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Ford's 4.6L DOHC V-8 Explained - Cammer Time
Ford's Hot-Blooded 32-Valve Double Overhead Cam Screamer Explained
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.
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.
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.
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.