Understanding GT40 Cylinder Heads

What Are GT40 heads?

Between 1993 and 1995, Mustang Cobras came equipped with a type of cylinder head called a GT40. These cylinder heads were a vast improvement over the E7TE cylinder heads that came on most Fox Body Mustangs. That might have been the end of the discussion if the Cobra were the only vehicle that the GT40 head came equipped on but several other Ford vehicles, including the ubiquitous Explorer and Mountaineer, were also equipped with GT40Ps, a variant of the GT40 cylinder head.

The GT40 cylinder heads quickly gained a reputation as being “Yard Gold,” the type of thing that only enthusiasts knew to poke around for in a scrapyard and could pick up for a song. Upgrading Fox Body Mustangs to GT40 heads became a popular modification.

GT40 Cylinder Heads

How Cylinder Heads Impact Performance

All of the power in your engine comes from fuel and air igniting in the combustion chamber. Though you might expect that the fuel is the limiting factor here, more often than not it comes down to air. This is why so many performance-boosting modifications from cold air intakes to different types of hood scoops focus on allowing more air to enter the engine, but the cylinder heads can frequently end up creating a bottleneck to this process.

Having better cylinder heads that improve flow becomes an essential part of optimizing your engine. Beyond that, it’s also important to have reliable cylinder heads since they’re responsible for handling such an important part of the engine’s process.

It’s pretty well-known by now that the E7s that came on the Fox Body Mustangs aren’t ideal, which is why so many Fox Body owners are eager to find replacement cylinder heads or improve the quality of their own.

E7 Cylinder Head

Upgrading to GT40 Heads

If you can obtain them inexpensively, GT40 heads are dependable and have improved flow when compared to the E7s. However, though they were once available in just about every junkyard, it’s become more difficult to find GT40s in good condition.

Though a lot of the GT40’s scarcity is simply due to age, as their reputation has grown, more people looking to purchase them has driven up the price. Though a lot of manufacturers, including Ford, used to make an aftermarket version of the GT40, they’re not available new anymore (you can still sometimes find them on eBay or in a pawn shop). Though it may be tempting to hit the pavement and start looking for the GT40s, it’s hard to recommend given their increased cost and the alternative options.

The actual performance boost of the GT40 head isn’t as noteworthy as you might think given the reputation this cylinder head has amassed. Really, it was only at its best when coupled with the GT40 intake manifold. Ultimately, you could expect to get a horsepower increase of around 30 when you upgraded from the E7 heads.

With new GT40 heads not being produced but being consumed at an increasing rate, it’s unlikely that you’ll strike this particular brand of yard gold in the near future, but there are a lot of other alternatives for improving the performance of your Fox Mustang by upgrading the cylinder heads.

Inside a Cylinder Head

Improving Your Existing Cylinder Heads

It’s unfortunate, but while as many other cylinder heads can be improved through porting, it simply isn’t worth the time investment for E7 cylinder heads. Because porting isn’t something that anyone with a Dremel can do, you’ll need to hire a professional to port and polish your cylinder heads, and that will cost a pretty significant amount that could be better invested in a new set of cylinder heads.

Ultimately, when people have ported their E7s they’ve been able to achieve between 20 and 40 horsepower, which is, once again, nothing to sneeze at and about as good as the GT40s would be able to achieve but ultimately is outclassed pretty quickly by modern improvements to cylinder head flow.

Porting Cylinder Heads

Additionally, either the E7 or the GT40 cylinder heads are going to be made of cast iron. While cast iron is durable, and certainly may account for some of their resiliency, they’re also exceptionally heavy.

If you’re interested primarily in restoration or nostalgia, it makes sense that you would want to either improve the E7s or upgrade them.

Even by the standards of the time, E7s weren’t exceptional heads, which is why the Cobras came with the improved GT40s. Since then, people have tried just about everything to get the E7TEs to a point where they’re competitive with the GT40s and though it’s possible at the end of the day, it just isn’t worth it.

Essentially, you can port your E7TE heads, which will earn you about thirty horsepower, or you can upgrade to GT40 heads, which will also earn you about thirty horsepower. Or you can use modern heads and actually gain significant horsepower and not have a two-decade-old piece of cast iron inside your engine waiting to break down.

