Supercharger Snapshot

Rob’s Blog

Their is no doubt that supercharging is back in fashion with manufacturers like Yella Terra, Walkinshaw,  Harrop and CAPA  not only producing kits to suit our local vehicles but they have made them more affordable and more effective by utilising the latest supercharger technology.  A common question we get asked nearly every time we talk to a customer who is looking to supercharge their car, is what options do I have and what are the differences between them? I thought it might be worthwhile giving you a brief snapshot of the different types of superchargers on the market.

First some history… In the late 1800s a German engineer named Gottlieb Daimler registered a patent for a pump to aid in the delivery of increased amounts of air and fuel to the cylinder and although he didn’t call it a “supercharger” in his patent application that’s what he was describing – We believe this to be birth of the automotive supercharger as we know it.

Superchargers also took to the air as World War I with military engineers looking for ways to make more powerful airplanes. Meanwhile, back in Germany, Mercedes was hard at work and by 1921 they released the first production supercharged vehicle utilizing a roots-type supercharger.

By the mid 1930′s Robert Paxton McCulloch started McCulloch Engineering Company and began manufacturing superchargers and by 1950, McCulloch had formed Paxton Engineering which took over the supercharger development and took on the task of creating an inexpensive supercharger fit for use by the general public. After $700,000US(allot of money in those days) in research and two years of testing, the VS57 supercharger was ready for the public in 1953 and by 1954 kits were made for nearly every commercially available US made 6 and 8 cylinder engine.

The rest is history, as Paxton developed newer and better superchargers until they became a part of life, not only in the world of racing, but also in the street-legal aftermarket world. Today it’s hard to keep track of all the supercharger brands and models, but I guess that is good news for the consumer as the choices are wide spread.

So there are three main types of superchargers – Roots, Twin Screw and Centrifugal. Each has its own set of advantages and disadvantages so let’s look at them in more detail…

First let’s look at the roots supercharger which was originally designed as an air moving device for industrial buildings. The roots supercharger features two counter-rotating lobes that trap air from the intake side of the supercharger and move it around the outside casing of the lobes and out the bottom of the supercharger through an outlet.

It is a “positive displacement” meaning that it moves a fixed volume of air per rotation and cannot flow backwards like a centrifugal supercharger. It has the ability to pump air at low rpm as it does at high rpm, which makes it very capable of making large amounts of boost even when the engine rpm’s are very low. This makes it great for people looking for low-end and mid-range power.

The main disadvantage to the roots supercharger is that it creates a lot of heat and it is difficult to intercool effectively. The roots supercharger does not compress air; it only moves it from the intake port to the discharge port so all of the compression is done in the intake manifold where laws of thermodynamics kick in and heat is generated.

Next is the Twin screw supercharger, at first glance it appears to look similar to a roots supercharger but there are significant differences. The twin-screw supercharger has two rotors that rotate towards each other and draw air from the back of the supercharger, the twisting rotors then move the air to the front of the supercharger while compressing the air, before discharging through a port at or near the front of the supercharger. Because the compression is done inside the supercharger, this design produces less heat than a roots supercharger and is almost as thermally efficient as a centrifugal design.

The twin-screw is also fixed displacement supercharger that pumps a fixed volume of air per revolution and because the tolerances between the rotating screws are very tight, its ability to create boost at low rpm’s is unparalleled. Unlike a roots supercharger, the rotors in a twin screw supercharger do not actually touch, so there are virtually no wearing parts making it very reliable, they are also self lubricated and do not tap into the engine’s oil supply.

Again there are a few disadvantages with the twin screw design, it is also hard to intercool effectively and because it has an internal compression ratio, the twin screw is compressing air even when it is not sending boost to the engine, so a bypass valve is required to release the pressurized air under cruising or deceleration. Because it takes work to pressurize the air in the first place the twin screw supercharger does draw more power from the engine even while it’s not under boost and like a roots the throttle body must be placed before the compressor. The Screw style supercharger would definitely be my choice for those people looking for bottom end and mid range power due to its improve thermal efficiencies over a roots style supercharger.

The centrifugal supercharger is not a positive displacement supercharger because it does not move a fixed volume of air per revolution. The centrifugal supercharger essentially operates like a high speed fan and it has several advantageous that make it popular in the aftermarket world. It produces very little heat because of its internal compression ratio and is small in size and very versatile because it can “free-wheel” and allow the engine to suck air through it or even flow air backwards. For this reason, unlike the roots and screw superchargers it can be placed anywhere in the intake tract and can even be used to “blow through” the throttle body, meaning it can be mounted nearly anywhere. It is also the most thermally efficient supercharger producing the lowest discharge temperatures.

The main disadvantage of the centrifugal supercharger is that it must be spinning at a relatively high speed before it begins to make a significant amount of boost. For this reason, it does not typically create boost and power at low engine rpm’s and normally only begins to create boost at around 3000 rpm with the boost curve gradually and increasing with engine RPM.

It is worth remembering that all the superchargers have to integrate with other critical engine systems like the ignition system and the fuel delivery system and these items need to be considered when fitting any supercharger to your car, most of the supercharger kits sold today are complete supercharger systems which makes things easier, however I would always recommend custom tuning you engine management to suit the new supercharger setup.

Power to the People!

Rob Vickery