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Why Upgrade Your 4.6-Liter Mustang’s Cylinder Heads?

Written By: Josh Honeycutt

The 1996-2004 Mustang GT makes a good amount of power from the factory, but if you want to make serous gains in horsepower, you'll need to increase airflow through the intake by replacing or porting the restrictive stock cylinder heads.

American Muscle

When grasping for more of those precious ponies from your Mustang, the cylinder heads are a very important aspect of gaining that extra horsepower. There are several different cylinder head designs Ford produced for the 4.6l engine. The majority of the heads covered in this article will be for the 1999-2004 Mustangs.

  • Replacing your 4.6L cylinder heads should occur when you are looking to significantly increase airflow efficiency outside of the standard bolt-on options, or you are rebuilding the engine
  • Upgrading to aftermarket heads can provide greater air flow and increased performance
  • The difference between PI and non-PI heads is largely due to the difference in sizes of the camshaft profiles

When is the right time to replace my 4.6L cylinder heads?

Ported PI Heads

Most Mustang owners upgrade their cylinder heads when they’ve either exhausted their bolt-on options, or are rebuilding the engine. It’s not an easy job to do if your not mechanically inclined, and most would prefer to let a shop deal with the install. When installing a new set of heads it’s important to make sure the other head components are up to snuff. Make sure you have the proper camshaft, valve springs, retainers, etc. for your application. Of course these components are all dependent on how extreme the application.

Why should I upgrade my SN95 Mustang’s cylinder heads?

Ported Mustang Cylinder Head

The airflow through the heads is measured in CFM’s (cubic feet per minute). The main goal in head porting is to improve airflow without losing velocity in airflow. If the heads are ported to the max the resulting power band will be very narrow. Not exactly ideal for a streetcar. This is why most shops offer head porting in different stages. Of course, simply opting for better flowing heads will achieve better flow.

What do aftermarket heads have to offer? There are some very nice aftermarket options for mustang cylinder heads. Some aftermarket companies make their heads with revised intake and exhaust runners to increase flow. However, there’s a lot of performance that can be gained by simply changing to a different set of factory heads. Such as upgrading to PI heads from non-PI heads, or changing from two valve (2v) to four valve (4v) heads as well.

Overview & Key Information: The Mustang GT two valve cylinder heads

The 2v (two valve) heads on the 4.6 before 1999 were the non-power improved heads. The 1999-2004 2v heads are the power improved (pi) heads. The non-pi heads rolled out on the 4.6 engine making only 210 horsepower. In 1999, the pi heads were introduced and horsepower rose to 260 horsepower. There is also the SVO 2v head. They are a bit pricey new. However, if a deal can be found on a set of SVO heads they are the best out there. They outflow both PI and non-PI heads at all lift points.

Is there a difference between PI and non PI camshafts?

One difference between the non-PI and PI heads is the camshaft profile. The non-PI heads came with a camshaft profile of roughly .480 inches lift on the intake and exhaust side. The valve duration at .050-valve lift came in at 204 degrees intake, and 208 degrees exhaust. 

The PI heads upgraded the cams to .535 inches lift on the intake, and .505 inches on the exhaust. The duration fell to 192 degrees on the intake, and 184 degrees on the exhaust. This combination yielded a very nice improvement in low- end torque and flow across the board. Both cams are compatible with all three cylinder head designs. But, enough about cam profiles.

What to expect from performing a PI swap 

The PI heads came with 42cc combustion chambers, as opposed to the 51cc non-PI heads. This will cause an increase in compression on non-PI engines to roughly 10.5:1. This really isn’t a problem unless you also plan running high boost or low quality gasoline (this will require premium grade gasoline). However, with the PI heads and matching intake manifold you can expect roughly a 30hp and 40lb/lb. torque increase. Not bad at all for a fairly simple swap.

I already have PI heads: How can I improve the PI head design and get more horsepower?

There is room for improvement on the stock PI heads as well. Many machine shops offer CNC porting to raise flow numbers on the intake and exhaust sides. Or, companies like Trick flow and Patriot offer aftermarket Mustang heads already CNC machined and assembled out of the box. These also flow more than the stock PI heads and should provide a good boost in efficiency and power. 

Upgrading your heads and camshaft 

Aftermarket Valve Spring
Aftermarket Valve Spring

Upgrading your camshafts to an aftermarket set with a bit more duration in conjunction with the Mustang PI heads can also offer some horsepower improvement. Combining a good port job with the proper cam will yield the best gains. Of course a nice set of headers to compliment these improvements will also increase horsepower considerably. 

What’s involved in swapping the 2-valve heads for 4-valve cylinder heads? 

OK this option is not for everyone. However, even a heavily ported 2v head does not match the flow numbers of a 4v head in its stock form. And the addition of bolt-on modifications (headers, cold air intake, etc.) is much more effective. Not to mention the exhaust note on the 4v sounds heavenly. 

The biggest argument against this swap is the cost and work involved. First off, it’s a tight squeeze, not something you want to try with the engine in the car. The 4v heads are a lot larger than the 2v pieces. Also the polarity of the coil-on-plug connections will have to be switched. And of course, there will be some custom tuning required. But, if you don’t mind the hard, work, sweat, and foul language this could be the upgrade for your Mustang GT.

Upgrading a Cobra's cylinder heads

2003-2004 Mustang Cobra Heads and Cams
2003-2004 Cobra Heads and Cams

For the cobra owners out there, you already have a leg up on the competition. The cobra comes from the factory with high flow four valve heads. The best bang for the buck on these heads is a good port job. Also, a cam with a little more lift and duration makes for a nice horsepower gain. Most engine builders recommend not exceeding .475 inches of gross lift. 

Plan a “head” – installation 

In my honest opinion, cylinder head upgrades are not mild bolt-ons. It is a task that can be handled by the home mechanic. But, if you do not possess decent mechanical skills, this job should be left to a qualified shop. On the other hand, a substantial amount of horsepower can be gained by upgrading the cylinder heads. If nothing more than a camshaft upgrade, the power increase will be noticeable. There are many reputable aftermarket companies that supply cylinder head components.