Somewhere around 2002 my cousin built a pair of NA 461cid engines which had the Merc Thunderbolt distributors that he took to Tom Earhart's dyno. These were very mildly cammed engines with FLAT tappet hydraulics w/8.6 comp ratio that made 461hp @5100rpm.

I was using the Crane HI-M6 ignition boxes on my own 565cid engines and I had a spare box so, I brought it along to try it for my cousin's dyno test engine---we were able to hook it up without any problems that I can remember to his Merc Tb distributors and saw NO HELP/NO GAIN.

I always have used the the Crane HI-M6 intion boxes on my NA 565's with MSD billet distributors and never did a BEFORE & AFTER/BACK TO BACK comparison test to see if there was any performance difference on my engines, but we had to hook up a relay to retard my advance only while the key was being turned during start-up of my engines. I have used the same Crane ignition boxes now for about the last 10 years or so, and one was even blasted with high pressure water and is still working today. So, they are very reliable in my experience with them vs the horror stories I hear about the MSD ignition boxes. I still have my spare Crane HI-M6 box that I have never used.

I have used an MSD billet distributor and Crane Hi-M6 box in conjunction with an MSD Crank Trigger system. I use the Crank Trigger system to avoid the plagues of the MSD distributors that quickly become rusted and create all kinds of grief and trouble I have had to find out shortly after I bought my AT in 2001.

Of course as someone has already mentioned, one of the things I like about an ignition box (be it an MSD or a Crane) is that they produce a multiple spark at idle speeds to keep the plugs from fowling during long idling periods.

Also, my engine builder likes the MSD ignition box as he has had no problems with them---this is because he always wires them directly to the battery as "rdoactive" has described in POST #2.