I have a 502 650 hp. should I go msd or thunderbold 4? msd has a big box when the thunderbolt looks eaiser what are your thoughts

TB IV dist with a Crane Box.

I've used the Tbolt on two performance engines so far with no problems. 1st was a 406 small block with 10:1 with solid lifter cam. 2nd was current 540 at about 550 hp. I would spend $$ on other items first.

I would be interested to hear who is using Tbolt on big HP engines, and what HP?

I second this. The Crane box with the Thunderbolt distributor gives a lot of adjustability for $$$. There is no doubt the Thuderbolt is a great ignition system but it is limited by its set advance modules. The Crane is wide open on what you want to do.

I have 625 hp NA 540s (9.3 c/r)with T-Bolts. Work fine and top of the chart for reliablity
MSD
T-Bolt distributor and Crane boxes are good too but I don't feel the advance curve is that critical on a boat. Biggest importance is you overall timing and let the idle timing fall where it may.

548 cid; 620+ hp @ 5500 rpm. Very happy with my T-bolt IV using 20* adv. module. --- Jer

Totally impessed with TB IV on hp marine engines.
Totally impressed with Crane HI-6M on marine engines.
Not to keen with the MSD system on marine engines.

The TBIV distributor does not have the corrosion problems that plague the MSD's.

Marine engine's - for the most part - don't turn mega rpm and/or create super high cylinder psi that over burdens a mild powered ignition system. A way too lean or way too rich condition will run better with a CD ignition, but, this is marine and we have things tuned correctly right ? If we have an application that idles a ton and tends to foul plugs and we have tried everything to tune this out, then a CD box will help big time. Just skip buying a corrosion prone distributor, keep the stock one, and add a much more reliable (then MSD) cd box from Crane.

I was told to use the T-Bolt system and that the NASCAR Guys run multiple systems for a reason.

and I've only seem MSD's in NASCAR. So if they are so bad, and Crane is so good, why are the guys who can have what they want using MSD?

TB4 with MSD. Benefit is, that if the MSD box breaks, you switch a couple of wires and keep going. TB4 does not need the box to run, where the MSD Dist (except the RTR) is dead with no box.