Internal combustion engines require air (oxygen)
to burn fuel, and therefore increasing the air flow
into the engine will theoretically increase
performance.  The process of combustion creates
a vacuum that naturally draws air into the engine.  
However, increasing the flow of air, or boost, is a
primary method for increasing horsepower.  This
is accomplished through three main methods:
chemically with nitrous oxide or mechanically
using air compressors.  
Nitrous oxide is a gas that when introduced into
an engine creates greater combustion through its
increased oxygen content.  Combustion 101
dictates that you need oxygen and fuel
compressed and ignited to produce power.  

Nitrous oxide contains a greater proportion of
oxygen than does normal ambient air.   Nitrous
oxide is bottled under pressure in liquid form and
introduced into the engine through small high
pressure nozzles at wide open throttle.  The
nitrous gas mixes with the fuel and combusts with
much greater intensity than the ordinary air-fuel
mix, creating much more horsepower.  While this
sounds like a perfect horsepower solution, nitrous
has several key limitations.  

Primarily, nitrous takes up a lot of space, a single
2 liter bottle may only provide enough gas for 2
quarter mile passes at the drag strip.  Mounting
any more than 2 bottles in a vehicle and the
weight begins to affect performance and offset
some of the gains.

Nitrous also isn’t free, the way air is—Nitrous
prices are approaching $8.00 to $10.00 per
pound, for a 10 pound bottle- that’s $80 to $100
for a few passes down the quarter mile.

Finally, the more powerful combustion creates
more strain on engine parts, increasing the
likelihood of failure.
Nitrous Oxide
A supercharger is simply an air compressor that is
driven by the engine crankshaft (via a belt) similar
to the way a water pump works.  Like a water
pump, the supercharger spins continuously, while
its speed varies with the engine RPM.  This direct
link is what makes a supercharger desirable; the
instant the engine RPMs rise, more air if forced
into the intake manifold by the supercharger,
instant boost.  
The downside of this design is the supercharger
cannibalizes some of the power created by the
engine to create boost.  The marginal horsepower
created by a supercharger comes with a
proportional cost.   This effect is particularly
unfavorable at low RPM when boost isn’t
necessary but the supercharger continues to draw
away horsepower.  
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