Open headers vrs. full exhaust

I have raced and dynoed my Dart full exhaust and open cutouts without any jet changes. Picked up 30lbft of torque, no hp gains. Track numbers were about the same 12.80's with my 60fts a little better..Figure the times and mph were same because of leaning out on the top end...
64 Dart w/ street 360,eddy heads,solid cam,airgap,750dp. exhaust-hooker 1 5/8 full length headers, 2 1/2" summit x pipe,dymomax ultraflows,tti tailpipes.

Hope this helps
Bob