Turbos (and the engines they're attached to) should maintain RPM/pressure/engine power at part-throttle. Here is a great real-world testimony of the effect turbos have on power modulation when spooled. (Skip to 4:45, I don't know how to make the embed work with timestamp links) --- Post updated --- Maybe all that needs to be done is feedback pressure into ExhaustPower? Diesels shouldn't be as affected as gas engines. Not sure how they would differ exactly.
See, I can't help you because I only use a keyboard. Having said that, I'd be shocked if the turbo boost was anything less than realistic to real life, as the engine still puts out exhaust gases.
When a turbocharger is spooled up, the feedback loop between impeller and compressor forcing air past the throttle plate, if I understand Sloppy correctly, lowering from 100% to 50% throttle only reduces turbo RPM/intake volume/engine output (at least over a few seconds), by 10-20%, where in an N/A application, it might be 35-40%--close enough to feel linear. Ballparking to convey the concept... With no BOV there should be "lift-throttle lag", basically.
I am not an expert on charged engines but if you enable the turbo debug app (not quite sure about the name) you will notice that Diesel engines "bleed" boost naturally when you decrease the engine load as their intake air quantity is usually not changed. Petrol engines - which are always throttled - loose boost immediately but you can see in the detailed debug app that turbocharger RPM actually only slows down instead of stopping (which makes sense as mechanical stress would be severe) so after you restore full throttle boost returns almost immediately as the turbo is still spinning at almost peak RPM. --> This is precisly the reason for the blow off valve being a thing, the - now - heavily throttled petrol engine cant handle the amount of intake air generated by wide open throttle + turbo and the resulting pressure rise - without a release valve - would likely put severe stress onto all related engine components. Turbos in BeamNG also work at partial throttle (you can test with the cruise control app) but at idle usually spin down completly with - I would assume - is somewhat unrealistic as even an engine on idle does create exhaust and as such the turbine will spin if not bypassed although naturally not at very high RPM. This is not about boost.... this is about actual turbine RPM mind you. Of course boost is poor/nonexistant when you are deep into turbo lag.... but in BeamNG the turbine actually comes to a complete stop when the engine is at idle. I would assume that in reality it will spin at low RPM in most cases. This shortcut however probably isnt very relevant. If your turbine is spinning at low RPM or not at all wont drastically affect spin up time once engine load increases. Also developers know about this and base all their calculations on this behavior.
Without a BOV, shouldn't turbo RPM and boost shouldn't be 1:1? Whereby, intake boost feeds back through to the turbo, keeping it spooled, sustaining boost, et cetera, lessening the effect of moderate throttle reduction on immediate output. Blow-off valves bypass boost pressure to reduce stress on the turbo when the throttle is closed quickly. As long as the wastegate is functioning correctly, boost (BOV or not) won't exceed a safe level for the fuel system and engine internals.
It's actually the opposite. when the throttle snaps closed, all that air having no were to go will force itself back through the compressor housing, effectively stalling the turbine as it trys to spin the opposite direction.
