Leif,
It has been awhile since I have studied this... Heat and pressure promote the N and O to combine into NO and NO2 (collectively known as NOx) The larger combustion chamber is more area for the reaction to take place. Fuel/ air ratio are also factors. If you are running lean, you will have more NOx and less HC. The converse is also true. Running stoichometric allows for the minimum combined HC and NOx at the expense of some fuel economy. In other words, you could operate leaner but you would fail NOx emmission requirements. Perhaps the flathead was operating rich, minimizing NOx production.
