As with all modifications to increase power, the use of nitrous oxide
carries with it concerns about the reliability and longevity of an
engine. Due to the greatly increased cylinder pressures, the engine as a
whole is placed under greater stress, especially the parts involved
with the combustion chamber. An engine with components not able to cope
with the increased stress imposed by the use of nitrous systems can
experience major engine damage, such as cracked or destroyed pistons,
connecting rods, or crankshafts.
Even if the engine is up to the task, severe damage can occur if a
problem occurs in the fuel system; an engine running with nitrous oxide
depends heavily on the proper air to fuel ratio to prevent detonation
from occurring. For example, if the engine's fuel supply were to be
reduced, this would cause the engine to run lean by whatever degree the
fuel delivery was reduced, which can lead to engine knock or detonation.
Depending on the engine, this may only need to occur for a matter of
seconds before major damage occurs.
It is essential not to reach a fuel cut rev limit as this will also
momentarily restrict the fuel flow to the engine and as nitrous is still
being injected into the engine without the additional fuel the engine
will again run lean and cause detonation.
Some mechanism to disable the nitrous system when knock is detected
by a knock sensor would be beneficial. Ignition timing must also be
watched closely when using nitrous oxide. It is said that, on a large
car engine, for every 50 horsepower of nitrous used, the ignition timing
must be retarded by two degrees.[citation needed]
This is recommended for any stock type application. It is also
recommended that high octane fuel (92 octane minimum) be used to avoid
detonation.
Good optimisation of enrichment fuel is essential otherwise the fuel
can 'drop out' and puddle in the intake tract, potentially causing a
backfire. With a properly designed nitrous injector and correct
placement of the nozzle (not too far from the intake entry point and
away from any abrupt bends and restrictions in the intake tract)
backfires can be avoided.
Cars with nitrous-equipped engines can be identified by the "purge" of
the delivery system that most drivers perform prior to reaching the
starting line. A separate electrically operated valve is used to release
air and gaseous nitrous oxide trapped in the delivery system. This
brings liquid nitrous oxide all the way up through the plumbing from the
storage tank to the solenoid
valve or valves that will release it into the engine's intake tract.
When the purge system is activated, one or more plumes of nitrous oxide
will be visible for a moment as the liquid flashes to vapor as it is
released. The purpose of a nitrous purge is to ensure that the correct
amount of nitrous oxide is delivered the moment the system is activated
as nitrous and fuel jets are sized to produce correct air / fuel ratios,
and as liquid nitrous is denser than gaseous nitrous, any nitrous vapor
in the lines will cause the car to "bog" for an instant (as the ratio
of nitrous / fuel will be too rich) until liquid nitrous oxide reaches
the intake. 
