Don, I'm running a 6A on my 77 Mustang and a 6AL (rev limiter) on the 57 Plymouth (my distributor has been converted to stock Chrysler electronic).

On the Ford (Duraspark) and Chrysler units the 6A and 6AL are stand alone and replace the factory control modul. In the case of GM HEI it works through the in distributor modual.

The Ford (351W) runs Edelbrock Preformer RPM heads with no provisions for a heat crossover package.

The Hemi in the Plymouth is 10:1 compression and a "bit" over camed.

I've had a chance to run both engines on both the stock iginition and with the MSD boxes.

In both cases each engine is cold blooded during the winter (we get to 15-20 degrees some mornings.) The MSD boxes do a LOT for cleaning up the idle and cold drivability problems. Yes I realise I'm only curing a symptom and not the real "problem" with either engine but the boxes do make a marked improvement.

In the case of the Ford there is no noticable difference between factory and MSD systems once the engine has reached full operating temperature.

On the Plymouth the idle seems to be a bit cleaner across the board with the MSD.