Paul,

In case you are still interested, there are 125 teeth on my RX-8 13b
flywheel.
... <snip ...
Gene

So, there's only 125 teeth in the flywheel starter ring?  That's real
good news for using the NXP KMI15/4 Rotational speed sensor.

Restating what I wrote Paul Lamar earlier:
I did a quick search and came up with a sensor that might work for
you:

NXP KMI15/4 Rotational speed sensor
http://www.nxp.com/acrobat_download/datasheets/KMI15_4_4.pdf

It puts out a digital signal that's already cleaned up.  I'll try to
order some samples. (I'm sure others can find something better.)

This part DOES NOT require a magnet, just something ferrous. (Like a
single bolt at the perimeter.)  About $4 each at Digikey.

It looks good for 25KHz tooth rate, WAY beyond your intended use.
That's almost fast enough to sense the flywheel starter teeth!
It could certainly count turbo blades.

Plugging in Gene's 125 flywheel teeth:

MATH:
10,000 RPM * 125 teeth/rev
--------------------------- = 20.8KHz  (or 20,833 teeth/sec)
       60 sec/min

CONCLUSION:
If someone wanted to use the flywheel as a Crank Angle Sensor (backup
or otherwise), this chip fills the bill.  Even at 10K RPM, there's a
17% margin for the chip's operating limit.

The nice thing is that when a belt inevitably comes off, it won't take
out your CAS, since this is on the other end of the engine.  In our
EFI application, the CAS is critical.

If the group is interested, I'll try to build a CAS demonstrator when
I finish the return-less EFI PWM fuel pump.  (Still trying to select
fuel pressure sensors.)

- Vince Orton

So how are you going to know when it is top dead center?

Paul Lamar