From what I have ready the engines themselves are identical insofar as the powerhead is concerned...so yeah, Merc must be doing some 'black magic' on a normally aspirated engine to get 50 more ponies...either that or they radically DE-tune the 250 hp version.

One only needs to look at the MAX RPM Limits of the 250R vs the 300R to see how the "Black Magic" is done to make 50 more HP. I am certainly no expert on this, but it seems like that it how it is done.

Hmmm...I must be missing something? Because all the specs look to be exactly the same. From the Mercury website:

I was looking at the "R" motors specs, they have different max rpms, for a 250R vs 300R, not the regular V8 motors. Sorry about that.

That is strange that they have same everything but HP rating.

Yeah...so it kind of implies that perhaps the 250 is a detuned 300? But how, exactly?

The PRO XS all rev to 6,200, the 250 Pro XS is the same as the 250R power wise (270 ish HP). The 300R has different cams, intake, ecu; makes significantly more than 300 HP. The regular four strokes have different exhaust and tuning. They may have different intakes but I don't know for sure.

The HP difference between a 250 ProXS and 300 ProXS is small, not worth messing with.

the 225 to 300 are all the same engine with the exception that the 225 has a smaller diameter airhorn on the intake to reduce power. The 300r has a lot of different parts. The 250 and 300!are identical. It’s 4.6 liter v8. So it’s a very mild state of tune @ 250hp. Hell most 1 liter sport bikes make 200 ponies lol

it’s a 4.6 liter v8. Only 250hp is weak af for that large an engine.

I’m in South America so I hope for export a remap is available

I know a lot of the smaller engines are identical except for restricting air flow.

I had a Yamaha 9.9 that I made into a 15 with just bending the Reed cage to let the reeds open more. After that instant 5hp.

Just bought a 20hp mercury 4 stroke. Both the 15 and 20 are the same except for air restrictor (washer) installed in air tract to restrict flow and a different programed ecu (efi). They charge quite a bit more for the 20 than the 15, but the cost of a new ecu was double what buying a new 20hp ecu was.

This is very common with out boards.

I could list a lot of engines which are just about identical with different hp ratings.

I don't know much about the engines you are asking about. I would post over on the hull truth. A lot of outboard guys there. I'm sure someone could give you the answer.