I'm curious what you're running? Not stock power with stock merc. Who's doing what with ignitions. What problems have you seen? What do you like and dislike?

I've been talking with Joe, MT about some 600hp 502s. We have not yet discussed this so I was curious what the concensus was.

I know that I own 4 Marine MSD boxes that are only good for boat anchors...lol. I had mercv6 module ignition on my 700 hp blower motors and they worked flawlessly. Now I have EFI merc power and I don't worry about it.

If you run MSD always carry a spare box....

If you run twins bring 2 spare boxes...lol

I'm running pertronix flame thrower dui's. Only reason why is because I did exactly what your doing and came to the conclusion KISS. Keep it simple stupid. Lol. Never chasing problems etc, Lots of good ignitions just what I chose. If your a gm guy you've obviously ran HEI at some point. Ever have any problems?
on dyno with all the latest and greatest msd ignition etc there was no difference between the two.

Engines dyno'd were 496's and the other ignitions used were in the dyno room. I would not have bought for testing purposes and turned out I was pleased with what I had purchased.

+1 to GETRDUNN...as you know I'm partial to HEI mod by DUI. Strong, Reliable, Simple...one wire...KISS.

I was told by my engine builder that the stock GM HEI distributor starts to spark scatter above 5000 rpm. He was part of the development team (at McLaren I believe it was) when Mercruiser was developing the 502 Mag.

I went with the MSD 8366 distributor on my build which is a direct replacement for the Computer Controlled GM HEI. The only problem with this distributor is the MSD module is junk. I replaced mine with a genuine GM module and have had no problems since.

I am running the Holley HP system on my boat and when I initially went to start my fresh build I was not getting an RPM signal from the distributor. Without an RPM signal the Holley will not fire the injectors. Swapped out Modules and fired right up.

For what you're building you can certainly stick with the stock Merc ignition. On my 454's I ran last year I used the V6-14 modules. I ran a Crane 741 cam, in what were pretty much stock Merc 420 hp engines. Even with that large of a cam I could idle the boat down to 7-800 rpms and never had any issues with docking. I've now installed engines very similar to what you want to build. 509's that are around 650-675 hp, I'm also running the Crane 741 cam in these engines as well, but here I'm using the Merc V8-HP module. So far I like the V-6 module better as far as idling, however I feel the V8-HP has a better advance curve. I believe the idle issue I'm having with the 509's is related to the carbs, so I'm going to go through them this weekend. In my option either module would serve you well, I've never had an issue with the Thunderbolt system...

I know that Joe really likes the Daytona Sensors set-up. It is a great piece, but not inexpensive, and I doubt you will need the special features it offers for your builds. I looked at them for my engines, and decided to stick with the stock set up.

I have seen merc v6 thunderbolt ignition that looks like it was picked off the bottom of a swamp, rusted to hell and still kept on firing. The MSD boxes that I have are potted and nice and shinny. However, no spark....lol. I can also tell you that the components that are in the MSD distributor just never seem to hold up in the marine environment. Just my 2 cents, KISS method, why introduce another variable that is prone to failure without noticeable gains..

I refuse to run anything MSD(may soon die) as to their crap is just that...last several setups I ran a H/P HEI and have had not spark scatter over 5000 by the way...

It's the stock in cap coil GM HEI's that have issues over 5k. They where designed to be put in motors that where done producing very weak hp at 4k. The aftermarket took care of that quickly. I ran one (aftermarket fortified) to 7k+ for years.

But, as soon as GM introduced the external coil hei (EST) set up that had no weights, I never looked back.