Where does Merc measure the HP on the MAG MPI engines? At the flywheel or prop shaft? I can’t seem to find a straight answer. I just swapped out a 350 MAG Carb engine for a 350 MAG MPI and I’m trying to guesstimate speed/prop based on HP ratings

It is supposed to be at the prop, but from what I've seen on other posts is that it closer to the hp at the crank.
It should not make any significant difference on prop choice.

Recently a guy posted up 4 old brochures on the Cobalt site when someone was asking about blue motor options.

All 4 list your 350 MAG MPI as 300 PHP

Oddly enough, they listed the HP 500 EFI as 500 PHP in 2001. Then in 2002 and 2004 they listed the HP 500 EFI as 470 PHP And in 2006 is lists the HP 525 EFI as 500 PHP

If motor is sold in a boat then its at the prop
If motor only is sold then its at the crank

These are my observations

If it has a drive that handles the shifting duties ( alpha, bravo) they knock off 30 hp from the crank, if it has a transmission behind the engine, it’s rated at the crank.

There are a lot of confusing answers here. Blue motors the HP is rated at the crank, black motors are rated at the propshaft. Both motors you have are black motors, they are both rated at the prop shaft. So it is an apples to apples comparison.

The carb 350 mag was rated at 270 hp, the EFI mag was rated 300. Will the extra 30 hp increase your RPMs enough to need to re-prop? I'm guessing no. Now if you were on the rev limiter, then maybe, but I'm guessing you might gain 100-200 rpms. A lot of that will be determined on how tired the old engine was and how fresh the new engine is.

Either way, I doubt you are going to need a new prop. Speed you may gain 1-1.5 mph, 2 mph if the engine you pulled was tired.

Over at Raylar they did dyno the 496HO and it was 430hp crank and 387 prop. My 2002 496HO is black and rated 425 which I presume is at prop

Mercury mystery???

gained 5mph loaded and it’s banging on the rev limiter. It’s gonna need more prop for sure