I use these all the time for professional purposes (although I prefer Sony ActionCams) and while I haven't mounted one on my 914 for personal use, I'd suggest trying it on the inside of the rear window facing forward. You should be able to see a nice combo of the road and hood along with the gauges and wheel/shifter and maybe a little bit of the passengers.
This should work well with the top on but might even be better with the top off. I much prefer window glass and a suction cup mount to anything. Paint on a flat surface as a second choice - also with a suction cup - and then the adhesive mounts are last.
If you can go on top of the roll bar with the top off - that might be better but would limit your mounting options and prevent you from putting the top on quickly. If it's inside, you just put the top on and you're done. Inside the cabin would limit wind noise and pick up a little engine sound along with cabin conversation to a point.
One thing - I almost always externally power any action cam. You might have to make sure that your unit can do that - it may just be a setting that needs to be activated. Batteries on those things are notoriously short-lived and seem to die at the worst time. a long usb cable with the appropriate connections to the cig lighter and you're good to go.
I'd shoot 1080/30fps (29.97) as it'll look a little more "cinematic" - 60fps (59.94) will look a little more "video-y" - but that's a matter of your preferences. 4k will chew card space and make unwieldy-sized files and probably make the camera overheat and shut down.
oh - and if you mount it on the outside, tether it.
trust me.
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