Considerations for transplanting a jet ski engine?

[strantor]

I've got a late 90's Sea Doo Jet Ski with a Bombadier/Rotax 787 2 stroke engine in it. I'm thinking of putting it in... something else. Not a golf cart, that would be ridiculous. Not sure what yet, but something other than a jet ski.

I know these are cooled from ambient water that is pressurized by the impeller in the rear and the cooling water branches off in several directions. Some of it cools the block, some of it cools different parts of the exhaust. I think this could be replaced with a closed loop cooling system, but I'm wondering if the exhaust would still need cooling. I assume the exhaust is water cooled because unlike on a motorcycle or a snowmobile it's sealed up inside a fiberglass coffin between your legs. But if it were free to bask in the cooling breezes of excess speed like all the other exhausts in the world, would it be happily air-cooled like they are too?

What else am I not thinking of, other than cooling, that might be a fly in my ointment?

[slonomo]

I'm thinking the exhaust manifold will need to be swapped for one designed for open air cooling. Using a water cooled exhaust manifold in open air with no water cooling it, I'm thinking it will heat up too much and if it's cast iron it will hold that heat, possibly transfering it to the engine.

The engine itself, provided it is being cooled by water, should not be an issue if the heat transfer of the cooling system is efficient enough. Don't expect a closed radiator system set at 150 deg F to cool as well as 75 degree water from a natural body of water. You'd have to have a very large radiator to make up the difference, and probably an electric cooling fan and very tight shroud. I'm sure the engine is tuned to run at a certain temperature and the open water cooling system is part of that formula.

[strantor]

Originally Posted by slonomo

I'm thinking the exhaust manifold will need to be swapped for one designed for open air cooling. Using a water cooled exhaust manifold in open air with no water cooling it, I'm thinking it will heat up too much and if it's cast iron it will hold that heat, possibly transfering it to the engine.

The engine itself, provided it is being cooled by water, should not be an issue if the heat transfer of the cooling system is efficient enough. Don't expect a closed radiator system set at 150 deg F to cool as well as 75 degree water from a natural body of water. You'd have to have a very large radiator to make up the difference, and probably an electric cooling fan and very tight shroud. I'm sure the engine is tuned to run at a certain temperature and the open water cooling system is part of that formula.

Regarding the water cooled exhaust I had thoughts along the same lines and was hoping someone would show me evidence that I'm wrong. But affirmation of my fears is equally valuable.

Regarding the engine being designed for constant cold water, I actually didn't expect this to be much of an issue. I thought it would be more of a benefit, as the cooling system could be simpler than what's found in a car. Since it isn't meant to get up to a certain temperature and stay there, No thermostat. Just a pump running constantly, continuously circulating water through the block, and as long as you keep the engine temp below <unknown number>, all good. But now that you mention it, that <unknown number> is probably shockingly low relative to what I'm used to. I need to find that out before I burn up any more brain cells on this dream.

Thanks for the input!