Examples can be seen here.
Installation requires some careful and precise drilling and threading in addition to removing and re-installing the turbo itself, which is something I don't want to do for my personal experimenting.
Now I am wondering if the speed of the turbine/compressor can be determined acoustically. I mean, you can easily hear the distinctive sound of the turbo when it revs up/down.
The question to me is, what kind of sensor could be used to pick up the turbo's sound?
I'm not sure what frequency you hear from the turbo, but it must be either its rotational frequency (up to about 200000rpm/60s~3.3kHz), or the same multiplied by the number of blades (~13 -> 40kHz).
Installation of the pickup inside the engine bay will require, among other things, mechanical ruggedness and temperature resistance to at least 100°C.
Can a cheap piezo ceramic plate do the trick?
Are there special microphones for this kind of environment?
Knock sensors come to mind, but I'd prefer something contactless, especially not needing to be bolted to the turbo, see above :)