I have an MSD 8920 tach adapter. I purchased it thinking it would fix my tach problem. After hooking it up correctly I thought it was defective, I sent it back to msd and they tested it and there was nothing wrong with it. It turned out to be my tach output on my summit ignition box. Now I got a new box, and the tach works without the 8920. I have the receipt for when it was purchased, and the slip from MSD when they tested it. If anyone wants it let me know.
I can take pics if needed.