- 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.
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.
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
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%
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
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
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
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.
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
like Paxton, Vortech and Powerdyne have become well respected in developing
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
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.
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
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.
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
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.
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.
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
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