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What is a Mustang PI Engine, Heads or Intake?

By:  Connor MC  / Jun 13 2019
What is a Mustang PI Engine, Heads or Intake?

If you've heard of the terms NPI, PI or performance improved Mustangs and want to learn more, this guide will break them down. We explain the non power improved engines and how Ford came to add an additional boost in ponies with the PI engines.

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Well now that you know what a PI engine is you can go ahead a grab a pair of heads for your Mustang. Along with the heads a set of cams, cold air intake and a shiny new exhaust will help your new heads breath even better!

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What is a PI Engine?

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.

1999-2004 Mustang Front End

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.




NPI (non performance improved)

Original design

205-225 HP at crank


PI Engine

Featured improved heads & intake manifold

260 HP at crank


Are PI Heads/Intake Different Than a PI Engine?

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.

Mustang GT Cylinder Heads

I Have a NPI Mustang GT (1996-1998), Can I Upgrade?

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.

Under the Hood of a GT 1999-2004 Mustang

Wait a Minute… I Just Heard About PI Cams?! Do I Need Those Too?

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.

Mustang GT Camshafts

Is There a Difference in the PI Exhaust Systems?

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.

Fitment includes: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, GT