Looking to upgrade to a set of GT40/GT40P Heads? Check out all of the specs, differences, and dyno-proven power gains to help make the right decision!
The Ford GT40 heads are a factory small block ford head found on certain vehicles, most notably, the Ford Mustang. They were most famous for their debut on the 1993 Cobra and were also found on the 1993-95 Ford SVT Lightning truck. GT40 cylinder heads were a more free-flowing high-performance cylinder head that outperformed the factory E7 heads that were found on most 5.0 Fox Body Mustangs.
There are two variants of these cylinder heads, one being the GT40 already mentioned and the other being the Ford GT40P heads. How to identify your GT40 heads look for a "GT" or "GT40P" stamp inside the head casting or a three-bar (GT40) or 4 bar (GT40P) stamped on the side of the head. GT40 heads can be found pretty much anywhere you look.
| Head Type | Intake Port | Combustion Chamber | Intake Valve | Exhaust Valve |
|---|---|---|---|---|
| E7 | 124cc | 61cc | 1.78 | 1.46 |
| GT40P | 145cc | 59cc | 1.85 | 1.46 |
| GT40 - Explorer | 145cc | 65cc | 1.85 | 1.54 |
| GT40 - Cobra | 145cc | 61cc | 1.85 | 1.54 |
The GT40 heads can be found specifically on the 1993-1995 SVT Cobra Mustang as well as the 1993-1995 SVT Lightning. These can also be found on the 1996-Mid 1997 Ford Explorer & Mercury Mountaineer SUV.
The GT40P cylinder heads arrived on the scene in 1997 with Ford’s revision to the GT40 head. These heads were installed on the '97-and-later Ford Explorer and Mercury Mountaineer SUVs. As you can imagine, in the late ’90s and early 2000s, and even today, the GT40P heads became an inexpensive OE style upgrade to Ford’s Pony Car and other 5.0L powered vehicles.
Some changes Ford performed on the GT40P included reducing the combustion chamber from roughly 65cc to roughly 60cc. This would increase the compression ratio to help increase horsepower. A big change that has plagued the Mustang community to the GT40P heads was the spark-plug relocation. Though this placement improved combustion efficiency, it caused interference when using standard small-block Ford headers with the GT40P head with the header and the spark plug. Ford Performance used to offer an option to remedy this but they have since been discontinued and can only be found by local or online sellers.
If you decide to use the GT40 or GT40P heads found on any of the Explorer or Mountaineer for your Mustang GT or other small block powered Ford, you will want to upgrade your valve springs. The stock valve springs found on the SUV heads will not be strong enough to support even an OEM Fox Body Mustang’s camshaft. LMR.com has you covered with a set of upgraded valve springs for cast iron heads from Ford Performance!
Are GT40P heads worth it? If you can get your hands on the special headers required to run them, the “P” variant is a viable upgrade over the OEM E7 heads.
The GT40 & GT40P heads had a few noteworthy differences, the most important being the bump in compression as previously mentioned. The GT40P cylinder heads had a small combustion chamber ranging from 58-61cc while the GT40 heads had a 63-66cc combustion chamber.
The GT40 heads have thermactor holes built in to connect to the EGR/smog equipment whereas the GT40P heads use an external EGR connective system. The GT40P heads also had a different spark plug placement making header choice important. GT40P specific headers are needed unless you use spark plug wires with 90-degree boots to clear the heads.
Although you will see different gains from car to car and with more mods, we can give you a ballpark power gain that you will see from doing the swap. Most stock Fox Body Mustangs can expect an increase of 20-40 horsepower at the crank over stock heads. That will equal somewhere around 15-30 rear wheel horsepower. These power numbers will be very similar with both the GT40 and the GT40p Cylinder heads.
Pair these with a GT40 intake manifold from a 1993 SVT Cobra, the Ford Explorer or Mercury Mountaineer, or even an aftermarket intake, and your Fox Mustang will pick up some good power you will be able to feel! While you will not break records with your newfound horsepower, your Fox Body Mustang will be much more fun to drive!
All in all, if you are looking for a cylinder head upgrade and a set of new aluminum heads is not in the books at this time, the Ford GT40 heads or GT40P cylinder heads are a great option to wake up your Mustang!
