Engine sounds are not output as individual effects that we can control but contained within the audio mix. In most racing games 5.1 / 7.1 is rather overrated. Often the center channel is a mono mix of the LR and the .1 channel is underused. Stereo or indeed Quad Surround (front/rear stereo pairs) can be excellent, especially in such a confined area as a sim rig.
If applying a front/rear based fader control, (via software or hardware) this would enable higher/lower volume to the front or rear speakers but it will do this for
all sounds, contained within the mix and output to those channels. not just the engine sounds.
Here is an example of a compact potentiometer-based fader control I can recommend that requires no power adapter to adjust the gain how you want on 4 channels.
Creative Solution?
To give you some perspective on the experimentation work I've been developing.
Custom profiles applying my own (IP) that I have been working on via Simhubs tone generation. It is possible with a lot of time/effort, that I can monitor the audio output generated from a specific car's, engine character.
The reason is to create an advanced, Simhub based RPM/SPEED set of effects that tonally match well with the specific car driven. This then can be sent to transducers and exciters to generate, felt vibrational energy and audible harmonics that match well with the car's engine sounds. In doing this we, generate a 3D type audio/vibration experience from the cockpit that aligns with what you hear from the cars engine sounds.
Because Simhub gives us full control of mapping effects to any channels we want, then yes in this scenario we can place the accompanying felt engine energy and audible harmonics to the front or rear tactile using whatever volumes we want to apply to suit front-rear or mid-engine mounted cars.
From my own perspective in pushing tactile further, this is one element we can apply to give different cars more unique felt sensations and not just typical and sometimes a bit boring tactile effects that feel very similar on all cars.
Currently, however, this level of immersion is something I have not seen anyone manage to accomplish or any Sim cockpit company offers. But who knows, maybe we will see something in the near future?