Forget turbos, as we investigate the other side of forced induction: supercharging with our Fast Car Superchargers Guide…
There are plenty of people out there who argue that superchargers are better than turbos. The truth is, there are positives and negatives for both camps, but luckily, the basics are pretty similar.
In some instances, all that separates a turbo from a supercharger is how it’s driven. Both force in air to increase density, but a supercharger does it with a crank-driven belt rather than spent exhaust gas. The result is that you get instant power, rather than lag while you wait for the turbo to spin up.
But there’s more than one type of supercharger – we’ve just described the centrifugal sort. A positive displacement (PD) supercharger produces a similar yet contrasting engine character. The difference is that the ‘turbo’ type favours high revs.
However, technological development moves quickly, and the PD type is quickly catching up. Suffice to say, there’s more than one way to skin the supercharger cat, so our next job is to demystify this technology…
ROOTS SUPERCHARGERS: HISTORY
We’ve all heard of an Eaton – the supercharger type most widely used by current motor manufacturers – but this is actually a Roots blower. The principle was invented in the mid-19th century by the Roots brothers, and was originally intended for use as a water mill. It proved more successful at pumping air into a furnace, hence the name ‘blower.’ Roots-type superchargers typically consist of two meshing rotors, which grab air and shift it quickly from the intake side to the exit. Early blowers carried dead-straight rotors with just two inter-meshing lobes – they boosted, but the problem was that they pulsed, too.
The big break came with the infamous GM 71-series, which was designed in the 1930s and introduced on the 6-71 diesel engine. This is the sort you’ll see sitting on top of Top Fuel dragsters. They’d thought about it by now and this series had three-lobe, twisted rotors, which counteracted the pulsing effect and so produced smooth, continuous boost. The range varies according to the amount of air displaced, from 2-71 to 14-71. They’re American, so 71 refers to the engine size in cubic inches (CI) and the figure before, 6, means it’s suitable for six cylinders each, totalling 71 CI (1.16 litres) – resulting in 426 CI (6.7 litres). However, you’re not limited to fitting them only to six-cylinder engines – it’s just that the total capacity should be around that sum.
There are plenty of Roots variations around. The most common in Europe is the aforementioned Eaton, which is found on the Mini Cooper S and on supercharged Jaguars and Mercedes’. Again, loads of development work has moved the principle on – they now have Teflon-coated rotors and Eaton have recently announced their TVS (Twin Vortices Series) line, too. Like the GMC superchargers, the different models in the Eaton range are determined by the amount of air they displace. The most common is the M42 in the Mini Cooper S, which shifts 42 CI of air per revolution. Supercharged Jaguars and Mercedes are typically fitted with M62s (suitable for engines of 2.5ltr–4ltr) or M90s (3ltr–5ltr). There’s also an M112 Eaton. M-series Eatons have three-lobe rotors and 60 degrees of end-to-end twist. The newer TVS range has fourlobe rotors with 160 degrees of end-to end twist. Why do you need that? Read on below…
ROOTS: THE LYSHOLM SCREW
Add more twist, closer tolerances and mismatch the rotors and you get the screw supercharger, which is designed so that the air is directed down the centre with a compressing effect. This principle was patented by Svenska Rotor Maskiner AB (SRM) of Sweden in the 1930s, although the name comes from their chief engineer, a Mr Lysholm. This Lysholm Screw is still a Roots-type blower, but with greatly increased efficiency. A good Roots is about 50 percent efficient, whereas a Lysholm whacks it up to 80–85 percent.
In addition, because they have no parts in contact with each other, Lysholms don’t wear out. This type is commonly used to great effect in Top Alcohol drag racing, although Top Fuel regulations still insist on traditional Roots blowers – just think what the effect would be if they didn’t! There is a range of superchargers available from Lysholm but a more common make, especially in the USA, is Whipple.
Kenne Bell is a name associated with Whipple, and they’re famous for their bolt-on supercharger kits that greatly increase the performance of 5.0ltr Mustangs. Lysholms are so efficient that you can see why other manufacturers have introduced improvements to their range in order to compete.
