Agree

http://www.rinda.com/acro/EFI_Article1.pdf page 2 bottom left. How could an ecm retard the timing if a bunch of springs and weights or a different controller like a Thunderbolt IV were providing the advance?

ok, landfish, where were we? you need a distributor for this MEFI setup. I assume since you mentioned thunderbolt 4 it was originally a carb engine and this efi system is new, at least to this boat?

The external coil GM all electronic HEI distributor and coil are used on the Mercruiser 502MPI engines + many of their other fuel injected engines. They can be found around $180 -$190 new. I believe ACDelco's # is 1104060. Double check. $184.00 Amazon Prime - note I believe a 1-2 week backorder. The coil for this is ACDelco D560 GM Original Equipment Ignition Coil ($59.92 Amzaon Prime) and comes with the bracket.

If you have the 502MPI wiring harness, then is all a no brainer.

Ok, that number comes up as pass car distributor. Is there any functional or LEGAL diff between auto and marine hei distributors? I run auto stuff, but every time I suggest it people start screaming. Merc's number is 805222a. 600 bucks. I think the only diff is that shiny black paint. It seems to triple the price of everything.

No. The external coil HEI Merc distributor is the auto distributor with no modifications. Bonus !

If you want to use it in a carb'd application you can, just make sure to use ACDelco D1965A ignition module. It has an advance curve. Short quick one, but it does have one. It is the one used in the Voyager systems.
ACDelco D1965A GM Original Equipment Ignition Control Module
by ACDelco
Link: http://a.co/7q8RS2v

Just wire the distributor accordingly.

I have been using a MEFI 1 for 22 years with the stock Merc distributor and coil that SB displayed. I have also been tuning it for the past 7 years. I can assure you the entire advance curve, or more specifically 3D curve is controlled by the MEFI unit. You merely set base timing mechanically then tell the MEFI what that number is and it references the Ignition Advance vs RPM vs MAP table to control the ignition.

Beyond basic advance it has spark idle stabilization until a certain rpm threshold at which point it references the advance table. Some units also have mean best timing (MBT) at stabilized rpm points, F1 slope extension etc.

Thanks for the help guys. Learned a lot of things. Going with the AC distributor and coil. Shop I'm using for work is great, but they wanted to use Sierra. Had a few bad sierra electronic items, and glad to get the help to get the correct Delco parts.