Jim Smart
March 12, 2014

Monster Mash

For 2000, Ford built the ultimate Cobra-R with the tall-deck 5.4L DOHC V-8. Where the 5.4L DOHC differs from the 4.6L is its iron block topped with the Ford GT supercar's cylinder heads. It produced a healthy 385 horsepower and 385 pound-feet of torque. Though the Cobra R's 5.4L DOHC engine used the iron Windsor truck block, it was fitted with special pieces like a steel crank, Carillo rods, forged pistons, and specially modified 32-valve heads. This is a DOHC you can build yourself using off the shelf parts, though the intake manifolds are very hard to come by these days.

All 4.6L DOHC engines came from the factory with cracked powdered metal connecting rods except the ’03-’04 supercharged Cobras, which received Manley rods and forged pistons for obvious reasons. Cobra engines prior to ’03 got hypereutectic pistons just like the SOHC engines. This hypereutectic piston/crack powdered metal rod combo has been very successful in high-performance applications. If your goal is 500-plus horsepower, go with forged pistons and forged steel rods.
Aftermarket companies such as Coast High Performance and Eagle Specialty Products offer complete stroker kits for SOHC and DOHC alike, including steel cranks, H-beam rods, and forged pistons, enabling you to increase engine performance.


Lubrication

Instead of being driven off the camshaft like a small-block or FE-series, the DOHC Modular's 13mm G-rotor oil pump is driven by the crankshaft. According to Sean Hyland, every new oil pump should be inspected right out of the box for casting flaws.

Pump rotor clearances (cover and rotors) should be 0.002-inch according to Hyland. If clearances are any tighter than 0.002-inch, do not use the pump. If clearances are too loose, Hyland suggests lightly working the pump surfaces on a perfectly flat surface with 600 grit sandpaper and a petroleum based solvent. Plate glass makes a perfect working surface beneath the sandpaper to achieve a the desired results. While you have the pump apart, check the relief valve for proper operation. It should be installed in the right direction and move freely within its bore. For heavy duty applications, billet pump gears or Melling's cast iron pump (PN 10227), is recommended.

Another often overlooked pump issue is getting the pump centered on the crankshaft. Bolts and holes do not provide perfect pump and crankshaft alignment, which must be performed by hand before you tighten bolts. Pump misalignment causes unnecessary side loading and stress, which can lead to pump failure.

8 When installing the Modular’s oil pump, it should be perfectly centered on the crank so there is no side load. Side loading will cause pump failure and engine damage. The factory F8OZ-6600-AA aluminum Cobra oil pump features a 15⁄16-inch pick-up and 13mm wide rotor and cavity.
9 This is Melling’s #10227 cast-iron Cobra pump with 13mm wide steel billet rotors and 15⁄16-inch pick-up. It provides an 8-percent increase in pump volume. It’s a must for severe-duty applications.

Cylinder Heads

The DOHC's factory cylinder heads have continually eveolved over the years. In its infancy, the DOHC Modular was fitted with twin-port heads also known as B-series heads. Ford referred to this early DOHC head as the Swirl Port, but the twin or "B" designation is more representative of the port's shape. The square, throttle-operated primary port operates below 3,000 rpm. The round, vacuum-operated secondary port comes into play above 3,000 rpm, much like secondary throttle plates do in a carburetor. The Twin Port heads are preferred by some Modular enthusiasts, however, there are better cylinder heads.

In 1999, Ford pressed the Tumble Port head into service, which featured a large single intake port that did a better job of keeping fuel droplets in suspension, hence the word "Tumble Port." Hyland tells us this head works very well at up to 0.450-inches of valve lift. Exhaust ports on the Twin Port and Tumble Port are virtually the same. The Tumble Port gives the 32-valve Modular better low-to-mid-range torque and more horsepower on the high end.

In 2002, the C heads were updated for greater exhaust flow, with a mild increase on the intake side. These were employed in the '03-'04 Cobra, Mach 1, Aviator, and Marauder. The 2000 Cobra R had it's own specific head design that later grew into the Ford GT and GT500 designs.

Induction

The DOHC's original induction system installed in the Mark VIII was produced and designed for the Twin Port head and consisted of a throttled primary port and a vacuum-operated secondary throttle assembly known as the Intake Manifold Runner Control (IMRC). Hyland discourages the use of Twin Port heads and induction, especially if you're going above 6,000 rpm. At low- to mid-range rpm, you need long intake runners that make air velocity and torque. At high rpm, you need short intake runners for top-end horsepower. The twin port/secondary throttle approach is restrictive, which is why it is ineffective above 6,000 rpm.

The Cobra's Twin Port induction package uses revised computer programming to open the secondary IMRC throttle plates at 3,200 rpm for improved airflow and power. For 1998, Ford went to a plastic IMRC for less heat transfer according to Hyland, and this is the IMRC to use. You want F8ZZ-9U531-AA for the righthand side and F8ZZ-9U531-BA for the left. Gaskets are F7LZ-9461-AA. Hyland mentions the IMRC eliminators, which were once available from Ford Racing. When you eliminate the IMRCs, you gain horsepower because you're operating on both ports all of the time; the tradeoff is a reduction in low-end torque.

The '99-up Tumble Port head utilizes a different induction package with eight intake runners instead of 16. Hyland says a 20-cfm gain can be had with porting, but it is of little benefit in terms of power. The FR500 intake manifold fits the FR500 cylinder heads perfectly and can be used with the Tumble Port and Mach 1/Cobra cylinder heads with minor modifications. The FR500 head/induction package works on a principle of long and short intake runners separated by secondary throttle assemblies similar to the Twin Port. The aftermarket, of course, offers a variety of induction systems for the Twin Port, Tumble Port, Mach 1/Cobra, and FR500 cylinder heads.

10 The reason why Ford’s overhead cam Modular makes more power is cam profile and the nature of these very large, low-friction cam lobes. This cam technology spreads the load over a larger surface area and allows for a more aggressive profile while reducing friction. Valves and springs are much smaller and lighter, on par with a motorcycle engine where you can spin this guy high and make more power.
11 The DOHC’s timing chain system works exactly the same way as the SOHC except there is an additional chain up top on each side to drive the second camshaft. Oil-pressure-fed tensioners maintain pressure on chain guides.
12 Tumble Port (left) and Twin Port (right) demonstrate DOHC cylinder head function. The Twin Port operates on the square primary port below 3,000 rpm. Punch the throttle and you get round secondary ports for high rpm horsepower. The revised Tumble Port head from 1999-up improves low- to mid-range torque and gives you more high-rpm horsepower.
13 On the left is the 1999-up Tumble Port with a larger 54cc combustion chamber. On the right is the original Twin Port with a 52cc heart-shaped chamber. The Twin Port offers a combined flow of 231 cfm from both intake ports at 0.500-inch valve lift. The Tumble Port yields 225 cfm intake flow at 0.500-inch valve lift. Exhaust flow is the same at 0.500-inch. The later ’03-’04 Cobra/Mach 1 head delivers the same intake port volume at 177cc, but greater compression thanks to a smaller 52cc chamber. Intake flow is 233 cfm. Exhaust flow is greater than the Tumble Port at 169 cfm.
14 There are a number of induction systems available for the DOHC Modular induction factory intake manifolds. If you’re building a Twin Port DOHC, your options are few, which is incentive to consider the Tumble Port, Mach 1/Cobra, or FR500 head. This is a dual-purpose DOHC intake manifold from Sullivan Performance. It works with electronic fuel injection and it will accommodate a Holley carburetor flange.