Finding a good apples to apples comparison is hard for the GT-40x head. Some magazines have tested them against AFR185's. This is not a true apples to apples comparison in my opinion.
The GT-40x head should be compared to the AFR165. The Z304 FMS head is a better head to stand up against the AFR185.
I had to dig, but this is what I found from two different Muscle Mustang and Fast Ford articles:
Ford Racing GT-40x Cylinder Heads:
Peak HP: 390 hp @ 5,400 rpm
Peak Torque: 416 lb-ft @ 4,400 rpm
Average HP: 315 hp
Average TQ: 386.20 lb-ft
Peak Airflow Intake: 234 cfm @ .650 lift
Peak Airflow Exhaust: 157 cfm @ .450 lift
Average Airflow Combined: 275.38 cfm
Cost Per HP: $32.03
link:
http://airflowresearch.com/articles/...Part4/A-P4.htm
AFR 165 Cylinder Heads:
Peak HP: 396 hp @ 5,400 rpm
Peak Torque: 378 lb-ft @ 4,400 rpm
Average HP: 288.89 hp
Average TQ: 353.86 lb-ft
Peak Airflow Intake: 240 cfm @ .550 lift
Peak Airflow Exhaust: 200 cfm @ .550 lift
Average Airflow Combined: 293.97 cfm
link:
http://airflowresearch.com/articles/...art2/A-P11.htm
It's not just about HP. It's about torque and usable rpm's. If the car is 90% street use go with a head that provides higher port velocity thus higher torque numbers.
As you can see from these numbers, the GT-40x head trounced the AFR165 in the tourqe department. This is the important figure. The GT-40x head develops this torque advantage through exceptional port velocity. The guys at Ford Racing aren't completely in the dark
.
People have been bashing the GT-40x head for years because of that one article where the AFR185 made something like 75 more hp over the 40's. Not a fair comparison but if you look closely the torque numbers for the 40's were not that far off. Torque is where it's at baby!
It's the force we all love! You can have all the HP in the world but if the force is not with you...well you know the rest young Skywalker.