A 2003 496 mag. is it a closed cooling system? Is it even an option or are they all closed systems. Also, how do you convert a 496mag to a 496HO? The boat in question does not have captains call I am assuming that it is not neccessary. Thanks

All 496's have closed cooling. There are a few threads here on converting to HO. All it takes is a cam and computer. The captians call has nothing to do with it.

I was thinking a 496ho would run better with thru hull exhaust that is why I mentioned it. Thanks for the help I will do a search

all merc 496 are closed cooling
some volvo 496 are not

I have two cams 375each 850 each to get your ecu redone

If you go to a 496 HO you may need to increase your cooling capacity. Check your engine temps and if they take a spike, you'll need to increase the size of the heat exchanger unless you can increase your seawater flow to overcome it. We can get you the heat exchangers if you need them.

Closed cooling has nothing to with thru hull exhaust.

Yes, it would make more power with thru hulls.