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Modified Ports in a 16 valve head, showing
valves in place |
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The
more efficient an inlet port is the less the pressure drop
is between the carburettor and cylinder effecting the resulting
power output. Port reshaping and resizing improves the air
fuel mixture flow allowing the combustion chamber to fill
up quicker. This increases both power and torque, especially
at higher revs. There are also less energy losses throughout
the ports due to their improved consistency in size. |
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Cross Section of Cylinder Head Showing
Mixture Flow Through Standard Port
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Cross Section of Cylinder
Head Showing
Mixture Flow Through Modified Port |
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The
amount that an inlet port is opened up is dependent upon
the inlet valve size. The port is made a fraction of the
valve size based on empirical (tried and tested) results.
The reason of this is to compensate for the valve stem blockage,
shown below, as well as the fact that the valve is reciprocating.
Also as the shrouding factor
is increased the optimum port size is reduced. |
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Combustion
Chamber Reshaping |
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The
combustion chamber has an effect upon the breathing potential
of an engine. The shape can be modified to increase the
flow of the air fuel mixture throughout the engine. This
must be done when fitting larger valves to gain full benefit
from extra fuel air mixture flowing into the engine.
When
larger valves are fitted to an engine this results in the
shrouding factor being increased. If the chamber remained
the standard size then the shrouding factor would increase,
reducing the breathing potential of the engine and effectively
reducing the power output. In some cases this is to less
than with the standard valves.
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| Combustion
Chamber Reshaping on a Mini A-Series Cylinder Head |
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Unmodified combustion chamber with
standard valves |
Modified combustion chamber with increased
sized valves fitted |
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