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'79 to '81 E12, Ignition control unit ( aluminum box inside fender, above charcoal filter ) look the same. Same size, same part number.

'78 633, or 635 Euro (swap hybrid) the Ignition control unit is larger. As I've gotten the idea L jet big sixes share a great many parts ( ECU's, wiring harness, distributors (?), depending upon specific set up details,...

Are the Ignition control units interchangeable ? Distributors ?

Not sure what ICU's do, to swap experiment and toast things, or what tiny differences lurk under the cap and rotor.

Thank you,

I would guess they would work if the plug is the same. For troubleshooting I tried an E12 unit on an E21 (which uses a much smaller one) one time and it worked. Well, I didn't run the car, but it told me the control unit I was using was bad. It's possible they use different electronic parts in the distributor, but my guess would be those are the same and the difference is in the advance curve, both vacuum and centrifugal. Different springs and weights mostly. It could be a very early euro electronic ignition distributor might be a nice pickup and add some torque.