this may not help anyone but let me tell you what happened to me and maybe someone will be able to tell me what happened.
I had a moroso crank case / header evacuation setup on my 68 GTX.Car ran mid- high 11's and we went to a track for a muffler race. I had an exhaust system put on the car 2-1/2 " with flowmasters.Man it sounded good and when I made a pass it was a SECOND slower,and it blew the intake valley pan up like some stovetop popcorn,all bulged up and blew out the valve cover gaskets!! Unhooked the exhaust and back to 11.70--