[mpgmike]

That makes my point perfectly! It isn't enough to simply burn the fuel, it really matters when it burns in the combustion cycle, and how quickly. Burning out the exhaust manifold contributes absolutely nothing towards making power at the wheels. However, the emissions analyzer will show that it got burned.

Engineers use the term Critical Crank Angle (CCA) to denote the optimal phase of crank angle where highest peak cylinder pressures do the most good; usually 17-18 degrees ATDC. With a slow burning charge, it takes more ignition timing advance to build pressures by CCA (which can have disastrous side effects). Faster flame propagation and oxidation rates allow more conservative ignition timing while still delivering the goods in time. Furthermore, a faster burn rate is more likely to oxidize more fuel within the combustion cycle than a slow burn. In other words, a faster burn is able to release more of the chemical energy within the "magic window" when the engine can convert it to usable kinetic energy.

So your observation that it is not an issue of unburned fuel is correct; it's an issue of when-burned fuel.