Ok, I think I understand this.
I'll go add those on my drawing...that should help me understand your changes better...

Thanks!

PS - Any ideas on an inductive pickup for this ? So I don't have to splice into the wire with the pulse train ?

An inductive pickup off the tach wire, if it were even possible (which I doubt, because the current is probably too low), would give you a differentiated output (short alternating positive and negative pulses) which would be too narrow to work with the LM2907. You could use additional circuitry to reconstitute the square wave, or add a oneshot, but it might not be worth it (even if it would work).

 

Ok, here is the updated drawing.

I included the components you mentioned and added on your drawing.
And I think I know why you suggested those.

I also added the values needed for the LM2917 so that I have a trigger range of between about 262Hz and 550Hz (which would give me adjustment from 7,800 RPM to 16,500RPM).

Or so I think based on my rusty math...

Let me know what you guys think.

(and if you have ideas about inductive pickup of the pulse train instead of splicing)

 

An inductive pickup off the tach wire, if it were even possible (which I doubt, because the current is probably too low), would give you a differentiated output (short alternating positive and negative pulses) which would be too narrow to work with the LM2907. You could use additional circuitry to reconstitute the square wave, or add a oneshot, but it might not be worth it (even if it would work).

Ahh, I posted another message just after you posted this.

New drawing above this post.

Hmm, ok.
Maybe something to try later on as this progresses and I figure out how its working in real life...

 

From LINEAR APPLICATIONS HANDBOOK 2, NATIONAL SEMICONDUCTOR,1977 RADIO SHACK # 62-1374

 

I think Napoleon's speedo signal is a 5V p-p square wave. Speed is pretty tough to derive from the ignition system.