How can I give a constant time to physics engine by command

Dear fellows,

I am working on a simulation for virtual creatures. There I sometimes need a game mode in which
the game loop runs as fast as possible without rendering or sound (dark, silent screen) or so (lets say 1000fps).
Just at each of these 1000 times per second, the pysics engine should be stepped by a fixed time of 16.666ms.
That means that I can simulate physics with 1000 times the real speed (with a timestep of 16.666ms).

I know that Unreal has no fixed timestep but can I tell the physics engine in unreal by command to step with a certain (fixed) time?
Do I have to change the source code?

Any help is greatly appreciated as I am really stuck.

Thank you guys!!!

Alex