Horizontal vs vertical vs sliding vs other partial grill closure?
 

Which one offers the best aero benefits when partially and when fully closed?

which
 

It might be the 'longitudinal' form which is best.Like Lockheed chose for the Pratt & Whitney engines of the SR-71,or hydro dams have for the turbine wheel jet nozzles.The Messerschmitt 262, Junkers Jumo 004 turbojets also used this 'bullet' valve in their tailpipe nozzle.
It's probably also why they're not used,do the the length available in a production vehicle.
I'm running this on the T-100.There's zero vena-contracta entry loss regardless of valve position,which cannot be said for any other type.

Thank you

Would you mind posting some images or links for a better understanding?

image
 

Here's a try at an image
The opening I originally had was too large so I blocked half of it.It was enough to allow wide-open-throttle acceleration for 1-mile with no cooling issues.I went ahead and drove another 2,500 miles with it on the interstate.
http://i1271.photobucket.com/albums/...2/HPIM1874.jpg