What is a pi swap?

In 1996 Ford converted the Mustang GT base engine from a 5.0 liter pushrod engine to a 4.6 liter modular engine "overhead cam design". This was Ford's first go with the modular engine and the new 4.6 liter was less than pleasing with only 225 hp to the wheels. This version of the 4.6 liter 2v engine was used in the GT's until 1999 when Ford released the 4.6 liter power improved "p.i." engine, thus deeming the previous 4.6 liter 2v engine "non p.i." The improvements consisted of larger and straighter intake runners "the cambers through which air is delivered to the combustion chamber" in the heads, larger intake manifold runners "the chambers that deliver the air to the heads" and larger intake and exhaust valves. With these improvements of head and intake manifold air flow as well as exhaust flow the P.I. engine yielded an additional 35 hp to the wheels laying down a respectable 260 hp. Other than these improvements the engines are identical, leaving the option for the 1996-98 Mustang GT owners to swap these heads and intake to their Mustangs, which is often referred to as a "P.I. swap". (NOTE: The P.I. swap will raise the compression ratio of the non P.I. engine to 10:1, making forced induction almost impossible without running a higher octane fuel. If you plan to do this to you're 1996-98 Mustang GT and use a supercharger or turbo it'd be in your best interest to use a dish style piston.)