Ken,

When an engine is designed, there are power gains to be made by
increasing compression ratio.  One of the determining factors that goes
into achieving higher compression ratio is minimizing the volume (air
space) captured in the combustion chamber (defined as the space enclosed
by the top of the piston, the exposed cylinder walls, and the cylinder
head (with valves).  This same space that you are trying to minimize
also gets invaded by the valves as they open (projecting into the
combustion chamber).  If you move the piston up further to reduce the
volume, it moves closer to the valves that are trying to open as far as
possible for good breathing.  So - bottom line - when optimizing the
performance, along with optimizing the shape of the chamber boundaries
for proper flow for performance and emissions, you eventually will hit a
point where the valves and pistons hit if they are not synchronized
(i.e., timing belt breaks).  Normally (with good timing belt), that is
not a problem as the valves are closed or only partially open when the
piston is all the way up (at "TDC" - top dead center).

The manufacturer can do things like machine divots (reliefs) into the
tops of the pistons to allow higher compression while making room for
the valves to be open with piston at TDC, or add more valves per
cylinder to allow good breathing with lower valve opening height - but
such measures can add cost and/or degrade emissions.

So, basically, everything is a compromise.  The manufacturers bet that
there is more gain to their bottom line by giving higher performance,
while minimizing their costs even if it increases maintenance costs or
risks of damage (which is probably a safe bet since 90+% of consumers
are ignorant of the risk anyway) - the old "What the customer doesn't
know won't hurt them" philosophy.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
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