Supercharging for More Power
Turbos and superchargers
both perform the same basic function - they’re pumps that force-feed an
engine its fuel air mixture at a greater than atmospheric pressure. But
the difference between them lies in how they go about getting the job
A supercharger is driven
mechanically from the engine’s crankshaft to provide the necessary
huff, while a turbocharger utilizes exhaust waste gases to spin an
impeller at tremendously high speed and generate the puff. The
supercharger looses out to the turbo in some ways because it drains
power from the crank, while the turbocharger, driven by waste exhaust
gases, gets it power for free. But because the supercharger is driven
mechanically with no slippage, it’s already ready to deliver buckets of
extra grunt at virtually any revs, from idle upwards, while the turbo
is at a disadvantage at slow engine speeds, when the exhaust gases lack
the energy to get the impeller really spinning.
A supercharger is connected
directly to the crankshaft by a belt, unlike a turbocharger, which is
driven by exhaust gases. A supercharger provides improved horsepower
and torque, at lower rpm's, by pumping extra air into the engine in
direct relationship to crankshaft speed. The positive connection yields
instant response, in contrast to turbochargers, which must overcome
inertia and spin up to speed as the flow of exhaust gases increases. The supercharger is a way to get around "turbo lag". The lubrication
system also differs, in that the supercharger is self-contained whereas
the turbocharger requires engine oil.
provide more fuel to burn, there’s a huge increase in horsepower and
torque, at low and midrange engine speed. The output of a supercharged
engine can be easily varied by simply changing the size of pulleys
between the engine’s crankshaft and the blower.
Whatever charging system
you use, whenever building an engine for big power, you must always
remember, too many horses and bad tuning can destroy an engine, even if
the engine is built up. Therefore, it is most important to increasing
power and torque for reliably, rather than for peak power.
Heat is not good and
compressing air produces heat!
Forced induction compresses
air, and as a law of physics the temperature of the air increases as a
direct counterpart to its compression. A lot of engineering goes into
trying to compensate for this fact in supercharging and turbocharging
The word "adiabatic"
describes a process in which no heat is gained or lost - 100% adiabatic
efficiency would be the perfect forced induction device, creating no
heat gain whatsoever, probably impossible to achieve ever. And the
closest anyone can come yet is around 80% efficiency.
The problem with heat is it
defeats the original purpose - the hotter the air, the lower the
density possible, and the extra power comes from dense air. Another
problem from heat is ignition - the hotter the inlet air, the more
tendency the engine will have towards detonation and pre-ignition
(knock and ping), which damages the engine, besides diminishing
performance. Drivers of blown vehicles tend to keep an eye on the
The goal of efficient
charging is to compress the air and to keep it cool, for maximum power.
The cooler the intake charge, the denser the air and the more
The greater the adiabatic
efficiency with which a supercharger compresses air, the less the heat
that gets added to the intake manifold. Efficiency is measured by the
discharge air temperature at a given pressure. For 6 pounds of boost, a
supercharger with intake air temperature of 185 degrees is more
efficient than another at 190 degrees. Boost itself is only the measure
of pressure the intake air is under, not an indication of the power
produced as horsepower.
Which has greatest
The Roots blower has the
lowest adiabatic efficiency of all the forced induction designs
(including the turbocharger, which has to start off with hot exhaust
gases to deal with) - generally around 50 percent. The roots type is so
inefficient because it doesn't compress the air directly, but delivers
uncompressed air which wells into the intake manifold, becoming more
compressed, but with additional heat gain from the turbulence and
reverse flows of air mixing.
can vary from 60% up to perhaps approaching 80%+ efficiency, as can
turbochargers; both are more efficient at higher rpm, which is another
way of calling them more inefficient at lower rpm.
The twin-screw supercharger
normally delivers lower output temperatures, for adiabatic efficiencies
of 70-80%+ across the whole rpm range.
Twin Screw Supercharger -
a positive displacement compressor
The twin-screw supercharger
is a positive displacement air mover, in that it moves fixed amounts of
air per revolution, like the roots type blower. Unlike the roots
however, which is only an air delivery system, the twin-screw
supercharger is also a compressor. The counter rotating lobes and
chambers of the twin-screw are designed for a screw-like tapering
effect which runs its intake air into a smaller space for output, thus
compressing it. The rotors have very close tolerances yet never touch.
Compressed air is delivered into the compression environment of the
intake manifold with very little leakage or energy loss.
Because of the increased
mechanical efficiencies of the superior design, the output air
temperatures of the twin-screw positive displacement supercharger are
radically improved from the roots type. The twin-screw quotes adiabatic
efficiency of 70%-80%+ range across the whole powerband.
As with the roots, since
the supercharger is under continual drive, and since it delivers boost
practically from idle, overboosting is prevented by the use of an
intake bypass system, which allows the engine to breathe normally at
cruising or idle: the bypass closes on throttle use, delivering full
Full boost by 2000 rpm
The twin-screw supercharger
creates boost the instant the throttle is touched, and generally
reaches full boost by 2000 to 2400 rpm. Full boost is then available
all the way to redline. A positive displacement compressor is ideal for
street performance cars.
Performance vehicles are
very responsive with positive control using this type of supercharger.
The instant torque for accelerating, passing, and hill-climbing
diminishes the strain on the engine and increases the safety factor.
The twin-screw compressor is especially useful at high altitude, where
physics dictates that all engines lose power.
The twin-screw supercharger
is essentially silent, producing discernible sound no greater than
whisper strength. Of all the forced induction systems, the twin-screw
compressor supercharger might be the most awesome direction for the guy
who wants a sleeper.
