I currently have a pair of 454 inboards that I need to repower. I may have an opportunity to get a couple of Merc 8.1 HO's coming out of a boat that has IO's.

I know I'll need to convert them to accept an inboard transmission, does anyone know what that would be? Are there any other differences I need to be aware of such as cooling. I currently have freshwater cooled engines that suck the water up via thru hull sea cocks, would that be the same with the 8.1s. I also have wet exhaust manifolds going thru mufflers then out the back. The new 8.1's have silent choice. Will I need to get new exhaust manifolds and does the 8.1 cooling work the same as my 454's. I do know that the newer 8.1's are fuel injected so I may need to add a return fuel line but are there any other major obsticles that I should be aware of before going this route?

Your bellhousings should work. The 8.1's have sea pumps and will hook right in to your current thru hull pick up. The exhaust system are the same mercury designed it to be a direct replacement for the 454. If they are merc's? The 8.1's do not need a return fuel line. it circles back into its own fuel system. It should be an easy swap.

You can use your existing top starter mount bell housings and starter setup but you will have to get the 8.1L Mercury Horizon or Crusader flywheels for the 8.1L because the top mount starter has a different length. The flywheels must be internal balance type no offset counter weights like the 454 types. Do Not interchange the flywheels!
You can remove the power steering pump if its not needed and then use the inboard serpentine belt as its shorter in length.If you don't use power steering then just remove the power steering hoses from the rear mount cooler and leave the cooler in place on the raw water hoses unless it has to be removed to clear the top mount starter and such.

Best Regards,
Ray @ Raylar

You may also need to deal with the wiring. The I/O has a gear sensor and the engine will not crank if the PCM registers "in gear". If it registers "in neutral" it will not rev past about 2,000RPM. Whilst the inboard transmission has a gear sensor of its own, it is unlikely it will have the same calibration - at least that is what I found. I tried (a) to leave the wiring disconnected, (b) connect the the loom wires and (c) add an original sensor but none worked. My PCM registered "in gear" so I just bypassed the key crank and installed a push button start with in gear protection via the new gearbox switch!

You may also need to add an oil cooler for your new transmission and adapt the hose fittings.

Also remember the 496 is all metric so no bolts will interchange.

also how do your current 454's counter rotate ? if you have BW 72c trans and have a reverse rotation engine you are in for a nightmare...if you have hurth trans and they reverse there you will be ok....been down this road with my gibson...Rob