According to the MSD literature that came with my coil, they said that for best performance to mount it vertical, but there was no warning about premature failure.
I mount mine on the front of the cylinderhead next to the distributor so that the main coil wire stays as short as possible.

 

Yes vertical, if it's an oil filled type ignition coil (I don't believe it's necessary if it's epoxy filled).

 

most of the oem coils from FoMoCo were either horizontal or at a slight angle.

 

I probably shouldn't jinx myself like this, but my MSD coil has been mounted horizontal for over three years now.

Still working!

Greg

 

One thing I see happening is the screw in the center terminal coming loose and the oil dripping out. Happened to mine.

The other thing I see happening is with the coil mounted sideways AND if the oil level is low then one entire length of the coil is not submerged in the oil therefore not getting cooled and it would ultimately lead to coil failure.

Just my thoughts.

 

FWIW I ran one horizonal for about 2 years on my old wagon. Hit the wall at ~100 mph. Then ran the same coil (vertical) and MSD box on my 65 Mustang for another 2 yrs before I sold that car.

 

I had an Accell Coil mounted horizontal and the wire kept melting off...turned it vertical and problem went away.