Selecting an Intercooling System
Both air/air and water/air
systems have their own benefits and disadvantages. Air/air systems are
generally lighter than water/air, especially when the mass of the water
(1kg a litre!) is taken into account. An air/air system is less complex
and if something does go wrong (the intercooler develops a leak for
example), the engine behaviour will normally change noticeably. This is
not the case with water/air, where if a water hose springs a leak or
the pump ceases to work it will not be immediately obvious. However, an
air/air intercooler uses much longer ducting and it can be very
difficult to package a bulky air/air core at the front of the car - and
get the ducts to it! Finally, an air/air intercooler is normally
cheaper than a water/air system.
A water/air intercooler is
very suitable where the engine bay is tight. Getting a couple of
flexible water hoses to a front radiator is easy and the heat exchanger
core can be made quite compact. A water/air system is very suitable for
a road car, with the thermal mass of the water meaning that temperature
spikes are absorbed with ease. However, note that if driven hard and
then parked, the water within the system will normally become quite
warm through underbonnet heat soak. This results in high intake air
temperatures after the car is re-started as the hot water takes some
time to cool down.
- Efficient at constant high speed
- Cores readily available
- Bigger is better
- Longer induction air path
- Packaging of large intercoolers difficult
- Large pipes to and from intercooler required
- Ambient air as the cooling medium, less
efficient when in traffic: heat soak
- Short induction air path
- Easy to package
- No heat soak
- Excellent for short power bursts
- Consistant efficiency for every day driving:
stop and go
- More complex
- More expensive
- Heat exchangers harder to source
Technically, a water/air intercooler
has some distinct cooling advantages on road cars. Water has a much
higher specific heat value than air. The 'specific heat value' figure
shows how much energy a substance can absorb for each degree temp it
rises by. A substance good at absorbing energy has a high specific heat
value, while one that gets hot quickly has a low specific heat.
Something with a high specific heat value can obviously absorb (and
then later get rid of) lots of energy - good for cooling down the air.
Air has a specific heat value of 1.01
(at a constant pressure), while the figure for water is 4.18. In other
words, for each increase in temp by one degree, the same mass of water
can absorb some four times more energy than air. Or, there can be
vastly less flow of water than air to get the same job done.
Incidentally, note that pure water is best - its specific heat value is
actually degraded by 6 per cent when 23 per cent anti-freeze is added!
Other commonly-available fluids don't even come close to water's
specific heat value.
The high specific heat value of water
has a real advantage in its heat sinking affect. An air/water heat
exchanger designed so that it has a reasonable volume of water within
it can absorb a great deal of heat during a boost spike. Even before
the water pump has a chance to transfer in cool water, the heat
exchanger has absorbed considerable heat from the intake airstream.
It's this characteristic that makes a water/air intercooling system as
efficient in normal urban driving with the pump stopped as it is with
it running! To explain, the water in the heat exchanger absorbs the
heat from the boosted air, feeding it back into the airstream once the
car is off boost and the intake air is cooler. I am not suggesting that
you don't worry about fitting a water pump, but it is a reminder that
in normal driving the intercooler works in a quite different way to how
it needs to perform during sustained full throttle. However, the
downside of this is once the water in the system has got hot (for
example, after you've been driving and then parked for a while), it
takes some time for the water to cool down once you again drive off.
Which is better an
air-to-air intercooler or a water-to-air intercooler?
It really depends on the application. In
order for an intercooler to effectively cool the air that passes
through it, the intercooler itself must be cooled by some external
means. Most intercoolers are cooled just like your engine's radiator -
air flows over the outside of the intercooler's fins, which in turn
cool the air inside the intercooler - hence the name air-to-air
Intercooler. Some intercoolers, however, are cooled by water instead of
air, in which case they are generally called aftercoolers, or
water-to-air intercoolers. The benefit to an aftercooler is that air
passing through it can be cooled more than in a traditional air/air
intercooler if very cold water and ice are used to cool the intercooler
- in fact, some aftercoolers chill the air to below ambient air
temperatures even after it has been compressed by the
The reason aftercoolers are more
effective in cooling the air charge is because water is a much better
conductor of heat than air - in fact water conducts 4 times as much
heat (energy per pound) as air! The obvious drawback is that with time,
the water will heat up to the temperature of the air passing through
it, and its ability to cool incoming air goes away. Some aftercoolers,
however, use a small radiator to cool the water that runs through the
system, making them ideal for street use as well as racing. The water
is constantly pump whilst the ignition is on and is cooled as it
travels through the water ratiator. The cool water travels into the
charge cooler and cools the boost by absorbing heat energy. The hot
water exits the cooler and back to the water radiator via the
reservoir. This method of cooling is regarded as more efficient as the
cooling action of the water is more consistent than air to air
intercooling. The water drops to temperatures lower than ambient and
therefore cools the boost with greater efficiency. However charge
cooler systems require the installation of more components with a
slightly increased cost. Charge cooling is commonly used for high
compression engines where efficiency and temperature consistency are
key requirements. For drag racing applications aftercoolers packed with
ice work very well because they only need to work for around ten
seconds or so before you shut down and head to the victory podium. For
milder racing and street applications air/air intercoolers or
aftercoolers with radiators are more practical as their ability to cool
incoming air is not reduced with time.
For drag racing, autocross
or performance street applications the Liquid/Air system can provide
sizeable competitive advantages. Through the use of chilled coolant,
you can reach intercooler efficiencies well in excess of 100%,
unleashing dramatic performance gains.