George,
I'm using a Rod Bower Filter since 120h now. The systems works very well.
I used to start with an older design and upgraded to a bigger flapper valve and
bigger holes for filtered air intake. The pictures are made before the upgrade.
I actually use a Lycoming IO-390. The aluminum intake ring in the cowling is also
available from Rod.
The difference from filtered to ram air is about 1.2" in cruise and you really feel
a difference when you open the valve.
There is some very fine dirt sticking on the inside of the fuel meter intake from the
unfiltered ramair use. But I don't think this will harm the engine...
When the valve is closed the engine will get warm filtered air from inside the cowling.
As soon as I'm airborn I open the ramair valve.
I'm happy with the system!
Christian