Shah Mat

I'm in the process of purchasing a complete 502 carb motor for my boat and I'm going to need an ignition system. I guess it's pretty close to a HP500. I have heard bad things about MSD, and good things about the Thunderbolt IV system. What components do I need? Is the module in the dist? Do I need a 6cyl module? What year should I be looking for? My old motor is efi and will be sold complete, so as you can see that I need everything. I see stuff for sale on here and ebay all the time but don't know what I'm looking for.

z.zuperboat

NEVER HAD A PROBLEM WITH THE MERC STUFF.VERY RELIABLE IN THE MARINE ENVIRONMENT.SENT U A PM

Cord

iTrader: (2)

No, the module is external. It was mounted on the exhaust riser so you'd need the bracket on the riser. Now, I just mounted my module to a aluminum plate on the back of the engine, so it's pretty easy to come up with a mounting alternative. The distributor is specific to the system, so you'll need that also. The wiring is pretty simple, but it would help greatly if you had the correct harness. If not, get a diagram and wire it up. I believe the system used bullet and ring connectors so it's nothing special. I added a MSD rev limiter to my system. I've had no problems so far. The only problems I've ever heard about the MSD stuff is that the distributor parts will rust. For what it's worth, I've seen Tbolt4 distributor parts also rust.

powerguy

Shah Mat

I have a Thunderbolt IV V8-24 module available that I bought for my 502 mag but sold the boat. The stock module for a 502 mag is V8-20 with the 20 deg. max advance but not in until 5000 rpm. Based on Dennis Moore's book and advice here on OSO (you can search for the posts)I had planned to change the module out to the V8-24 which is a 24 deg. advance module but the main difference is that the full 24 deg advance is in by 3750 rpm. With the 24 deg. module you set your initial advance to 10 and ou are at 34 deg. advance above 3750rpm which I am told is where big blocks run their best at. Let me know if it's something that you need.
Dave

P.S my original post is here:

http://www.offshoreonly.com/forums/s...ht=thunderbolt

Kidnova

Shah Mat, I switched from a 7.4 to a 502, also about 500 hp {ZZ502}. My engine guy used the Thunderbolt IV ignition system off the 7.4. The distributor works fine with the 502. However we had to switch to a different module because the rev limiter was buit in to the existing one, so rpm was limited to 4800.

Check with powerguy. I think he has what you need. I'm pretty sure the V8 24 mod is what we ended up using though, becuase of the reasons mentioned by powerguy. There is no rev limiter in the V8 24 module that I have, so be aware of that.

If you're in no big hurry and can wait a few days, I can open up the boat this week end and check to be sure of what module is on it. Let me know.

Cord

iTrader: (2)

Kidnova-you proably have a V system. The V has the rev limiter, the IV did not.

Kidnova

Cord, I believe you're right on that. Think we were looking for a IV module to replace the one with the rev limiter.

Griff

If you have an engine close to HP500 specs, then you should probably try to get the V8-HP module that was used on the HP500's. It has 20* of advance and is at full advance at 3000rpms.

Other than that, you just need a distributor and the wiring harness that plugs in to the module. You can mount the module anywhere as long as its grounded.

Griff

Oh yeah, there is no rev limiter on TB IV's. Merc racing/blue engines had rev limiters on TB IV's, but it was a separate component.

Mbam

I have a bunch of brand new V8-HP Tbolt IV modules on the shelf, they are no longer made but I stocked up before they were discontinued. I hate it when Merc does that with something we use a lot of.