Hi Derek,
The sound you are listening to is a short sound recording of a real engine (so it could be ANY engine type) - which is then digitally manipulated (bit rate changed/playback volume adjusted) that gives the illusion of the engine running faster/slower.
The same sound file in this simple example is being played twice simultaneously at different bit rates to simulate 2 engines running at the same time but at different speeds.
The sound produced is linked and proportional to the RC handset throttle (some AI will be added to this at some point to make acceleration/de-acceleration more realistic) Engine start-up/stop sequences will be included. I have some great recordings of engines struggling to start-up which will hopefully add to certain engine types sounding authentic.
The biggest challenge is finding good quality recordings of engine sounds without "interference" - e.g. Wind/Voices/Music etc - If I get to a point that it seems worth it I would commission some high definition recordings of various engine types.
Regarding volume that is as always simply down to how much space you have in any particular model to place your kit and how easy it is to "vent" the speaker to the outside world
For now I am going to use 2 x 5" inch speakers and a 20watt Class D amp - you should be able to hear this @ 100feet!!! - easy to get sound out of the this boat under the dashboard


Do you have an engine sound/type that you think would be a good example/test to try?
Regards
C-3PO