is it possible to any new-stock-mercruiser engine to work 20 hours non-stop at 3000rpm?!?
if not which engine can?

As long as you have a BIG gas tank

LOL insptech

20 hours at 3000 rpm. I don't see why not. I certainly don't consider 3000rpm , beating on the engine. Although I did read in a Merc. manual that running at 3000rpm is comparable to driving your car up hill , pulling a trailer , at 80 mph. Now thats kind of a scary thought.

And like insptech said "as long as you have a BIG gas tank" or was that "a BIG ass tank"

hoe about 5500 all day i run that all day with my blower motors. YOU KNOW WHAT THEY SAY **** TO BE THE MAN YOU GOT TO BEAT THE MAN*** ask the guys from indian lake they will tell you !!!!!!!!!!

There are some kit airplanes on the market, 3/4 scale Mustangs that run basically 502 / 502's that cruise at 4500rpm for hours. They MUST be reliable for this application.

I have seen BBC motors used to power irrigation pumps. These things burn propane or natural gas and run for days at a time, nonstop. Not sure what rpm they are turning but, I would bet it is at least 3000 rpm.

Macdad, I am familiar with those BBC generator motors. Where I used to live I had a buddy that raced Super Modified oval track cars, one of his sponsors was a generator company and he would get those motors for free. We were told that in some hospitals the generator motors were replaced on a set hour limit weather they ran fine or not. I had got several from him over the years, They were 454 LS6`s!!!! Similar to the 454 MAG. and since they were run on propane, they were extermely clean inside.

Biggus,
I saw a BB generator not long ago. Had a hard time IDing the engine at first , because of the intake and carb , it was a ONE barrel and matching intake...no adapter.
That certainly wouldn't be run too hard

Didnt mercury run 50k miles non stop at lake x back in the 70's? changed gas while driving, changed drivers while going....