<<If
you're running lean of peak, you don't have any oxygen molecules in the
exhaust.
 If you're running rich of peak, you don't have any unburned gasoline in the
exhaust.

What you *really* have is an exhaust product mixture containing largely
carbon dioxide and water vapor, but also partially reacted ingredients and
partially decomposed (the precise term is "dissociated") products.>>

There is a germ of truth in the above statement, but in the big picture, yes
there is free oxygen in a lean exhaust and unburned fuel in a rich exhaust.
Right at stoichiometric there is a combination of CO and NOX (oxides of
nitrogen) so as you go slightly rich of stoic there will likely be no
totally unburned fuel.  It will consist of a fair amount of CO, H2O and some
free HC - and a small amount of free oxygen.  Just on the lean side the CO
will, as George said, go essentially to zero and there will be more NOX, but
there will still be some free oxygen present.  But we're not talking about
within a percent or two of stoic, but some distance away.  The question that
has existed over the years is whether the free oxygen will attack (oxidize)
exposed high temperature metal.  I think the evidence shows that with modern
exhaust valve materials it doesn't.  There are lots of high-output engines
that routinely run LOP.  All diesels, for example.  Large natural gas
engines run very high manifold pressures and LOP.  And don't forget the
R3350's.

Gary Casey