What is a Mustang PI Engine, Heads or Intake?

Ford introduced their 4.6L single overhead camshaft (SOHC) V8 engine into the Mustang starting in 1996. The first edition packed 205 HP and eventually saw an increase up to 225 HP for the year of 1998. These numbers are not particularly great, especially when you consider the fact that the old 5.0 pushrod V8 produced the same numbers. However, GT Mustangs from 1999-2004 saw a pretty sizeable increase in power, now up to 260 HP, thanks to some engine improvements.

In 1999, Ford revised their 4.6L Mustang V8 to produce an additional 35 HP for a grand total of 260 HP at the crank. This revised motor, installed in all regular Mustang GT’s from 1999-2004 was dubbed the PI engine, where PI stands for performance improved or power improved. There is great debate over which term is actually correct (even Ford occasionally uses both terminologies in reference), but one thing is certain, the improvement part.

The performance improvements came via a revised intake manifold and revised cylinder heads. The new PI heads raised the compression a small amount, received changes in port shape and also featured better heat dissipation. The new intake highlighted a more durable design (96-98 intakes were prone to cracking) as well as increased runner length and size. The combination of the two, heads and intake, resulted in the 35 HP increase.

Today, all Mustang GT V8 engines from 1999-2004 are the PI engine, whereas 1996-1998 4.6’s are referred to as NPI (non performance improved) engines.

To clarify, if you have PI heads and a PI intake on your motor, then you have a PI engine. The term ‘PI engine’ is reliant on having those two upgraded components. The heads and intake are what make it a PI engine. If you only have the PI intake installed, then you have a PI intake, but not a PI engine. It is a fine line.

Yes, you certainly can! In fact, this is a very popular upgrade for 96-98 GT owners. So popular in fact, there are many guides scattered all over the World Wide Web that detail the process in-depth.

To complete the swap (a quick Google search for ‘pi swap’ will yield a TON of results), you’ll need to pick up the PI heads, intake, and a few odds-and-ends (hoses, brackets, gaskets – that sort of thing) in order to complete the swap. You can buy both the heads and intake brand new as they are still produced (in fact, even aftermarket companies make their own version of this head) or purchased used off a fellow Mustang enthusiast, from the scrapyard. These heads are available in plentiful numbers. If you decide to perform this upgrade, sourcing the parts is not going to be a problem. It is a pretty in-depth procedure based on the fact that the top end has to come off, but with attention to detail and much patience, the average Joe can do it in their driveway. The trickiest part may be getting the timing set up correctly after the install. Other than that, it’s nuts and bolts.

Yes, you do. However, this is a trick question. Recall that the 4.6L V8 is an overhead camshaft engine. If you’re buying the heads used from a wrecked car/scrap yard, chances are the cam will still be in the heads. If buying new, you’ll have the option to purchase the heads fully-assembled (includes valvetrain & cam) or bare (no cam, no valvetrain).

In the event that you don’t get matching PI cams with the heads, don’t fret - the regular NPI cams will still work. However, it is recommended to have all the matching PI parts, as this way you’ll receive the maximum benefit this upgrade has to offer.

There is no difference in exhaust manifolds between NPI and PI models. That being said, while everything is apart, it is a good idea to change out the manifolds for some headers. It greatly helps with power and is much easier while everything is already apart.