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Overview of Forced Induction

- forced induction refers to the amount of oxygen forced into the engine.
- this additional oxygen is mixed with extra fuel to produce more power.
- it’s added by mechanical and/or chemical means
- mechanical refers to supercharging (incl turbos and blowers).
- chemical refers to nitrous oxide injection or a similar method.


The amount of power an engine produces is determined by how much fuel it burns. Maximum power is delivered when the engine burns 1 part of petrol for every 13 parts of air. When we get the engine to consume more air, we can introduce more fuel into the cylinders, thus generating more horsepower.

There are two main options for increasing the horsepower of a car. You can increase the cubic capacity, which increases the amount of fuel the engine will burn. But bigger cylinders mean more weight. More fuel is burned but performance is lost because of the weight penalty. Internal friction in the engine also increases. A 50% increase in cubic capacity will give a maximum of 40% power increase.
Another option is to improve the breathing of the engine by enabling it to draw more air into the engine. This will give a small increase at the top end of the rev range but at the expense of power at the bottom end.

Forced induction is the other option available for increasing the power of your engine. We can supercharge the engine by mechanical or chemical means.

The supercharger can be belt or gear driven or driven by the engines exhaust gases (turbo). Many different designs exist, but whatever the design, the objective is to force more air into the cylinders so that the engine performs as if it were much larger.

When a piston moves down it leaves a vacuum in its wake. Because the inlet valve is open, air at 14.7psi (normal air pressure) rushes in. For very high performance engines, the cylinders will completely fill with air (100% volumetric efficiency).
With supercharging, the aim is to raise the air pressure above atmospheric pressure. If we add boost of 14.7psi then we can force twice the volume of air into the inlet valve, and theoretically get twice the horsepower. That’s why the F1 engines of the late 80’s achieved huge amounts of bhp.

The BMW 1.5 litre 4 cylinder and the Honda 6 cylinders achieved over 1200bhp at 60psi of boost. In 1988, boost was limited to 2.5bar, so engines wouldn’t produce more than 700bhp.

Chemical means also exist to increase the amount of air (oxygen) available for combustion. Nitro methane is 53% oxygen by weight, and is used to get in excess of 1500bhp out of 8-litre dragsters without the use of blowers. With blowers, the power is closer to 6000bhp (sweet). Other liquids used are nitro propane, hydrogen peroxide, nitrogen peroxide. However, the most common liquid used is dinitrogen monoxide (nitrous).

Nitrous is stored in gas bottles at 800psi at normal ambient temperature. When released from the bottle, it converts from liquid to gas with an oxygen content of 36% by weight. In small doses, this is the minimum power increase. At max levels, the power increase will be double this, i.e. 72%. This means a 2.0 litre engine can make about 500bhp and a 10 litre drag engine can make about 2000bhp.

Blown Race Cars
The first blowers were mechanically driven superchargers called centrifugal superchargers and were driven via gears off the front of the crankshaft or by a belt off the flywheel at around five times engine speed.

In 1921, Mercedes started using a roots-type blower operating at 7psi and only engaged at full throttle.
In 1923, Fiat started using a vane type blower on their 2.0litre F1 engines. This also employed part time operation in the interests of longevity. Fiat subsequently ditched their vane (Wittig) blower in favour of a roots type blower.
By 1940, Mercedes and AutoUnion were putting out 520bhp at 5000rpm using 26psi boost and 9.2:1 compression ratio on methanol.

The era of blown F1 engines came to a halt in 1951 with the 1.5litre V16 BRM. They couldn’t compete with the naturally aspirated 4.5 litre Ferraris.

Blowers continued to be used in other formulas and were used to great advantage in the 1960s in America on the drag strips.

