Has anyone had problem with back pressure in the head?

My ITR seems to be pushing a lot of pressure out of the head, whenever take off the oil cap... Lots of back pressure... car is retarded don't know how many degrees but when i put it back to normal BTDC 16 degs it sputters and knocks... I don't know what could be causing this but i do have exhaust 2.5 diameter piping until end of cat... then goes back to 2.25..... Could it also be because of my EVAP leak?.... or my EGR not working correctly (where is this located?).... can't seem to find it.....