Exhaust
There is a major part you are forgetting the intake manifold and throttle body. F150 s use large long runner intake manifolds , split throttle bodies and long tube air inlet. All of this has an effect on the exhaust note. As well as pipe diameter, length, and layout. Mustang are shorter than F150 S so the pipes are shorter.
One is build for speed and acceleration the other for hauling a large load of stuff its always going to sound different besides I think the truck exhaust sounds better then the mustangs. after all aren't all the mustang owners looking to sound like the f150s? LOL