Roots Blowers

Fiat, Toyota, ford, Mazda etc… have strapped roots type blowers onto their production cars. The majority of these car makers chose blowers by Eaton who set about redesigning the roots type blower in the 1980s.
Eaton twisted the three lobe rotors through 60degrees. The rotors were given a special coating and were machined to close tolerances, which meant the rotor tolerances were down to 0.2mm which reduced leakage and improved pumping efficiency. This allowed rotor speed to reach up to 15,600rpm.

Eaton also added a bypass valve at the blower inlet. This allows the blower to draw air from the air filter, bypassing the blower at idle and at cruise where it’s not needed. This gave the manufacturers the option of using an electro-magnetic clutch (like an air-con clutch) to disconnect the blower. Toyota has long used this system and Mercedes also used it on the C230 model in 1996.

Centrifugal Blowers
Companies like Paxton, Vortech and Powerdyne have become well respected in developing Centrifugal blowers.

The compressor half of a turbo is basically a centrifugal compressor. With all the development that took place in developing the turbo during the 1980s(F1 and rally), it was a simple task to copy the turbo compressor technology and come up with a way to add a suitable drive to spin the blower at extremely high speed(7 to 15 times engine speed).

With centrifugal blowers, there is more boost produced lower down but less maximum boost at the top end of the rev range. A solution to this would be to fit a dump valve so that boost is vented higher up the rev range, therefore a high compressor speed could be applied, but this would be a waste.


A better solution would be to fit a variable drive to spin the compressor very fast at low rpm and to progressively reduce the boost higher up the rev range to avoid excessive boost levels.

Lysholm Screw Blower

The screw type blower is the newest type of blower to be developed. On the outside, the screw type looks similar to the roots type, but inside, the two rotors are twisted into a complex screw pattern (called a helix pattern). The helix pattern was used in industrial applications to pump oil, but was only used in automotive applications to pump air when tolerances were brought down to 0.002mm.

Fleming Thermodynamics of Scotland licence out other manufacturers to produce the Lysholm Blower. The most well known are IHI in Japan (who also make turbos), other companies are Opcon in Switzerland, PSI Corp in USA and Whipple in USA. The Mazda 2.3litre Miller cycle v6 was the first production car to use the Lysholm Screw Blower.

This appears to be the blower of the future and its only problem at the moment is the cost of manufacture. It has efficiency of 75-85% which far exceeds the roots (35-60%) and is well in front of the centrifugal. It is lighter and smaller then the roots and requires less power to drive it. It gives the best of both worlds in boost delivery. It provides substantial boost at low levels just like the roots, but continues to supply good boost further up the rev range like the centrifugal. It was banned from top fuel drag racing in the early 90s but it continues to dominate the 500cu top alcohol class (3000bhp) and there is nothing which looks like beating it.

Other Blowers

German company KKK are producing a promising blower called a Ro-charger (rotating piston). Brown Boveri developed a Comprex pressure wave blower. It was used by Ferrari in 1981 in F1 and gas been used on small diesel engines since. Volkswagen uses their own G-later spiral blower. They use the G40 (40mm spirals) on the Polo and the G60 on the 1.8 golf. The G-later is 55-68% efficient and produces good low down boost, but turbo charging has superseded it in the VW group.

Turbo charging

Turbos go back as far as 1905. In 1917 they were used on French aircraft. In 1923 Brown Boveri were producing turbochargers on a regular basis. Early turbo development concentrated on diesel engines because the higher exhaust temperatures of petrol engines led to turbo failure. But General Electric kept developing turbos for petrol engines and by 1938; they were supplying turbos for the American air force for use in various bombers.

The development of hi-tec metals and materials for use in jet engines, led to rapid development of turbos they were highly resistant to temperatures and lightweight. Turbos become ultra reliable and later became light enough to have low spool up time (turbo lag).