What Makes a Cylinder Head Effective

There are several ways to appraise the effectiveness of a cylinder head, though there are also some aspects that are harder to quantify specifically. Essentially, a good cylinder head doesn’t impede the flow of air in and out of the engine and creates a compression ratio that’s ideal for the task that it’s meant to perform. Not everyone wants a high compression vehicle because there’s more opportunity for rod knock and pinging in a high compression engine, and they require premium gasoline. But if you want a lot of horsepower, typically you want a lot of compression.

When you’re looking at cylinder heads, you’re presented with a lot of numbers that are meant to help you figure out if the flow will be adequate for your vehicle.

Intake Port

Typically, smaller intake ports will help to create more torque, and larger intake ports will create more horsepower. This would mean in an ideal world you would have a small intake port for your day-to-day driving activities and a large intake port when you want to go for higher revs.

In the real world, you have to strike a balance and remember this is one tiny part of a much larger equation.

Combustion Chamber

The combustion chamber is where the magic happens. Obviously, there are a lot of factors that impact the ideal size of a combustion chamber for your particular engine, but in general, a smaller combustion chamber equals more compression which equals a higher-powered engine. Higher compression is also what requires a higher octane gas though, so it’s not entirely without cost.

Intake Valve Diameter and Exhaust Valve Diameter

The intake valve diameter gives you a rough idea of the flow potential through a cylinder head. Because this is part of a much larger (and more complex) equation, it shouldn’t be assumed that a 10% increase in intake valve diameter results in a 10% increase of flow, but all other things being equal, a larger intake valve diameter certainly improves flow.

The intake valve diameter will always be larger than the exhaust valve diameter. Exhaust is hotter than the air entering into an engine, so forcing it into a smaller area helps to reduce temperature and reduce pre-ignition.

Valve Lift

Valve lift is a complex number, but it does impact airflow and is included in the spec sheet for many cylinder heads. Essentially, valve lift is the maximum distance, in inches, that a valve can rise above its seat. This is going to have a variety of limiting factors, including the springs, the camshaft, and the piston head clearance. Ultimately, more valve lift is better from an airflow perspective.

Intake and Exhaust CFM

CFM is the number that tries to address airflow the most directly. CFM stands for cubic feet per minute, and it’s meant to refer to, literally, how many 12”x12” blocks of air could flow through your engine in sixty seconds. This will vary significantly based on factors like valve lift, and unfortunately, this is a number that’s easy for manufacturers to manipulate to make their cylinder heads look better than they are.

As mentioned before, valve lift has a number of limiting factors, and just because the valve could theoretically lift .7 inches doesn’t mean that it actually will, and yet some manufacturers will quote the CFM at the max valve lift. Thankfully, a more standardized method of reporting shows CFM at valve lift of .5, which is still not a perfect measurement of airflow but does help when you’re comparing cylinder heads to each other.

Modern vs Fox Body Mustang Cylinder Heads

Though there are a lot of other factors to consider based on your individual build that we can’t easily determine, when you look at the individual components of modern cylinder heads compared to the stock E7s and GT40s it becomes quite clear that we’ve come a long way with regards to cylinder head technology, and are now able to make cylinder heads with greater flow and a lighter weight.

Continued Popularity of GT40 Heads

So, with so many great alternatives, why are GT40s still the talk of so many modification communities? Some of it, at least initially, was cost. When GT40s were easy to find in junkyards, they were a great inexpensive upgrade. And to this day, if you can get a set of GT40s inexpensively, they are absolutely better than your stock heads. Unfortunately, scarcity has driven the price up to almost the equivalent of modern heads in some cases.

The reason why people are willing to pay so much for GT40 heads may come down to nostalgia. One crucial piece of evidence for this can be observed in the GT40 intake cover. The intake cover that was sold for the GT40 is attractive, but it doesn’t offer any kind of performance gains. When it was offered new on Cobras, very few people actually purchased it, adding to its rarity.

The current going price for these covers is significant and only rising (so, if you’re looking for scrapyard gold, these would be a good pickup). The reason for the price increases comes down to rarity and nostalgia. People love the idea of owning Mustang history, and for some people, a restoration means using completely authentic parts even when superior parts are available. If you’re looking for horsepower and performance gains, there are a million better ways to get there, but if you want the nostalgia of driving a real Cobra, well, there’s really nothing else for it.

Source: GT40 Heads for Your Fox Mustang, Foxstang | What Is Cylinder Head Porting, and How Does It Increase Horsepower?, CarThrottleImage Credit: Car Tech | Engine Builders Mag | Stangnet