I put a plastic bag over my aircleaner, and then I just hose it off with my garden hose. No soap, no touch, I've done it since it's new and only a couple of times I get it a little wet and spray carwash soap on the whole thing, I use a soft brush with long bristles to get in all the areas and I brush all that I can reach, and THEN, I rinse it down with my hose and drive it to nice and hot and then after it cools I'll clean the few spots and use my Cal. Duster to shine up the black stuff. That's only IF I get too dirty, usually I just hose it down and drive away, like when I'm going to the carwash. by the time I get there it's dry and clean.
My enging bay has looked brand new since day 1 for so little effort is ridiculous.
When I see a dirty engine bay I just freak, I can't believe it, but then again, you should see my underwear right now....Whew...go figure.