It seems to me it would be easy to determine on a dyno. At the end of the run, clutch and calculate loss from the time it takes to spin to a stop. Maybe almost nobody cares enough to waste the time since what matters is at the wheels.
15% loss comes from SAE norm. It's probabably too general and bit less for mustangs, also depends on manual vs automatic, manual has lower loss. If dyno for stock Mach 1 would say 410 rwhp and Ford says 480 hp, then yeah, simple math 410/480 gives 15% loss.
I think this is about correct. the stock M1 dyno pulls I have seen are all 408-410 at the wheels. Advertised is 480 so the 15% loss seems accurate. I am not sure it remains a constant either as a percentage or a fixed number (~70hp) as modifications are made however. If we have someone here who has had a motor built and run on an engine dyno before install and then ran their car on the rollers we could figure it out. I am skeptical of claimed HP gains by manufacturers they always seem very optimistic and when people add mods on top of mods and think the numbers simply add on top of each other, it rarely works that way.
Blame Ford. For example BMW's dyno numbers are almost as high as crank paper numbers, it makes the owners lot more happier. Can you imagine if Ford said mustangs have only 400 hp? It's marketing in this case. And EU owners are even more salty considering the power drop when the BMW's etc are made in our backyard and have same power for USA too. But this is easier with turbos than N/A, I guess. At least the V8 and exhaust make up for it 1000x
