Combustion 101
The Timing

So how does the camshaft know when to
open the valves?  The answer is that it is
connected to the crankshaft (which is spun
by the cylinders) via a chain or a belt.  The
crankshaft and camshaft spin together.  
However we learned that the valves only
open in two of the four strokes of the
cylinder cycle.  If the camshaft and
crankshaft spin together they will open the
valves every time they rotate.  The solution
is that the crankshaft spins exactly twice as
fast as the camshaft.  So for each four
stroke cycle the camshaft spins only once
while the crankshaft spins twice.  

The timing of the valve opening and
closing must be established precisely.  If
the intake valve opens to late or closes too
early- it won’t let enough air and fuel into
the cylinder.  If the exhaust valve opens
too early- it could let the air fuel out before
its fully combusted- losing horsepower.  

Now if that isn’t enough pressure, the valve
timing must work well when the engine is
spinning slowly or when its spinning very
fast.  In fact, that is impossible to do.  The
valve timing is always a compromise and
set for conditions a majority of the time.   
For daily drivers- the valve timing is set for
an engine spinning around 1,500 to 4,000
RPM—typical driving.  For a race car the
engine is optimized for engine speeds in
excess of 5000 RPM.  An engine that is set
up for racing usually runs worse than a
comparable daily driver at low rpm.  Engine
designers and hot rodders can make
adjustments to the valve timing-by
changing the camshaft depending on their
RPM needs.  
The Camshaft
The Camshaft
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