ROOTS: HOW THEY WORK
Superchargers are known as air pumps and a Roots simply shifts air quickly from one side to the other – it has no compressing action and is known as a positive displacement supercharger because it moves a fixed amount of air. We’ll see the difference when we come to discuss centrifugal blowers later. Traditionally, this type of supercharger is relatively inefficient – the air is directed around the outside of the rotors against the case walls.
You can see that early Roots with just two-lobe straight rotors aren’t going to shift much – it’s pretty obvious, then, that increasing the rotor count in the same space shifts the air quicker for the same work done. Hence the modern developments in units such as Eaton’s. But, there’s a better principle…
CENTRIFUGAL SUPERCHARGERS: HOW THEY WORK
A centrifugal supercharger is similar to a turbo – the only difference is the way it’s driven and, of course, the need to control a turbo via a wastegate. A centrifugal supercharger produces boost differently to a Roots supercharger – it actually compresses the charge by centrifugal force and, as such, creates more boost in greater proportion to the amount of revs.
We’ve already discussed at length how a turbo produces boost in our previous article, so we don’t need to cover old ground here. But the main point to be aware of is the difference between this type and the Roots, as well as its suitability for different driving conditions. The truth is, centrifugal superchargers need revs before they produce boost. As they basically consist of a fan, they’re simpler than Roots blowers, although they’re boosted by gears inside, which means the fan reaches supersonic speeds.
However, the blades have trim, which encourages centrifugal action and compression against the housing walls. But you need to get the thing going before the centrifugal action can take place, so they are more suited to higher-revving cars. Because there isn’t much boost created low down in the rev range, centrifugal superchargers are perfect for smaller displacement and virtually standard compression engines – plenty of kits are available where the compression ratio can still be quite high.
This is because the supercharger will counteract the effect of low vacuum at higher revs associated with naturally aspirated engines and thus force in a denser charge. The bigger the blower, though, the more boost it will produce and you will need to take the same steps of dropping compression that you would with a turbo.
In contrast, a Roots blower makes instant horsepower and torque – phenomenal torque, in fact – making it ideal for drag racing where you need as much torque as possible to quickly get off the line. But Roots blowers tend to operate in a narrower band – their construction is such that there’s a lot of moving parts and mass to spin up.
Therefore, they need power to drive them. They can sap as much as a third of the engine’s power, although the net gain is vastly superior to that of the traditional methods of N/A tuning. By comparison, centrifugal superchargers are less power-sapping, although this is still a major factor in the turbo vs supercharger debate.
CENTRIFUGAL SUPERCHARGERS: HISTORY
The development of all types of supercharger happened at roughly the same time – Europe mostly used the Roots type, since it’s ideal for road use and power in the low-to-mid range, which suits the sort of racing we favour. For example, Blower Bentleys used front mounted Roots blowers and so did the infamous Mercedes SS and SSK. In contrast, the centrifugal supercharger is better suited to high-revving endurance events such as the Indianapolis 500, so it was mostly used in the States and featured on cars such as Auburns, Cords and Deusenbergs.
But there was plenty of crossover, too. In 1902, for example, Louis Renault patented a centrifugal supercharging system that blew through a carburettor. Many other examples exist, but in the 1930s, Robert Paxton McCulloch was one of the first to offer a bolt-on centrifugal blower in America. His name is significant, because Paxton and McCulloch are both modern-day aftermarket supercharger brands.
You’re also likely to see Vortech, ProCharger and, more recently, Rotrex, which is taking the supercharger world by storm, as fitted to cars like the S2000 pictured.
We can’t dismiss these, although they aren’t actually produced any more. A vane supercharger is basically an offset rotor within a casing, which acts as a sort of paddle wheel, shifting air from one side to another. The vanes’ length varies by sliding in and out of the central hub, whilst their tips actually touch the inside face of the casing. I
n the UK, the Shorrock C75 supercharger was popular during the late ’60s/early ’70s ‘tuning boom’ on fairly small displacement engines. The problem was, unless they were fed with copious amounts of oil (imagine the effect on the intake charge!) vane superchargers wore out extremely quickly. In the US, a similar type, the Judson, existed and was quite popular, but it suffered from the same lubrication woes.
