On a couple of RV upper cowls I have seen a fiberglass insert that seems to help make the air flow better into to engine cooling fins. What are they called? Where do you get them? Do they help?
Try James Aircraft. Plenums work by tightly sealing the cooling area. No leaking around baffle seals. My -9A is so equipped along with a Sam James speed cowl.
If you aren't speaking of a plenum but the original setup, there is a cowl inlet ramp on each side that helps the airflow into the cowl. Look at post #31 on this thread:
Dan H explains this very well, I'll try, the air entering into the cowl needs to expand and slow down from speed to pressure....or something like that, and the ramps inside the inlet help to do this, (diffuse?) I think is the word to help the air flow velocity and direction with out tumbling and causing weird stuff to happen to the air flow.......or something like that ;-)
