Yes, this is quite do-able. You will however need to retro-fit either an inductive or optical pick-up to your distributor in order to direct the single "common collector" output (that's what it's actually called) from the Motronic to whichever coil is to be fired. I guess that you could (in principle) use the distributor rotor to form some form of crude mechanical switch in order to direct the motronic output to which coil you want to fire, but I have never seen this done, and I suspect that it would be unreliable. Rotors and distributor caps are really only any good for routing the HT (high tension) generated by a single coil to individual plugs.
One way that I believe would be both functional and reliable, would be to fit six optical sensors around the inside of a modified distributor cap. When the rotor tip interrupts the beam from the LED to the opto-transistor it will indicate which plug needs to be fired, but you would still use the output from the Motronic to time the firing of the actual coil. Since the accuracy of timing is down to the output from the Motronic, the "routing" signals from the optical sensors need not be that accurate. In other words, they only indicate
which plug to fire, not
when to fire it. Optical sensors are commonly available and take the form of a little plastic ”aU”a with an LED on one side, and a light sensitive transistor (switch) on the other. As the rotor tip swings through the ”aU”a it will interrupt the light beam, providing and indication as to which coil to fire. This sounds like a lot of PT to me though, and could turn out very Heath Robinson if not careful. You would have to mill a distributor cap to get this right. TBH, I can’t see the point to all the effort and expense.
A more professional procedure for doing what you want, would be to fit a six-pole optical or inductive (magnetic) pick-up around the shaft at the base of the distributor, and then use the output from the pick-up to decide which coil needs to be fired. These are available commercially I believe, and there are companies that specialise in this sort of work. The distributor cap is then replaced with a blanking plate. I can’t imagine that this would be cheap though.
Whichever route you followed, however, the six outputs from the newly installed sensor in the distributor will be both low voltage and low current, and cannot be used to fire the coils directly. To make your 6-coil ignition system work you will need to purchase six ignitor circuits or a six-channel CDI (capacitive discharge ignition). I'd only recommend the CDI if you are running forced induction or very high compression ratio, otherwise six little "ignitors" will be cheaper and no less effective than a CDI. You'll then need to combine the output of the Motronic with the six ”arouting”a signals to the trigger inputs on the six ignitor circuits, which then, in turn, connect to the six coils.
I can provide detailed info on how to do this conversion if you decide to go ahead, but TBH, it’s more PT and money than getting rid of the distributor is worth. The quickest and cheapest route to what you want is to install an aftermarket ECU which already has multiple ignition outputs. Tim_Haynes, for example, has a Gotech-Pro on his supercharged M20B25 with a 6-channel CDI that I built for him (see
here) that fires six pencil-coils (COP), but you could use any ECU of your choice with a set of ignitors.
HTH
Geoff
