In many states and countries, it is necessary as to not violate any liter requirement in an automobile, both from the OEM & aftermarket modifications. However, there can be a lot gained in the way of performing a 3.0L M60 swap.
As you originally saw in the introduction, the 4.0L makes approximately 282hp & 295tq whereas the 3.0L makes 215hp and 214tq.
There have been proven theories as well as documented dyno results that show if one swaps a few simple parts over from the 4.0L M60, significant power gains can be realized. This includes:
Intake Manifold (it has larger runners to the intake
Throttle Body & MAF
This can easily take the advertised 215hp to 250hp and 229tq! This increase in power is noticeable in the form of more responsive pedal feel and more throttle response. Take note that in tests ran by other members of whom have performed this modification, curve comparisons showed the version with the modification had better performance throughout the entire RPM range.
