Hello cz26m,
Quote: |
| Could it be that the TDI already runs with so much excess air in its standard configuration that adding more air doesn't improve combustion any further, and that increasing the fuel quantity is only effective when the boost pressure is also increased? |
That's exactly how it is

, you've already provided your own explanation.
The only improvement you achieve is the reduction of particulate matter in the exhaust (less soot).
Instead of an increase in engine power, a slight decrease in power is more likely, as the additional drive energy provided by the turbocharger causes a higher exhaust back pressure.
Regards,
Alex.
AUDI A3 1.9 TDI, EZ 12/96, ursprüglich MKB AGR, umgebaut zum AHF mit GT1749V-Lader, verkauft mit 250tkm
Golf 4 1.9 TDI, EZ 1/98, MKB ALH, jetzt auch mit GT1749V-Lader, verkauft mit 300tkm
Touran 1.9 TDI, EZ 09/2004
Audi A4 Avant 2.0 TDI, EZ 03/2010