If you are having your turbo overhauled anyways,
consider having flats milled on the shaft at it's
mid-section.  Machine the center casting to accept an
inductive prox sensor that senses the flats as the
pass the shaft.  Now you've got a rugged, reliable
method of measuring shaft speed.  This is the method
Allied Signal used when we requested instrumented
turbos for test purposes.

Roger Enns.

There are many ways to skin the "turbo tach" cat.

I've read about opposing black/white paint on the shaft end, painted nut
faces, shaft magnets being picked up with inductors, etc.  They all have
their drawbacks.

OPTICAL
-------
Optical solutions usually have dirt/dust/grit problems, and involve an
illuminator and detector (LED and photosensor) in the intake path.  This
interrupts airflow, can be ingested, or can move in fast airflows.  The
best solution I've seen used drilled holes in the turbocharger housing
surface to mount the optics/components rigidly, to one side, out of the
intake airflow.

MAGNETIC
--------
Magnetic solutions usually involve magnetizing the shaft or
embedding/attaching a magnet.  Any shaft modifications require precise
balancing, since this thing will spin FAST.  Then we have to get a
sensor in there to detect the magnet.  Close, but out of the airflow,
otherwise we end up with movement and ingesting problems again.
Somewhere between the compressor and turbine sections seems reasonable.

Roger's idea of a flat on the shaft should work, but again, that shaft
would have to be precisely balanced.

*****

I'm a fan of the optical method.

1) Drill a hole in the shaft end, as if to put in safety wire.  The hole
needs to be big enough to let light through easily.

2) Drill opposing holes in the compressor intake housing, 180 degrees
apart, big enough to see through.  More importantly, big enough for a
photosensor to see a LED/laser diode through.

3) MAKE SURE the shaft hole lines up with the housing holes.

As the shaft spins, you will get TWO pulses through for each rotation.

From there, it's a simple counting and timing game.  Simple circuits.

I like it because there is nothing in the airflow, and the components
are securely mounted on the exterior of the intake housing.

Now, go ahead and shoot this one up guys.  :)
1) Can an LED/photosensor work 3-5 inches apart?

2) Are there any convenient housing hole locations that will line up
with the end-of-the-shaft hole?

3) Will dirt/dust/grit be a problem?

I'll do the circuits if someone drills up turbo charger for testing.  :)

Paul - If you would, please 3-D this for everyone.  Your pictures are
worth a thousand words.