There is a rule on I/O Hp that Merc and Volvo follow by agreement. Rated Hp must be within 10% of the real number.

Thus, any example of a engine sold as rated for 300 Hp can really make as little as 270 Hp or as much as 330 Hp.

Another example, a 280 Hp rated engine can make as much as 308 Hp and as little as 252 Hp.

Someone should get them all on a dyno once and for all.

320 Hp can be off 10% also.

I understand from a marketing side the mfg would like to be able to move the advertised hp around a bit to show a bigger gap between engine versions than might actually exist...that makes sense on some level. Maybe this 10% rule made more sense in an era of 125 hp engines (12.5 hp slop, 25 hp range) than in the current era of 250+ hp engines (25 hp slop, 50 hp range!). Also, I have to believe current hp measurement equipment is fully capable of measuring hp to a tighter spec than 10%...if that was ever an issue.

Where on the engine/drivetrain do marine engine mfgs measure hp these days?...is that standard among mfgs?

At the prop, is my understanding.

It's best to advertise your engine's HP at the lower side--

I don't know all the details, but some years ago a lot of people bought Mustang GT's, and I guess Ford didn't expect so many of the owners to find a dyno and see if their engines had the poop Ford said they had. Turns out a whole bunch didn't, and a class action lawsuit was filed, and I believe Ford had to do whatever repairs were necessary to get the engines to make advertised power. A few guys here more in the know than me will have better details, but as I remember, it wasn't pretty for Ford....

Lowrider's orriginal spec 454 was 330HP.
A friend's jet boat mod business has dyno'd many over the years and told me a good one MIGHT make 290HP, 280 was more the norm.

When I overhauled it, and got a good look at the specs of the cam, intake and heads, he was right, 280-290 was about it.
The factory GM "high-lift hydraulic" camshaft, is hydraulic, and lifts, bu there's nothing "high" abou the lift. Nothing good in the iron dual-plane intake either.

That was 1978 tho.

1978 wasn't a great year for big engines (or ANY engines for that matter)

FR, The Ford episode you are referring to had to do with the SVT Cobra. They had to retrofit an exhaust sytem to gain something like an additional 25-30 HP.