For some reason, I've ended up pondering whether the K75 & K100 Fi modules could be interchangeable  .

I'm thinking that fitting a K75 module onto a K100 would increase the fuel delivery by 33%.....ish, very ish 

And LP records turned at 33rpm ...ish.

Coincidence?

To remember your very very interesting link, I'm filing in my ethics folder, after my latest entry "Never date a woman who doesn't respect your wife"

"And LP records turned at 33rpm ...ish.
Coincidence?"

I arrive outside my front door, my dog is on the inside, he sees me, he wags his tail.
I knock on the door, he sees me knock, he barks because the knock tells him a stranger has arrived.
His learning is based on links he has made, he is hungry for links, we are hungry for links, all learning is links. What separates the dog from us is that he has no need and hence no ability to rationalise, and this prevents his further learning.

What you are doing is good, very good, but you need to rationalise a bit 

No.

 "Y'see George, if ya connect a K75 FI module to the AFM on a K100 doin' three thousand rpm, surely it's gonna think it's on a K75 doin' four thousand rpm an' it'll just bank fire the injectors the same!"

Are you going to try it? I first thought it was a joke.

I'm not sure, but I think the difference that may cause problems is that the K75 has three coils, one for each cylinder, while the K100 has two coils with each one firing two cylinders. Ignition sync signals are probably different between models. Also, see the Hall Sensor Plates are different, so timing pulses may be incorrect.

Might it confuse 4 and 3 and sound like a V12?

chris846 wrote: we are hungry for links, . . . 

A Lorne sausage in a buttery roll is just as satisfying; furthermore, you can use it to train your dog when to bark and at what.

Hey if it sounds like a V12, that's a result in my book!

I think the Fi module on the LE Jetronic gets a firing signal from the ignition module, and somewhere in the dim and distant I sort of think this is once per revolution - the Jetronic bank fires on the exhaust stroke as well as the firing stroke.
How this works out with a 120' 3 cylinder I dunno. There's only 2 hall sensors on the '75, so I'd guess that the ignition module sends a firing signal to the FI module once per revolution just the same?

There's only one way to find out...

It seems such a waste to have a wasted spark when you have three cylinders and three coils.

Well it seems the postulation is a little bit right, but mainly very wrong.

After a bit of research - reading, not doing, yet - I now think that the AFM doesn't in fact contribute a great deal. The ECU is built so's the injectors fire the right amount of fuel for a cylinder's worth of air every time the ECU receives a crank pulse from the ignition module (okay, actually half a cylinder's because they bank fire every revolution) and this amount of fuel is pretty much constant for much of the rev range. The rate of fuel consumption goes up as revs increase, but this is due to the injectors firing more rapidly. Makes sense when you look at it like that.
According to the Corky Bell book on supercharging (sold to me as the 'bible') the AFM is only there to modify pulse width a bit at lower revs, in order to improve driveability.

I was thinking more that the Le Jetronic was an electronic progression from the K-Jetronic: where an air flow meter mechanically opens constant flow injectors. In K-Jetronic the rate of fuel delivery is driven by air flow, not crank pulse. Still think it's worth pursuing the Le Jetronic a bit further for the supercharger though.

It'll be interesting to see what's happening on an oscilloscope. That's next 

EDIT: after years of struggling to find detailed info about the secretive world of Bosch fuel injection, I've just discovered a PDF link on the M***brick site (probs on here on this forum too, knowing my idiocy) that diagrams the 'map' linking airflow and pulse width. This suggests that the airflow meter's output is used by the ECU to determine engine load (ie. low opening/high rpm = low load, high opening/high rpm = high load etc.) So it's more instructive to view the AFM as a load meter....I think I'm getting it 

I suspect that the airflow sensor is adjusting pulse width to provide the correct amount of fuel for the air being pulled into the cylinder.  Since the amount of air being sucked into the cylinder on the intake stroke varies as the throttle opening, the fuel delivery needs to be matched to that volume of air.  

The Motronic takes throttle opening directly from the shaft the butterflies are on and calculates fuel from that.  The Jetronic, on the other hand, infers throttle opening(and the resulting airflow) by the deflection of it's flapper.

I think this airflow information is more important than just for fine tuning the fuel mix.  The air taken into the cylinder on the intake stroke can vary over a pretty wide range depending on throttle opening.  A constant flow of fuel would result in a mixture that would be incorrect most of the time.  

I think you are correct in thinking that the airflow meter is determining the engine load.  Under heavy load/wide throttle/hard acceleration there is maximum air flowing into the cylinder and thus maximum compression stroke air pressure.  Without the airflow information, there is danger of detonation and serious engine damage because of a lean fuel mixture.  That is the reason carburetors had acceleration pumps that gave a squirt of fuel when the throttle was opened rapidly.

If it was my project, I would try to use the Motronic.  Not only does it eliminate the airflow sensor, but my experience with OEM engine control systems tells me that the airflow sensor's barn door doesn't give the throttle response of the Motronic's throttle position sensor.  There is a noticeable difference between the two systems when the throttle is snapped open.  My experience, especially with my K75, is that the Jetronic doesn't respond well to snapping the throttle open, but rather prefers that I roll it on.  The Motronic does not have the Jetronic's momentary hesitation on rapid throttle opening.