Running too lean?

Motordyne

^+1

Soot can deposit on the tips and bumper even if the engine is running lean. And yes, running rich will make more soot but its not all about the A/F ratio, combustion efficiency is why its still an issue with lean running engines.

The reason there is soot is because of incomplete combustion. Even if run lean, not all of the gasoline is completely reacted with the oxygen. The partially oxidized gasoline creates carbonacious particles or soot.

This is where the stock catalytic converters do their job of cleaning up the exhaust flow stream. The stock cats "catalyses" the remainder of unreacted gasoline and force it to react with any remaining free oxygen to form water vapor and carbon dioxide. Both of which are much better in terms of cleanliness.

In the case of the 370Z engine, it won't allow the A/F to go to any extreme for very long. The objective of the ECU and stock cats is to burn all the fuel. Between the ECU and the stock cats very little soot is emitted.

But once test pipes, ART pipes or HFC's are used, some soot will make it through. The best that can be done is to tune it on the slightly lean side of optimum and go with it.