Does anyone know exactly how wide the HEI tach reference pulse is?

Also, to verify: it's a square wave that goes high (5V) once per cylinder, with a falling edge that occurs at 10* BTDC (after alignment with a timing light).

 

To be clear, the TACH signal in most L-98 type cars was taken from the coil
primary. The plug fires when the coil goes from 12 v 'conducting' to 'open'.


If you're talking about the HEI module ECM reference pulse (DRP) this may be
what you're looking for:
http://www.megasquirt.info/ms2/heiest.gif

It's from this site - look under the 'Ignition - GM HEI section' :
MegaSquirt-II - Advanced Electronic Fuel Injection Computer by Bowling & Grippo ©2005

The falling edge of the DRP corresponds to firing the plug. Reference pulse
duration looks to be about 50% duty cycle on a 7-pin HEI ICM.


Further notes:
For more discussion of the 'zero-crossing' reluctor trigger, read the application
notes for National Semiconductor's LM1815 chip. It appears to be similar to
the circuit that GM use on their VR triggers:
http://www.national.com/ds/LM/LM1815EP.pdf#page=1


HTH.