Justin Fivella
November 6, 2014

As gearheads, we should rejoice for the second coming of the golden age of hot rodding. Don't believe me, look around. Nobody could have guessed that 800, 900 and 1,000-plus horsepower cars would be the norm. And if you ask me, the '03-'04 Mustang Cobra, aka the Terminator, helped pave the way.

Simply put, when the Terminator hit the streets over a decade ago, it annihilated the competition. And a well-modified Terminator can still hold its own today. Their impressive stock performance immediately made them noteworthy, but their extreme capabilities in the hands of the aftermarket earned them legendary status. Never before had 500 rwhp been so easy.

The aftermarket wasted little time pumping up the latest Four-Valve mod motor with more boost and even more power. First it was 450, then 500, and eventually over 600 hp at the wheels; but the higher enthusiasts pushed the envelope, the more the stock components began to fail.

We paid a visit to AED in Shingle Springs, California, for the installation of the Cobra Engineering cylinder head cooling mod for a Terminator Cobra motor that was out for a rebuild. Although the installation can be done with the motor in place (but with the transmission removed), we thought it better to illustrate the install on a motor that was happily resting on an engine stand.

The almighty Terminator might be a beast, but it isn't perfect. When pushed hard enough, the No. 7 and No. 8 cylinders are known to fail. We'll save the tight piston-to-wall tolerances and piston failures from long fifth gear pulls for another story. Today we're here to hash up an old remedy that helps balance the coolant flow between the cylinder heads and prevent heat spikes on the driver-side head on the '99 and newer Four-Valve mod motors—it's even claimed to help combat the infamous "tick."

As I'm sure you already know, the tick is distinguishable above the standard injector noise and only gets worse as the motor reaches normal operating temperature. This tick is said to stem from worn exhaust valve guides that caused the valve stems to move, a problem exacerbated by the extreme heat of the tight tolerances and the Eaton supercharger. Call it coincidence, but the tick is most often found in the rear two cylinders on the driver-side head—the two that have insufficient coolant flow, causing those chambers on the head to overheat and, at times, taking the valve guides with them.

The Cobra Engineering head cooling mod comes with ultrahigh-quality components like billet aluminum coolant fixtures and precisely cut mounting plates. It is important to note that you’ll still need to source your own fittings and hose with the Cobra Engineering units. You can opt for AN or NPT fittings, whichever best fits your needs.

In January 2005, FoMoCo released an updated head with revised coolant passages to keep the exhaust valve guides cooler, but the aftermarket found its remedy much sooner with what's called the cylinder head cooling mod. Several companies, including Lethal Performance, D'Agostino Racing, Modular Motorsports Racing (MMR), and Cobra Engineering, all have different remedies for the fix.

"The factory driver's side head on the Cobras is a dead head," said James Hellermann of Cobra Engineering, "which means the coolant flows to the back of the head where it has no place to exit, unlike the passenger's side head that gets coolant plumbed through the head and out the backside."

In other words, hot coolant can't exit the rear of the factory driver-side cylinder head, creating hot spots and even steam pockets as the coolants fights its way back to the front of the cylinder head and toward the radiator. As we mentioned earlier, it just so happens that pistons 7 and 8, the two most likely to fail, are located at the rear of the driver-side cylinder head.

The fix? Simple. Uncork the back of the driver-side cylinder head and plumb the coolant either to the other head and into the heater core like the stock setup or from the dead head up toward the radiator. This not only balances the coolant flow between the heads, but it also helps keep the driver-side head cool by diminishing temperature spikes. Cooler cylinder heads means less detonation and safer power.

"It's not a guarantee that a mod motor won't develop the tick after the head cooling mod, but I've never seen one," said Hellerman.

As we mentioned, multiple kits on the market take different approaches. Some route the coolant from the dead head to the other head and into the heater core just as if it were factory, while others route it from the deadhead up near the radiator. Some kits use modified OEM components, while others utilize CNC-machined aluminum, giving you the choice to pick what's best for your setup.

