https://github.com/EpicGames/UnrealEngine/pull/3972
Hi, right now the only way to change the substepping code was to build the engine from source and change what you wanted to.
Today I took some time to make it more modular, with my changes you can edit how the substepping behaves, for example add fixed timestep without the need to modify the engine. I added a new class which can be inherited from and assigned in the project settings and functions can be overridden to change the behavior. I plan to send this as a PR tomorrow.