Engine rev down speed

how does one make engine rev down quicker, but also rev up normally, engine friction?

 

Try dynamic friction https://wiki.beamng.com/CombustionEngine

In simple terms, that adds more friction as rpm increases, also affects to engine braking, so you get more engine braking at higher rpm.

Friction is constant along rpm range, but you can use that too along with inertia to find out perfect balance for your needs.