I bought the GT40x heads a few years ago for my 89 gt auto from LR. I paired them up with the 1.6 roller rockers and the cobra intake manifold. The car seems to be down on power. I know I've installed it all correctly. Any ideas? Motor only had 60000 original miles and was immaculate internally.
I always think of flow in 3 areas, intake or induction, engine and exhaust. If you change on you will generally have to do something to address all three areas. On the intake side, did you get the egr spacer and throttle body for the cobra upper/lower also? I would run a fresh air kit on something like what you are working on as well. A lot of the flow potential you are after will be held back by the stock exhaust. The computer needs to be dialed in also after you make changes.
Did you change out camshafts as well when you swapped the heads? First thing that comes to mind is to make sure the timing is set correctly. Having your timing retarded to much will definately effect the power output. as far as the heads themselves go they flow better than the e7TE heads but you are really leaving alot of potential on the table if you are still running a stock cam. The GT40X heads have a 64cc combustion chamber which is slightly larger than the Stock E7 head. Depending on the thickness of head gasket you are running, that could have dropped your compression ratio a little from what the OEM motor was.