I thought it was 4"... built a homemade Ram Air setup kind of like demon x2s, somebody cut two holes in there but they're not centered up and they're not 4" or 4 1/8", one of the guys on here built one and he is three holes which actually works out pretty good due to the fact you need 30 square inches of air according to the Mopar performance chassis manual for it to do any good for a fresh air induction... if my memory is right to 4-in Circle gives you like 12 in or 12 and a half inches per hole so three holes would be plenty. My Ford Buddy told me to take the whole triangle thing out between the braces but then if it rains I think you're screwed at that point and it rains here all the time in the summer, and the spring, and the fall... and sometimes in the winter here. ..