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

Heads are the piece of the engine that help make everything happen. Heads sit on top of the engine block itself; they are at the "head" of the engine. Essentially, they are used to flow air through the engine, both in and out. They hold the valves that open and close. These valves not only let air in and out, but also close to allow the engine to compress said air. This compresssion is a crucial piece in creating power. In newer mustangs (1996-current), the heads also have the pleasure of housing camshafts in them. Generally speaking, the camshafts are responsible for telling the heads when to open and close the valves.

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

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.

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.

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.

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.

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.

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.

You don't have to stop at the valve train either. In depth work like this requires complimentary mods such as a cold air intake, exhaust, and a custom tune. An aftermarket intake will make the most of the ported heads, allowing your engine to breathe efficiently and stack on horsepower. An exhaust system will help purge the engine of burned gases, and if you top it all off with a custom tune you can make sure your engine is dialed in for peak power.

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.

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.

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.

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.