy on driveline components (they don't deliver bulk torque at low revs) and - most of all - they're docile. A centrifugal blower progressively builds boost - and therefore torque - so maximum driver control can be maintained. The harder you rev the engine, the more boost and the faster it wants to go - down low, though, don't expect huge things.
Positive Displacement Blowers
Positive displacement superchargers fall into two categories - Roots and screw-type (aka Lysholm).
Roots type blowers force air into an engine using two counter-rotating rotors. In modern Roots blowers, the rotors are helically formed to help reduce pulsing from outlet. Even so, Roots blowers are relatively inefficient, producing the most charge air heat for a given amount of boost pressure. Their attraction is their ability to produce boost pressure from idle rpm, turning any engine into an instant torque monster.
There are a number of manufacturers of Roots blowers, but one of the most well known brands - Eaton - is used on a number of production engines from Ford, Buick, Holden, Mercedes and Jaguar. Being able to meet Original Equipment manufacturer's durability demands speaks volumes about performance and reliability.
Japanese Import Blowers
If you're after a genuine supercharging bargain, the second-hand Roots-type blowers coming in through Japanese importers are hard to ignore.
The most common variety are those hauled off Japanese market Toyota 4A-GZE and 1G-GZE motors. Both of these lil' superchargers contain straight aluminium rotors coated with what appears to be Teflon, plus an electromagnetic operated clutch system similar to that found on an air-con compressor. Lubrication is via a stand-alone oil system (ie separate to the engine). The blower unit off the 121kW 1.6-litre 4A-GZE apparently displaces 1200cc per revolution, and that off the 125kW 2.0-litre 1G-GZE sweeps a larger 1420cc.
While these Toyota blowers are relatively common, there are other units to be unearthed also...
The 660cc Subaru EN07 engine packs a truly tiny positive displacement blower, as does the 930cc Nissan MA09 turbo/supercharged engine found in the Nissan March. We have also seen a blower unit similar to those on the Toyota models stripped from a Japanese Mitsubishi V6.
Twin-screw (aka Lysholm) superchargers operate similarly to a Roots unit, but their rotors and drive system is more complex. Twin-screw rotors have a twisted lobe profile and each one rotates at different speed. Induction air is compressed as it travels along the length of the rotors, giving less charge-air temperature increase than a similar Roots design. These are probably the most desirable of all blower types.
Some of the more high-profile twin-screw supercharger manufacturers are Sprintex, Opcon Autorotor and Whipple.
Centrifugal blowers have become extremely popular in the last decade or so. Centris are best imagined as a belt-driven turbocharger - they have a compressor wheel and cover, but forget about the turbocharger's exhaust side.
The fundamental difference between a positive displacement blower and a centrifugal unit is the quantity of air that they deliver per revolution. A displacement blower forces the same volume of air into the engine per crankshaft revolution, while the centrifugal cousins deliver airflow that rises as the square of their rotational speed (blower speed, not engine speed). Step-up gears within the blower make the compressor spin faster than the engine, while the external drive pulleys are geared so that the maximum desired boost level is attained at redline engine rpm.
A centrifugal blower doesn't give the neck-snapping instant torque that you can achieve using a positive displacement blower. Instead, the blower progressively builds boost pressure, and - therefore - torque. This means a given a car won't feel as grunty, but - ultimately - it might make similar top-end power as a displacement blower.
Note that some cheaper centrifugal blowers contain a simple flat bladed compressor wheel, but the more efficient designs use a complex curved blade profile to attain maximum airflow with the least possible 'working' of the air that leads to a charge-air temperature increase.
Turbo vs Supercharging
The energy to drive a supercharger - regardless what type it is - comes directly from the crankshaft. There's a greater power loss involved in driving a supercharger compared to a turbocharger - this is despite the significant backpressure induced by an exhaust mounted turbine.
On the positive side, the supercharger compressor is much more responsive than a turbocharger's. Instead of waiting for an exhaust pulse to arrive from the engine, the blower increases speed in direct relationship to engine rpm. This makes the belt driven blower generally more linear and better-feeling to drive than a turbo.
What's Best For You?
With all these choices it may seem like a toss-up decision to go one way or the other.
Your decision should be based on your application and the sort of torque delivery you're chasing. As mentioned, a positive displacement blower offers strong boost pressure from idle, while a centrifugal blower builds boost as revs rise. If your car has good traction and the driveline can handle the extra grunt, a positive displacement blower is probably the best approach for a snappy streetcar. With more balls down low, there is a fuel consumption penalty over a centrifugal blower, but the driving experience is much more rewarding.
On the other hand, centrifugal blowers certainly have their place. They're more driver-friendly in wet weather conditions as they don't really begin to haul until there's a few revs on board, installation is often neater and they offer arguably the best value for money. Making things easier are various off-the-shelf centrifugal blower kits designed to suit many popular performance (and non-performance!) cars. These kits typically include the blower unit, a heavy duty mounting arrangement, drive belt with idler and tensioner, a management system tweak, revised air intake and, often, an oil cooling system for the blower. There's all you need in one tight little bundle.
Intake air temperatures are a major concern when talking forced induction. If you're going the route of a cheaper, paddle style centrifugal blower (which produces high charge-air temperatures), it's almost mandatory to have an intercooler or water injection system installed. It adds significant cost to the overall price of a blower install, but - without one - you're not realising maximum power, it'll be damn difficult to release further power and you're walking the fine line of engine detonation.
Factory Blower Upgrades
Like the Jaguar XJ-R, Holden Australia's supercharged 3.8-litre V6 uses an Eaton positive displacement supercharger - a M90 model, delivering 0.5 Bar boost pressure.
Due to the lack of an intercooler, Holden has programmed the engine management to massively retard ignition timing with rising intake temperature. This helps to avoid detonation, but it's no good for making power. In this graph you can see the effect of just an ECU remap.
Yella Terra also has a kit for the supercharged V6, which replaces the front snout of the blower and the top pulley. This delivers more boost and, together with an exchange Memcal, gives up to a claimed 50 percent increase in torque.