See past thread on my filter/ram system
I posted a couple pictures and a story in a past thread.
I use the same ram air and conical K&N filter as the Bower system, but instead of pulling air for the filter from the lower cowl plenum through reed valves (which has 3 disadvantages) I provided an air source from the upper cooling plenum to the filter.
My filtered air source looses only 0.3 inches Hg at wide open throttle, 5000 ft cruise at 2500 rpm, compared with the straight unfiltered ram air source. Some of that is from the filter itself, some is from the intake hole in the upper cooling plenum, which is just a flanged hose attachment, no radiused transition or anything. I could probably get a small improvement by putting a radiused intake transition at the hole (note DanH's comment in my old post)
The Bower system pulls air from the lower cowl plenum, which is warm, has already lost a bunch of pressure going through the cylinder fins, etc. and there is also a pressure loss through the reed valves. These losses represent something like a full inch or more of manifold pressure drop.
If you think of the filtered air source as an emergency alternate air, then the losses are not important. If you are taking off from a short dirt strip at altitude with hills to climb over, you should worry about the power drop using the filtered source.
The other way to go is to stick with the standard fiberglass duct from the upper cooling plenum to the fuel servo with a rectangular filter box in the front floor of the cooling plenum. As Dan Checkaway showed, if you do a good job of this with radiused transitions into the filter, you only loose something like 0.25 inches Hg compared to full ram. Of course you are loosing this all the time, you never get to bypass the filter and get full ram. 0.25 inches Hg is probably about 3 Hp. But you get to keep the nice clean lines of the cowl with no ram intake, saves a bunch of glass work and looks nice. If I ever build another one, thats what I'm going to do.
