The BMW M4 GTS is an incredible automobile! Not merely quick, but legitimately fast and the S55 is just that much more aggressive in the GTS than the ‘normal’ M4. Part of this is down to the use of high pressure water injection into the intake manifold under high load and RPM. It allows the boost to be turned up to eleven (from 17.2 psi to 21.6 psi – an approximate 26% increase in max boost pressure).
But why inject water? Water isn’t compressible (it is – however, not by an automobile engine). The key to understanding why water is good it is useful to know what happens inside a cylinder during ignition of the air/fuel mixture. Ideally air and fuel are combined inside the cylinder during the intake (downward) stroke. The air/fuel ratio in a gasoline engine should, under ideal conditions be 14.7 parts air to one part fuel known as ‘stoichiometric’. As load and RPM build, the mixture should get richer, more fuel to air than stoichiometric. And the reason it gets richer is to help prevent ‘knock’.
So what’s ‘knock’? Knock is the uncontrolled/unwanted ignition of the air/fuel mixture before the proper time. In normal conditions the spark plug electrode will receive a high voltage charge timed to occur slightly before the piston reaches top dead center (TDC) in the compression stroke (upward piston movement).
If all goes according to plan, the electrical ‘spark’ between the electrode and ground causes ignition of the air/fuel mixture. Done correctly the ignition allows the fuel air mixture to create ‘flame fronts’ that consume the fuel. The gasses produced by burning the air fuel mixture expand and press the piston downward – the power stroke (after the piston has reached TDC).
When things don’t go according to plan, the pressure (and temperature) of the combustion chamber allow the air/fuel mixture to ignite uncontrollably. This can occur before the air fuel mixture should ignite, or in opposition to the planned ignition flame front, in which case the components (read bearings) of…
