[Power Hungry]

Here's the short version...

Diesels to not care about AFR. If you spray it, it'll burn it. If it runs out of air, what's left comes out as black smoke. No huge deal, but black smoke is simply a waste of good fuel.

EGTs are an indication of several things, but high EGTs can almost always be found on trucks that bellow black smoke because the fuel is still burning outside of the combustion chamber. To make power you need more fuel, but then you need more air to allow the fuel to burn completely in the cylinder. Turbos use heat energy to spin so when you improve the combustion process, the residual heat energy falls off and the turbo slows down causing the fuel to not burn completely and raising the EGTs again which spins the turbo up which reduces EGTs... and so on and so on.

The idea is to keep just enough fuel in the system to keep the turbo operating in its peak pressure map without introducing too much fuel and generating excessive EGTs. It's definitely a balancing act.

Hope this helps.