WAM
Thomas,
I'll try to answer your questions:
I don't speak for WAM, but I suspect it'll be this year that they'll have the 140 hp version available to homebuilders. I don't think they'll do anything with the 4 cyl. 160/180 hp until the 120/140 3 cyl. is up and going. As far as certification goes for the "storebought" market, I'm not sure, but I would imagine it will be within a year or so of availability in the experimental market.
Your idea to put one on an RV4 would be great. I've often thought about a P-51-style scoop, especially on a '4, but you have to remember that the sump actually hangs down below the belly of the plane, so you couldn't get a clean line between the cowling and the belly. I went with the turbine-style chin scoop, then routed my inlet plenums around the sump/head to a radiator mounted just behind the engine. The cooling system works well in this configuration.
Weight: my RV9/WAM came out lighter than the equivalent Lycoming O235 installation, and I have a CS prop. It was MUCH lighter than an O320.
Ken Krueger from Van's wrote an article about some testing we did between my plane and Van's factory RV9A/O320 (Kitplanes April 2010). The WAM performed well against the Lyc, especially considering that it has 40 less HP. Once I get the 140 hp version, I'll see if Van's wants to do another test. I'm almost positive that the WAM will match the LYC's performance, while burning considerably less fuel.
Any other questions, just let me know. I'm a real believer in the future of diesel for GA.
Kurt