"Just replacing the restrictive factory exhaust manifold should give a 20hp increase "
When I bought my Duratec plus 4 it had the full Williams Motorsport stainless manifold and exhaust/sports cat along with the ITG air filter conversion already fitted. I've had my car on a rolling road and it produced 155bhp at 6000rpm and 148lbft torque at 4800rpm.
Assuming Fords figures are correct that's a 10bhp increase only.
A few things to take into account here, I couldn't have the car tested prior to fitting so I'm not sure what figures would have been achieved in standard mode.
Not all rolling roads give similar results, as my tester said " some read high to please the client"
If you feel the car drives better than before any modification then the figures really don't matter!
Ford's engine figures will likely be theoretical, based on engine design. Assuming your rolling road calculated transmission drag that engine figure would seem a good improvement.
For comparison a friend put his near standard GDI Plus 4 on a rolling road recently and it achieved 140 bhp, 10 bhp down on Ford published figure but a good average according to Cain of Wolf Performance who happened to be there at the time and uses same dyno.
