Downshift or exhaust brake?

Me Again wrote:
OK, I found this on the TDR site.

https://www.turbodieselregister.com/threads/boost-readings-with-the-exhaust-brake-on.238694/

Claims 5-7 PSI of boost on VGT turbo's, which is pretty low.

"The boost is from 2 reasons. The main one is the turbo is spinning fast enough to move a bit of air. The seconds is the restriction from the turbo. The airflow that is making 5-7psi would not be enough airflow to make 5-7 psi if there were not an exhaust restriction that is leaving up to 60 psi in the cylinder. Boost is not a measure of flow, but of resistance.

You can still only have 60 psi of back-pressure, and that is what is creating the retarding horsepower.

There are other features, like cam profile, that will effect retarding power as well."

The retarding power of the exhaust brake is calculated by airflow x back pressure .... you need both. Double either one and you double the retarding horsepower.

Although the boost pressure increases airflow which contributes to increasing the back pressure it also counter acts some of the braking effect of the engine by pushing down on the piston during the intake stroke.

The more I think about this the more factors come into play. For instance there is a maximum back pressure that can be achieved based on the strength of the exhaust valves of an engine. When the intake valve opens the back pressure is wanting to push the exhaust valve open. I don't know how much back pressure can be generated before the exhaust valve would begin to float? If the turbo is creating boost, the positive pressure on the intake side of the engine would serve to raise the maximum back pressure that could be utilized. Overall there is a net benefit to an engine's braking ability by utilizing turbo boost.