Buddy of mine had one of these installed last year to replace a 6.2L MPI (377 cu in) that broke a piston in a 2001 272 Rinker. Boat has a BIII and was able to turn a 26" prop set past 5,000 RPM.

https://www.mercruiserparts.com/merc...61-383-mpi-bra

New 383 would only turn 4,400 - 4,600 RPM. Motor scans out good with 100% power, no codes. Merc tells his dealer it is the outdrive...needs rebuilt! Drive gets rebuilt as it was getting due anyway. No change. Then Merc tells dealer he needs to drop down in pitch on the props. WTF? More cu in, more HP, more torque and he needs to drop down in prop size?! OK, he finds a set of 24" props and now can turn 4,800 RPM but boat is still slower than with old engine and can't turn the RPM it used to.

Now to my question, is the ECM different between the new FWC 383 Merc offers vs. the old 6.2L MPI? Check out the ECM on his new engine shown below. Is this the real issue? Even if they use a 6.2L MPI ECM for the new 383's it doesn't cover the serial number range of the new engine. I know the two engines are close but... Thanks.

Most likely it is different however I doubt if the ECM is the problem. 4-600 RPM represents a ton of horsepower.

Thanks for the reply. It's been a head scratcher and Merc isn't helping.

Is spark plug gap set to .060" like the sticker calls for? Most peps here will tell you .035" is better regardless of what the sticker says.

Is it firing on all cylinders?
If it is only firing on 6 or 7, then it is down on power.
I suggest to pull all the spark plugs to check what they look like?

That was the first thing that came to my mind as well. It is indeed firing on all 8. Makes it even more of a head scratcher.

No idea what the gap is and I doubt the Merc tech nor Merc are focusing on plug gap. As long as it is under warranty I doubt my buddy would deviate from the factory spec. Don't want to give Merc any ammo to deny a warranty. I agree that gap seems needlessly large. I know I closed it up some when I put plugs in my 496 MAG.

In that case Mercury or the Mercury dealer should really test the engine using a dyno to check how much horsepower it produces.

If using a prop shaft dyno it should be fairly straight forward to test the engine.

If the engine is under warranty, a merc dealer should be dealing with the issue, if there is an internal issue - you would be wasting your time chasing your tail and it still has to go through merc dealers to get warranty

Trying to figure it out up to a point is natural but your buddy has already paid for the warranty.

+1 on the prop shaft dyno test while hooked to a laptop for diags

Neither available at the shop that installed the engine. It would have to be pulled and sent out. Right now Merc is handcuffing the dealer only authorizing them to do certain checks on the engine. They keep blaming everything but the engine itself.