There are no downsides to the head cooling mod, but it is important to note each kit's idiosyncrasies. The rear-plumbed units have less hose and are harder to spot with the hood up, but require that the transmission be removed if they're installed with the motor in the car. Some rear-plumbed units also require that the EGR tube be clearanced for proper fitment of both the EGR and the cooling mod. On the other hand, the forward-plumbed units may have a hose running from the back of the driver-side head up to the radiator, but these units can be installed in a matter of minutes with the transmission still in place. MMR even has a non-Terminator-friendly kit that installs on the top of the rear cylinder heads for a clean install that doesn't require the transmission be pulled, but unfortunately doesn't clear the positive displacement blower. Again, you can't go wrong whichever route you take; it's a matter of preference.

As you'll see, we chose to install the Cobra Engineering unit, but also included info on the others so you can make an informed decision. We paid a visit to the Mustang gurus at AED in Shingle Springs, California, for the install on a motor that was out for a rebuild. Although the motor doesn't have to be removed for the cooling mod, we used a freestanding motor to better illustrate the process.

Follow along as AED tech, Greg Wallace helps us install the Cobra Engineering cylinder head cooling mod.

01. We begin by removing the factory coolant tube off the back of the passenger-side head.

02. The Cobra Engineering unit pushes into place, but be sure that the O-ring doesn’t snag or tear.

03. Next, we slide the retention plate into place and tighten the bolts to secure the coolant fixture to the passenger-side head.

04. You’ll need to source your own fittings with the Cobra Engineering kit, so we sourced some quality Fore Innovations units for the job.

05. With the passenger-side unit in place, reinstall the factory hose that connects to the heater core.

06. Now onto the driver’s side and the dead head. This freeze plug is the reason installing the kit on the car with the transmission in place is impossible. Knocking it out can be a pain in tight places, but with the transmission out, there’s enough room to access it from underneath the car. Since our motor was on a stand waiting for a rebuild, it made it all the easier.

07. Don’t forget to lube the O-rings on the Cobra Engineering coolant adaptors so they don’t snag or tear.

08. In order to install the retention plate, the OEM coolant tube mount (the black metal tab on the right) must be trimmed—it uses two bolts from the factory, but one is more than sufficient. Note that it’s already trimmed in this photo. With it trimmed, simply install the retention plate and tighten the bolts just like the other side.

09. We used another Fore Innovations coupler on the passenger’s side for a super-sano install.

10. Here’s the finished product installed and ready for action. Both cylinders are now plumbed together for balanced coolant flow back into the heater core just like the factory system, only better. (Note: There’s a factory coolant restrictor in the feed tube to the heater core. James Hellermann of Cobra Engineering explained that removing it is optional.)

11. Cobra Engineering was nice enough to send us a photo of a kit installed on a car with the motor in place. With the transmission removed, there’s just enough space to slip the head cooling mod in place between the back of the heads and the firewall.

12. Here’s the one-sided factory system compared to the Cobra Engineering head cooling mod. Note that the driver’s side is a deadhead with only a freeze plug; but now both heads are connected for balanced flow between each bank and into the heater core.


Multiple Ways to Keep Your Cool

Over the years, several top companies have created their own cylinder head cooling mods. Each kit achieves the same results of balancing and improving coolant flow between the rear of both cylinder banks, but each has its own attributes. Here's a look at what Lethal Performance, Modular Motorsports Racing (MMR), and D'Agostino Racing have to offer. The old adage applies: "There's more than one way to skin a cat." So pick your poison.

The Lethal Performance Gen 2 head cooling mod is unique in the sense that it doesn’t require that the transmission or supercharger be removed for it to be installed with the motor still in place. It works on ’99 and later DOHC mod-motor cars and comes with quality components to plumb the dead head up front toward the radiator.
MMR offers multiple kits. This is the Professional mod and mounts to the backs of the heads to even the cooling flow and diminish the hot spots on the back cylinders. For nonpositive displacement four-valve mod motors like the Terminator, MMR also offers a unique kit that mounts to the top of cylinder heads at the back for a clean install that’s also an easy one too.
D’Agostino Racing takes a different approach with its DOHC cooling mod by offering modified OEM components that mount to the backs of the heads. Rather than billet aluminum, these are OEM components custom-modded to achieve the same results.