The one feature of a DOHC Ford modular engine that will smack you in the face is the massi
B Heads ('93-'97 Lincoln Mark VIII, pre-'99 Lincoln Continental, '96-'98 Cobra): These swirl-port castings were the first production Ford DOHC heads. With generously sized twin intake ports, square primary and round secondary, these heads do their best work after 8,000 rpm. Their best use seems to be on power-adder motors. It's probably not the way to go for a streetable, naturally aspirated motor.
C Heads ('99-'01 Cobra, '99 Lincoln Continental): Also known as tumble-ports, these heads feature a single-intake port with a smaller cross-sectional area that boosts airflow velocity. This allows for substantial increases in midrange torque production and superior horsepower production (compared to B heads) under 8,000 rpm. They also feature the same small exhaust ports as B and Navigator heads.
Navigator Heads ('99-present): These 5.4 DOHC heads feature essentially the same intake port design as C heads, but they have a much larger intake port volume than 4.6 castings. Despite the fact they have the same small exhaust port as B/C heads, the extra intake port volume could be beneficial in helping fill a motor of greater displacement. Expect slightly better midrange torque and sub-8,000-rpm horsepower production than even C heads. The larger intake port size, however, leaves a sparse selection of intakes from which to choose.
Ron Robart and his Fox Lake Power Products staff have truly pushed the envelope with Four-
FR500 Heads (Ford Racing Performance Parts): These high-flow after-market heads feature a modified C head intake port (raised roof) capable of flowing 330-cfm-plus (at 0.500 inch lift) on the intake side (with a Stage II port). But, with the same small standard exhaust port as most other DOHC heads, you will still have to remove a decent amount of material from the exhaust ports. Port entrance shape/size remains identical to C heads, so finding an intake isn't hard. The heads are capable of producing power beyond 8,000 rpm, unlike earlier versions of the tumble-port castings.
'03 DOHC Heads ('03 Aviator, Marauder, Cobra, Mach 1, Australian Boss 260/290): Featuring the same ultra-high-flowing intake port as FR500 heads, but combining it with a newly designed larger and more rectangular exhaust port, these may be the best all-around DOHC Ford heads yet. For those with a forced-induction street motor, they are without question the best heads available. As with the FR500s, they should produce great power even beyond 8,000 rpm, regardless of application.
Andy had this to say about his final choice of cylinder head. "The decision to use Navigator heads for the 5.3 featured in this story was a fairly simple one. The combination of a tumble-port intake design and slightly more intake port volume than a regular C head seemed like a winning combination for an 8,000-rpm naturally aspirated motor of larger-than-normal dimensions. If I could do it over, I would use '03 DOHC heads without question. The reason is that there is a much broader selection of intakes to use with the '03 DOHC heads than with Navigator (5.4) heads on a 4.6 block. The larger 5.4 heads don't leave a lot of room in the valley for the lower portion of an intake to sit, essentially requiring the use of either an ultrashort runner (highly modified) '99-'01 Cobra intake or a custom sheetmetal unit."