Forced induction is not a new idea — the French were supercharging race cars as far back as 1909
When a normally aspirated engine is running, air enters when a piston is pulled down inside its cylinder on the intake stroke because as the piston recedes, it enlarges the chamber, causing a vacuum. This vacuum is then filled by air rushing in through the carburettor, which charges it with atomised fuel. The fuel and air mixture is in turn compressed by the raising piston. Then the mixture is ignited, super-heating the air and causing it to expand and push down on the piston. The hot air is released through the exhaust system. That, in a nutshell, is how a reciprocating engine works.
Atmospheric pressure is about 14 pounds per square inch at sea level, and it is what causes the cylinders to fill with air. So it stands to reason then that if you increase the air pressure beyond atmospheric by forcing more air into the intake system, you will end up with a denser mixture, a bigger explosion in the cylinder, and more power as a result. It’s called supercharging.
Nothing else you can do to an engine will add as much power — and lest you bring up nitrous oxide injection, let me point out that NOS is just a chemical form of supercharging — albeit a temporary one. In fact, power gains of between 30 and 75 per cent are possible just by adding a supercharger to our engines.
Bucks for the bang
The major catch for most of us is cost. Installing a supercharger or turbocharger can set you back $5000 at time of writing, and that is more than most of us spend on an entire engine. There are a few other challenges for the bloke working at home too, such as installation, plumbing, and setting up the ignition. Also, blowers work best with fuel injection, and that is another expense.
And of course, like everything else, there are limits as to how much increase in pressure you can put into an engine. That’s because as you compress air, it gets hotter. And as air gets hotter it becomes less dense, so the power gains diminish. Also, if the air/ fuel mixture coming into the engine becomes too lean or too hot, it may explode on its own and cause detonation, which will ruin your engine in short order.
Too much boost, a high compression ratio, or a combination of the two will cause detonation in a supercharged engine. Detonation occurs when the combustion pressure gets so high that the inlet charge explodes before the spark plug arcs. When this happens combustion takes place while the piston is still moving up in the cylinder, and that causes a tremendous shock to the pistons, rods, and crankshaft.
The flame speed across a cylinder under normal circumstances is somewhere between 190 and 320kph (120 and 200mph). But when fuel detonates — that is explodes under pressure before the spark plug even fires — the flame speed goes up as high as 2000mph, and the shock destroys your ring lands and pistons. A richer mixture (with more fuel in suspension) or one that is less compressed is cooler, so it is less likely to detonate. Also, there are devices called intercoolers that cool the compressed air before it goes into the engine, and they help prevent detonation too.
But even when you can get a cool, dense mixture into an engine under high pressure, you need to take into account whether you engine’s bottom end can handle the extra power. So even though theoretically the more air and fuel you can run through your engine the more power it will make, there are practical considerations.
Four to seven pounds increase in boost is maximum for the street, and if you want a reliable car for daily use, keep it down around four or five max. That’s enough to give you good midrange torque, yet keep you from running over your own engine parts.
Roots-type blowers
Most engine builders are familiar the Roots-type blower, which has been around for a very long time. The basic design actually goes back to the late 19th century, and was developed to pump air into mines. Inside that big housing on top of the engine are two figure-eight-shaped metal vanes that rotate in close tolerance with one another to move air.
Since the device can move air much faster than an engine can consume it, the air in the intake manifold becomes compressed. The blowers you see most commonly were originally developed by GMC for diesel truck engines. Hot rodders were quick to see their advantages, and adapted them to their engines years ago. These old-style crankshaft-driven blowers can give you more boost than you’ll ever need, look impressive, and make a lot of noise, but they are not the best choice for street use.
Big blowers require a fair amount of power just to turn them over, and they don’t fit under the bonnets of most vehicles. They also give you poor fuel economy, because the fuel/ air mixture must be kept rich to avoid detonation. Of course you can mitigate this by driving conservatively, but why would you want a blower in that case? For a dragster there is nothing better than a big blower, but on the street they’re sort of like using a sledgehammer to crack a walnut.
Screw-type superchargers
There are some very nice screw-type blowers available that are a variation on the Roots-type supercharger. They use two rotors that are like interlocking screws. The original design comes from Sweden, and the advantage of it is that the air is compressed before it enters the intake manifold, so there is less heat.
Turbochargers
Turbochargers use the pressure of the escaping exhaust to spin a turbine that drives an impeller to force air into the intake manifold. Impressive power gains can be had with turbochargers, but they too have drawbacks. Like throttle lag for instance. You feel nothing at first, and then the engine comes on all at once when the turbocharger spools up.
Another interesting thing happens when you let off the throttle in a turbo-equipped car. The turbocharger keeps on pumping for a while longer as the last exhaust gas escapes, which is what causes that pop and the shot of flame out the exhaust.
Finally, turbochargers don’t provide boost at low revs, and their bearings and turbines can suffer from the extreme heat generated by the exhaust system if not properly cooled. They also require more extensive plumbing than superchargers.
Centrifugal superchargers
These too have been around for a long time but they have become popular again with American manufacturers. It is probably easiest to think of centrifugal superchargers as crankshaft-driven turbochargers. The advantages to them are that they give you boost at lower revolutions, and back off when you do, so there is no throttle lag. They are also easy to install, and they fit under the bonnets of most vehicles.
Blower Guide
| The big Roots-type blower on this Corvette is intimidating, but is gross overkill for street use |  |
| Neatly done turbocharger installation gives this Chev 670kW to play with |  |
| Pulling the air through the carbs causes its own problems. It would be better if the blower fed the carbs, instead of the other way around |  |
| Blown V8 certainly wakes up this old Kaiser, but it’s strictly a fair weather show car |  |
| This is what detonation looks like. Explosive shocks to pistons rapidly destroy them |  |
| Late model injected small-block engines have tremendous potential thanks to precise fuel metering |  |
| Here’s how a Roots-type blower works. Air is pumped into the intake manifold by twin internal vanes |  |
| Screw-type blower is a variation of Roots-type |  |
