In the 42083 Bugatti hipe here is my throw at a fake W16 engine. I wanted it to be small and easy to install. I spent some time on it over the weekend and after some trial and error I got it working.
It has more friction than a mini-V8, but I'm afraid that is inevitable. It all comes very precise. For the fake crankshaft I had to use mirrored cam-pairs, otherwise the sharp side of a cam piece would hit the piston under too much angle, which sometimes caused hick-ups. To make it run smoothly the pistons need a space of exactly the height of a flat plate, no more, no less. Also the end-stops of the 3L piston-axles are essential and the round sides of the bushes attached to the pistons need to face down.
If it wasn't because of the mirrored cam-pairs, a half-cylinder offset in the piston-rows would have been possible .
LXF-file here.
EDIT: I reworked the mini engine into a 1:8 scale version. Slightly different configuration.
If you build this:
Be sure to level the bottom-ends of the 3L piston-axles with the bottom-ends of the bushes by pressing them against the flat side of a beam.
Sink the piston-axle-end-stops all the way into the 12t gears.
Make sure the round side of the bushes face down.
Instructions can be found on Rebrickable.