I Got an MSD 6A off of eBay for $75.  It was originally installed in my 924, but because of the direction I am going with that car I am transferring all my upgrade parts to the 931.  I would recomend the MSD to all people with failing stock ignitions, or people who are going to be running high boost.  This could be taken a step further with the 931 by getting the MSD 6btm.  That unit has a pressure sensor and can be set to retard the ignition based on boost pressure (1-3 degrees per PSI, up to 15 degrees of retard), this may be a planned upgrade for me, if I don't go for the Electromotive HPX direct fire ignition system.  The diagram below is specific to the 1980 931, which does not have the digital ignition timing computer that later 931 models had, therefore this setup may not work on later 931s.  No tach adapter is needed for this installation.
			Above, the MSD unit is bolted in place of the original Bosch CD unit which makes it easy to wire.