In 1952 the first turbo car entered the Indy 500 with a diesel engine (how depressing). For the next 3 years turbos dominated the Indy series until the rules changed. Prior to boost restrictions been put in place, the 2.61litre 4cyl Offys in 1978 were producing over 800bhp on methanol with an 8.1:1 compression ratio with boost of 45psi(and this was without intercooling).By the late 80s the Porsche 2.65litre v8s were producing 700bhp at only 8.7psi boost on 9.5:1 compression ratio. A decade later the boost limit was set at only 5.5psi but engines by Honda were still developing 900bhp. At the end of 2001 boost levels were cut to just 2psi.

In the early 70s, Porsche were the first to introduce turbos to the sports car racing scene. The mighty 5.4 litre 917 model produced 1200bhp in 1974. But real development of turbos only took place after they were introduced to F1. Renault’s small 1.5 litre turbo made the paddock sit up and take note. Major problems were addressed and partly resolved such as charge cooling, engine cooling, and turbo lag. Fuelling and advance ignition were also major factors.

By 1981, Ferraris v6 engine and Tolemans 4cyl engine (Brian Hart) entered F1. In 1982, BMW entered with its 4cyl blown block. In 1983, Porsche, Alfa and Honda entered with their version of the turbo engine. Motori Modernis v6 and Ford Cosworths v6 were introduced in 1985 and 1986. During qualifying, boosts of up to 65psi were being used with power outputs of 1200bhp. (Compression ratios varied from 6:1 to 7:1 over the years).



Production car makers have invested millions into developing reliable and usable turbos for road applications.
GM produced the first road going turbo called the F85 Jetfire Turbo Rocket. It was a 3.5litre all alloy v8 (later used in Rovers and Land Rovers). It had a high compression ratio of 10.25:1 and max boost of 6.5psi at 2200rpm. Use of the turbo increased power by 30bhp to 215bhp and increased torque by 70 ftlbs to 300rpm. It used a Garrett turbo with 61mm turbine made of nickel steel with stellite and cobalt added.

A decade later Porsche developed an air cooled turbo engine for the 911, based on the lessons learned from the awesome 917 sports car. The engine produced 260bhp without intercooling and with a compression ratio of 6.5:1 and max boost of 11.4psi.

In 1978, capacity was increased from 3 to 3.3 litres and a small air to air intercooler was added which allowed a compression ratio of 8:1 to be used. Power increased to 300bhp and torque to 304ftlb. Intercooling provided a 5% increase in performance (non-intercooled air was at 130degrees Celsius)

Japanese motorcycle companies also used turbos on their bikes, Honda, Yamaha, Suzuki and Kawasaki all introduced turboed bikes in the 70s. The air-cooled 738cc 4cyl Kawasaki motor produced 96bhp and used a knock sensor.

Development by car companies continued throughout the 80s and by the late 90s turbos were much nicer to drive with almost no turbo lag.

Nitrous Oxide

In comparison to blowers and turbos, nitrous oxide injection is relatively unknown in Ireland. In the USA, nitrous started to be used in the 1960s. Ron Hammel started tuning engines based on the research done by Harry Ricardo. By 1969 Hammel was installing nitrous systems into sports cars. From then until 1976, nitrous was used widely in NASCAR and in drag racing by hiding it inside fake fire extinguisher bottles and by using it to overtake on the ovals. In 1976 nitrous was discovered in several competition cars and so it was revealed and this let to the widespread use of nitrous as an instant bhp provider. After this, nitrous was banned from most categories of motor sport, except drag racing and unlimited hydroplane boat racing.

However, it was through the use of nitrous to boost performance in illegal street racing that caused it to grow in popularity and by the 90s it was being widely used in all kinds of engines from mild to wild.

A one third power gain is the minimum available power gain available by using nitrous. Compared to turbos and blower applications, this doesn’t sound like much, but this gain is available through the entire rev range, from idle to max rpm. In street cars, the nitrous setup has to be triggered using a two stage method. In lower gears the ECU is programmed to introduce a small dose of nitrous to avoid uncontrollable wheel spin. In higher gears the full dose is delivered.

The above gives an overview of forced induction. For a more detailed and comprehensive explanation of each aspect of forced induction, use the links on the right.