Jim Smart
October 14, 2005

DOHC Engines
The Modular V-8's greatest asset has long been the deep-breathing four-valve head. The way this head works is simple. There are two intake valves and two exhaust valves in each chamber. Below 3,000 rpm, we're on the primary port in the intake manifold, with the secondary butterflies closed. When we mash the throttle and open the secondaries, we're on both intake valves, taking full advantage of airflow. This was a good idea in theory, but in actual use, it didn't work very well in the beginning. Low- to mid-range torque was poor with these engines in normal driving, despite a big burst of power at high rpm at wide-open throttle.

Beginning in 1999, Ford went with the Tumble Port head. As its name implies, the Tumble Port tumbles the air and fuel as it rolls into the chamber, keeping fuel droplets in suspension. Not much changed aside from the shape of the chamber, which improved low- to mid-range torque. The 5.4L DOHC got its own kind of Tumble Head at the same time for use in the Lincoln Navigator.

Ford brought performance enthusiasts the FR500 four-valve head in 2002 from Ford Racing Performance Parts. Although mostly a racing cylinder head, it does have a purpose on the street--yielding as much as 7,000 rpm and a whopping 500 hp. Bolt these onto your DOHC and make as much as 50 more horsepower from the heads alone.

The FR500 head is similar to the '03 Cobra cylinder head, which was introduced with an Eaton supercharger. This head's intake ports flow 233 cfm at .500-inch valve lift. Exhaust ports yield 168 cfm. These factors make the '03-'04 Cobra head a popular head with performance buffs that love Mod motors. Mercury copped this head for the '03-'04 Marauder; ditto for Ford with the Mustang Mach 1 and Lincoln's Aviator.

Like the SOHC engine, the DOHC has twin camshafts with pressed-on lobes. The primary lobe profile is different than the secondary lobe profile. This means two different cam profiles for each engine: one for intake and one for exhaust. The aftermarket offers billet camshafts for the Ford DOHC.

So, which head is best for your 4.6L DOHC project? It can be safely said Ford has a healthy variety of factory DOHC castings that will work well for your four-valve project.

Timing Set
The 4.6 and 5.4 engines have two timing chains: one for each bank of cylinders, driven off two sprockets at the crankshaft. Each chain wraps around a large cam sprocket at each head. Chain tensioners on each bank of cylinders tension the chains. Chain tensioners create tension from engine oil pressure, with each chain following plastic chain guides. There are two basic kinds of chain guides: all-plastic and plastic with an aluminum support. There are two basic types of crankshaft timing gears: one-piece and two separate gears. The one-piece gear is actually two gears in one, which Ford went to in 2001. Ask for XR3Z-6306-BA at your Ford dealer. The earlier two-piece, two-gear setup is more prone to failure if you're going to rev it hard.

Confusion abounds with the Modular V-8 timing sets because there were so many changes along the way. Best we can tell from our research, there have been four basic timing sets for the SOHC engines. From 1991-'92, the Romeo engines had true roller chains, plastic guides (tensioners), and were marked left and right for easier installation. In 1993, there was a second Romeo timing set style, with different tensioners. From 1994-'99, Romeo opted for a link-style timing chain for quieter operation. This means different sprockets.

As you might expect by now, Windsor did its timing system differently than Romeo. First, the cam sprockets are permanently tied to the camshafts, which is unlike Romeo's bolt-on sprockets. The Windsor engines also had the link-style timing chains and sprockets for quieter operation. Tensioners were different than Romeo. Again, Romeo everything, Windsor everything when you're building a Mod motor.

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