[patrickt]

Dave, don't over-think the MSD installation. The specs from MSD say the box pulls 1 amp per 1000 RPM. If you add a 50% safety factor to that the most that box will ever draw, and then only for a brief amount of time, is ten amps. That's it. If you run a 10 gauge wire to extend the leads that come with the box you'll be just fine. The internals of the circuit breaker add a small amount of resistance but, again, that's not going to amount to a significant factor.

Remember, MSD gets to issue one set of instructions, that will have to be interpreted by tens of thousands of people, doing tens of thousands different installations. Everything from mama's Biscayne to borderline-NASCAR revvers. I wouldn't worry about the condensation at all but, if you're obsessing, drill a couple of pin holes in the top (which is the bottom in an upside down installation).

Regarding the circuit breaker versus the fusible link, both have their respective advantages. I run the MSD standard 50 amp circuit breaker, but I have to agree with Rick that a fusible link is probably better, but a breaker is just a little more convenient. The reasoning is that if either one blows then you have a pretty serious problem. Now, if that problem is just transitory, and basically innocent, like a dropped wrench on the starter solenoid, then a circuit breaker is great. It just resets in a couple of seconds and you say "whew, I need to be more careful." With a fusible link you're saying "goddammitt, now I have to cut out and replace that fusible link -- that shoots the day." On the other hand, if you have a persistent serious short, the circuit breaker fires on and off until you can get to your battery cut off switch. The fusible link blows and your ohm meter tells you that the wire is shorted and you have a problem. Plus, circuit breakers weaken over time and pass less current before tripping when exposed to high underhood temperatures. That said, I run an upside down MSD box in the passenger foot well with no drainage holes and I feed it directly off the protected side of the 50 amp master circuit breaker. But, I don't drive in the rain and do no serious racing. Now, if I did heavy duty rain racing, I would have a waterproof box installation and run fusible links at the alternator and at the battery feed. I hope that helps.