Old thread, but i can write something here about my findings. In Europe most of dynos shows engine numbers, since after pull they measure drivetrain loss.
Usually it's Maha dyno. So my finding was some Shelby 07-09 thread that there was test at dynojet and same engine on engine dyno.
It showed 13% drivetrain loss. I measured my 4.6 turbo and current 2010 gt500 here and it shows roughly 16% drivetrain loss.
i was not there with 2013 yet.
Maha dyno shows rwhp 3% less than dynojet and shows same as superflow.
So my gt500 dynoed stock 506 rwhp and was measured 16% drivetrain loss, that's 603 engine hp, so just bit more than they wrote about engine dyno test here:
2013 only difference in drivetrain vs pre 2013 is driveshaft. How much drivetrail loss less is that? I bet no more than 3%, so according to dynojet, engine numbers in 2013 may be in 10-13% loss, that's about 670-680 crank HP for 604 rwhp.
I was searching lot, but i could not find engine dyno test for 2013...