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Back in the 60s and 70s, rallying was a fairly low-tech kind of sport. When the Mini Cooper S, the Lancia Fulvia and the Ford Escort Twin Cam ruled the roost in world rallying, reliabiltiy more than speed was the key essence to success. But as the 1970s progressed, rallying became more a sport where speed was the major ingredient to winning events. Events were still long and arduous, but without a fast car, you were never going to be in with much of a chance of victory. As the decade progressed, horsepower figures were regularly between the 250 and 300 mark. The Fiat Abarth 131, the Ford Escort RS1800 and the Lancia Stratos were the cars to have, and the fans loved it. Twin Weber carburettors sucking copious amounts of air into the engine made the cars sound tremendous. But time moved on. In the ever-present search for more power, the turbocharger was about to make its mark. Already well used in Formula 1 racing, German car maker Audi were about to revolutionalise rallying – in more ways than one. Audi were initially the butt of many jokes when they announced they would rally a four-wheel drive, turbocharged Quattro in 1981, but as history now shows, it was Audi who had the last laugh. Not only did four-wheel drive take off, but so did turbocharging. If you want to have a chance of outright victory in major events, you need to have a car with forced induction. Turbos are obviously important to success in the 21st century, but what are they, and how do they work? Let’s take a look, step by step, and bring you up to speed (no pun intended!). What is a turbocharger? A turbocharger is the simplest and most weight efficient way of significantly increasing a car’s engine performance. Whether you have a sports car, a rally rocket or a diesel engined four-wheel drive, the turbocharger is often the first addition to the engine bay when looking to increase power. A forced induction system (like a turbocharger) compresses the air flowing into the engine, allowing more air to be squeezed into each cylinder. The more air that can be added to the cylinder also means that more fuel can be added as well, resulting in more power from each explosion in the cylinders. A significant power-to-weight ratio is one of the benefits. The exhaust flow from the engine is used to spin a turbine, which in turn spins an air pump. The turbine will spin at speeds of up to 150,000 rotations per minute (rpm) – about 30 times faster than most car engines without a turbocharger – so naturally the temperatures in the turbine are also greatly increased. What are the power gains? The way to get more power from your engine is to increase the amount of air and fuel that the engine burns. Speak to a V8 lover and they’ll tell you the surest way to do this is to increase your four cylinder engine by another four cylinders! However, we don’t all want to go the V8 route, so turbos are an economical alternative. A turbo will allow your engine to burn more fuel and air by pushing more of each into the cylinders of your engine. With the typical turbo boosting at 6 to 8 pounds per square inch (psi), and atmospheric pressure at around 14.7psi at sea level, a turbocharger should give you an increase in power of around 50 per cent. In reality, though, the increase will be closer to 30 or 40% because having a turbo increases the restriction in the exhaust flow. On the exhaust stroke of the engine, the motor will need to push against a higher back-pressure than normal, subtracting a little of the potential power from the turbo. How does it work? Bolted to the exhaust manifold, the turbocharger uses the exhaust gases to spin a turbine. The turbine is connected by a shaft to the compressor (located between the air filter and the inlet manifold), which pressurises the air going into the pistons. The exhaust gases from the cylinders pass through the turbine blades, causing it to spin. The more air that passes through, the faster it spins. The compressor, a type of centrifugal pump, pumps air into the cylinders, which then draws air in at the centre of its blades and flings it outward as it spins. Because of the extraordinarily high speed that it spins at – up to 150,000rpm – most turbos use a fluid bearing set up as traditional bearings would explode at this speed. This not only cools the shaft and other turbo parts, but it allows the shaft to spin with very little friction. The Wastegate The wastegate is a valve that helps to reduce turbo lag, while preventing the turbo from spinning too quickly at high engine speeds. The valve allows the exhaust to bypass the turbine blades. The wastegate will also sense if the turbo pressure gets too high. When this happens, the wastegate bypasses some of the exhaust gases around the turbine blades, allowing the blades to slow down. The Intercooler When air is compressed, it heats up and expands, increasing the pressure in the turbo before the air actually gets into the engine. By getting more air molecules into each cylinder, rather than more air pressure, the power of the engine is increased. Looking very similar to a radiator, air passes both through and inside the intercooler. Intake air passes through the sealed passageways of the intercooler, while cooler air from outside is blown across fins by the engine cooling fan. By cooling the pressurised air coming out of the compressor before it goes into the engine, the intercooler will further increase engine power. Therefore, if a turbo is boosting at 7psi, the intercooled system will put in 7psi of cooler air, which is denser and contains more air molecules than warmer air. High rise power Driving at high altitude is a problem that will see an engine lose a considerable amount of power. At higher altitudes the air is less dense, so for each stroke of the piston, the engine will get a smaller mass of air. A turbocharger will help reduce this loss in power. While a decrease will still occur, it will be less than in a non-turbo engine because the turbo will find it easier to pump the thinner air. A carburettor engine at high altitudes will increase the fuel rate to match the increased airflow into the engine, as will more modern fuel injected engines. Oxygen sensors in the exhaust determine the correct air-to-fuel mixture, and this mixture will automatically increase the fuel flow if a turbo is on the engine.

Turbo sealing at a round of the World Rally Championship in the late 1990s. Photo: Martin Holmes

What is turbo lag? In a standard turbocharged car, when you put your foot on the power there is a delay of about a second before the turbine spins to a high enough speed to produce the required boost. The resultant feeling before the turbo kicks in is what’s known as ‘turbo lag’. In motorsport, this can be a real problem, and it’s one that manufacturers have worked long and hard to overcome. Fitting a turbo dump valve can be a help, but by far the most effective system is the Anti-Lag System (ALS), which is widely used in the rallying world. Many drivers have also used left-foot braking as a method to overcome turbo lag. By braking with the left foot, the driver is still able to keep his foot on the accelerator, thus keeping the turbo spinning to the optimum level. But while this method is effective, it is very hard on the brakes.

The days are long gone when the sport's top rally cars were normally aspirated.

How does ALS work? When a driver takes his foot off the accelerator, the ignition timing is retarded by as much as 40 degrees, making the intake air and fuel supply mixture much richer. The result is an air/fuel mixture that continues to penetrate the combustion chamber, even when the driver is off the power. The retardation of the ignition sees the air/fuel mixture entering the exhaust tubes mostly unburned and when the spark plug fires, the exhaust valves are already starting to open, due to the retardation. The increased exhaust temperature also means that the unburnt fuel explodes as it enters the exhaust tubes, keeping the turbo spinning and lowering response times dramatically. There are downsides though. Whenever ALS is activated the turbo’s temperature will soar from around 800 to 1100 degrees, it places a huge stress on the exhaust manifold and pipes, and it will also cause a reduction in engine braking. The benefits, however, far outweigh the negatives and rally teams the world over are more than willing to live with the consequences, provided they can get the power gain they require. So, there you have it – a brief, but to the point assessment of how a turbocharger works, and why they are so popular in the motorsport world. Many purists begrudge the turbo’s prominence in competition and the fact that the ‘throaty’ sound of carburettors sucking air is becoming less frequent, but there is no denying that the turbo is here to stay. And when you can quickly and easily secure a horsepower increase of around 40%, why wouldn’t it be popular?

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