You need to make sure the coil you select is designed for the type of ignition you have. There are different coils for points style, CD, and Induction and even at that there are different coil requirements even within the same manufacturers CD line. Coils are designed to work with different primary voltages, current and dwell. Make sure you know the specifications of the system driving the coil before you select one.

I've been using the Crane Hi-6M box and Crane PS-91 coil with thunderbolt distributor on my engine and never and issue. Now I agree with Steve there is nothing wrong with stock ingnition stuff. I needed to change my advance curve and it was cheaper to by two used crane boxes than it was to locate two V-6 modules. Now that said the V-6 module would have made an easier and cleaner install and most likely would have worked just as well.

In October/November I spent 2 weeks dynoing a 467 ci Pontiac that made 588 hp ( similar numbers to our N/A boats). I used a MEFI 4 ecm and ramjet 502 efi harness, I also converted a older hei to a newer style GM 8 pin "2 plug coil connector" module ( the same one most bbc boats used from 1994 to 2002) so the system would be plug and play. The coils that plug into my wiring are the same coils that my mpi boat uses, the gm 2 plug connector style. I had a DUI high saturation/higher voltage coil, Msd's "better" coil and the original one that came on my boat new in 2000 that I keep on the boat as a spare for the Msd I run. In back to back testing there was ZERO measurable difference between any of them, this was a 11.5-1 motor that turned as high as 6700 (hp peak was at 6200). There also was ZERO hp difference between the stock ecu firing the ignition and using a Crane hi 6 box except a cheap replacement stock ignition module in the distributor would overheat after1-2 pulls and make ignition break up around 5800 and after switching to a DUI ignition module this would happen around 6200 or so and because I was running a Crane hi 6 box I was able to turn the dwell setting/tables DOWN in the efi programming (which took some load off the ign module in the distributor) and rev motor as high as I wanted to without ign module overheating. That was the ONLY benefit I found from the ignition box, and again saw ZERO difference from the coil. I was really surprised, though I would see something somewhere.
Now on my boat, a 8.2-1 540 with a blower running as much as 14lbs of boost the Crane box and previous Msd box seemed to clean up the soggy idle but never saw any difference between after market coils and the stock one, fwiw, Smitty

I don't think anyone here is suggesting by adding a aftermarket coil that there is power to be gained.

I think the standard Mercruiser Thunderbolt IV is a good system, and works just fine for almost all stock applications.

As far as a replacement coil, we aren't talking about a major modification here. Its a coil. I will bet you, that very few of these coils, whether it be mercury, petronix, msd, standard motor products, sierra, omc, accel, mallory, etc, are being manufactured in house. At least when it comes to these standard round style oil filled coils.

There is no way these companies all have little "coil departments" with a team manufacturing the cases, windings, terminals, etc. Funny how a chrome coil cover or a coil bracket from jegs will fit 90% of these coils.

I look at it this way. Coils can go bad. Car, boat, etc. I choose to run a "MSD" coil, mainly because if one take's a crap on me while in the middle of a boating trip, I can go to any auto parts store, most of the time in walking distance, and get a replacement. I wont have to worry about it being sunday and the merc dealer is closed. Or, I can't tell you how many times I've made a trip to the local mercruiser dealer, to do nothing but waste my time, because they rarely have what I need in STOCK. I always get the "we can have it in a day or two" from the parts guy. Well, so can I if I order it online, and I don't have to go make a second trip to get the part, and chances are it will be cheaper anyhow online!

Just as much as I get tired of reading about the aftermarket "snake oil", I am equally as tired of reading about the "buy genuine merc stuff, its the best " snake oil advice too. Over the years, just about every single part on a mercruiser engine is outsourced. The engines are Stock GM, the water pumps, carburetors, intake manifolds, fuel pumps, alternators, starters, bearings, seals, and so on.
If you like taking it in the A$$ repeatedly, stick with merc part numbers and prices. They will surely be glad to provide with a 200% markup on the products they provide you with.

Oh, and stevesxm, in case you were wondering, I think your an @sshole.

As always, good info Smitty!

Smitty always has some useful information!! He's always got something interesting to talk about!

smitty you are spot on....same here, no diff in boxes etc as far as H/P....now with coils have to be careful as mentioned as some don't work with other componants and some designed for short runs(drag racing) and will over heat on longer run times...

We tried a MSD drag coil on dyno and lost a ton of hp (35 if I remember right) BUT not sure it was compatible with box/ecu, Smitty

As far as Stevesxm, I completely agree I wouldn't change a oem coil just to spend money BUT I wouldn't pay Merc a stupid amount for a replacement either when there are quality ones for 1/3 the price that perform exactly the same, Smitty

Anyway, back to what I was asking......basically reliability and value is what I am looking for. And to use with thunderbolt ignition on some mildly modded 509's