So, how do you supercharge your car? Well, the obvious way is to buy a kit! There’s plenty out there based on the various blower types. By now, you should have a pretty good idea which type is suitable for what application, so once you’ve decided what supercharger you’re going for, you’ll need to decide how to fit it. There are two basic configurations:
The first setup involves fitting the blower between the induction system and the engine and is known as a draw-through. Traditionally, the layout is carburettor-blower-engine. In this instance, the supercharger (typically a Roots) compresses both air and fuel. The rotor’s mashing effect also results in charge cooling. This is significant, because when a gas is compressed, its temperature goes up – hence why intercooling is a good idea and is achieved in exactly the same way as turbocharging.
Although, it’s not recommended with this type of layout, as you’d be filling the intercooler up with both air and fuel! Not only would this be a potential bomb strapped to the front of your car, but also, you’d get fuel dropping out of the charge if the intercooler was too big. This would lead to lag as you filled the intercooler up, which would defeat the purpose. Further intake charge temperature reduction can also be achieved with water injection, which effectively reduces the risk of detonation.
The other configuration involves blowing through the induction system, which, unsurprisingly, is known as a blow-through. With this setup, however, you need a method of bypass (a bit like a dump valve), which is a built-in feature on the latest Eaton superchargers. The bypass is necessary, as otherwise you’d have charge hammering against the butterfly when it closes, because a supercharger makes boost all the time – remember, it’s crank driven. This method is more suited to intercooling than a draw-through is, and the intercooling can be done in exactly the same way as with a turbo.
Mounting the supercharger has always been the biggest problem – with V8s, it’s not too tricky, since the blower traditionally sits on a manifold between engine and induction. These are available off the shelf, but they’re often a basic shape, so it’s not that tricky to make an adapter plate for one-off applications. The trouble is, this method also increases the height of the setup. Then again, there’s nothing more awesome than an old-school blower poking through a car’s bonnet topped with a whopping bug catcher! These days, though, it’s more about stealth.
Some engines, such as Chevy’s LS9 Corvette ZR1 lump, have the supercharger virtually built in – it’s so tucked in that it doesn’t even look supercharged. Yet the motor produces 638bhp at 6500rpm and 604lb/ft of torque at 3800rpm, all from almost 6.2 litres. Obviously, this is a production engine, but the various kits that are available all aim to keep the installation as low as possible so you can at least close the bonnet!
The problem isn’t so bad with a centrifugal supercharger – often these are similar in size to a big alternator, which means they lend themselves to compact installation in a similar way to a turbo. All that’s needed then is a mounting bracket – these can either be fabricated or are supplied with kits, which is the reason you buy one! On the subject of pipework, because of the massive induction noise, it’s normal that supercharger installations use solid rather than silicone hoses.
ENGINE INTERNALS: COPING WITH THE POWER
A gain, we’re not going to repeat our turbo article here – just mention that big boost needs a drop in compression ratio (CR), which can be achieved with the normal method of low-CR pistons and possibly stronger rods. If you have a cast crank, it’s also a good idea to upgrade to a forged one if you’re going to be revving past around 8000rpm.
The other consideration is ignition, which generally needs retarding, since you’re cramming in a denser charge. A too-hot burn will result in detonation and burnt pistons. Depending on the kit (or installer) you choose, it’s normal to have some sort of interface device to knock back the timing, along with increasing fuelling according to the amount of boost you run. However, there’s usually only so far you can go with ‘tricking’ the standard engine management. Often, you’ll need a complete standalone system, especially where massive boost increases are involved. It’s frequently a personal choice for the tuner, but any of the major ECU companies will have a suitable product – just don’t forget that it’ll need mapping, too.
Carb systems are slightly different. Traditionally, a vacuum-operated capsule is used to mechanically retard the timing. But again, you could use a managed ignition system that will work in conjunction with the carburettors. Plenty of American supercharged V8s use sophisticated MSD electronics to alter their timing levels when under boost.
TURBO OR SUPERCHARGER?
This debate will go on forever, but there’s no denying that once you’ve driven a supercharged car, you will want one! The pure grunt from the start – especially with a Roots blower – is seriously addictive, and the ability to simply fry the tyres from nothing is awesome! Then there’s the noise: the gear-drive whine powering the rotors is spine-tingling. Although it’s a bit oldschool these days, since older blowers used straight-cut gears, whereas quieter helicalcut gears are now the norm. When properly set up, a turbo car can give very similar performance, but there really is nothing quite like a supercharged kick…
Words Jon Hill