The way I figure it . . .
Our engine simply converts potential energy into mechanical energy at about a best of 35~40% efficiency (don't quote me on that number).
(Ignoring all the fine-print and speaking in broad, general terms
) . . .
- Advancing the injection event timing (start of combustion event in our application), allows more time for the heat of combustion (potential energy of the fuel) to be converted to mechanical energy at the crank.
- With more heat of combustion now being converted to mechanical energy at the crank, there's less energy (in the form of heat) going out the exhaust manifold. This is illustrated by the manifold EGTs now being lower.
Turbo-chargers simply convert potential energy into mechanical, then back into potential energy. This, is illustrated by the drop in EGTs across the turbine, and an increased pressure differential across the compressor.
With advanced injection event timing, there's now less energy (lower EGT) in the exhaust gases to power the turbine, thus, less boost.
Retarding the timing, means more heat in the exhaust (higher EGTs), resulting in more power to drive the turbine, thus a higher boost.
(insert lots of fine print here).
Five of the seven voices agree.