OK my 5 years are up so let's talk about water cooled HD heads. This was done along time ago and then put on the shelf. I was involved with it back in the 2005 time as I recall. Back then it was called Precision Cooling. It was developed for 2 options, water cooled or oil cooled. The oil cooled version of it came out on the XL line and the water cooled never made it out. I was told that at the time, Willie G. at HD stated "there will not be a water cooled HD as long as I'm here". Well I have no way to know if the statement was true or not but he is gone and now we have the water cooled head that was done way back when!
The heads themselves are interchangeable with the current air cooled heads. All the components will swap between the two from what I've been told and I can assure you the prototype stuff back in the day did. Now, its called Twin Cooled instead of Precision Cooled but it sure all looks the same to me as it did back then for the most part. The new lowers I hope solve some of the old test issues but we will have to wait and see on that one. The compression is the same for both the air cooled and water cooled 110. The 103 gets a slight compression bump 9.7:1 air cooled and 10:1 water cooled. The water cooled 103 also gets a different camshaft from the air cooled 103, not sure about camshafts on the 110 engines.
Does it work, yes it did back then and it worked nicely but rider comfort (heat) didn't really change any with or without it. Maybe the new shape of the lowers will help with that but like has been mentioned before most would remove the lowers in hotter weather and that's not an option with water cooled. When your sitting still in traffic I doubt that it's going to be any cooler and at highway speeds was never much of an issue anyways IMHO.
All new ECM and code in them and were hard at work on that already. Hope to be able to get it done quickly.