Muscle Mustangs & Fast Fords
The Four-Valve Swap Power Struggle Upgrade
When You Need 9-Second Muscle And Your Two-Valve Engine Can't Hack It, Swap It Out For A Four-Valve.
If your lottery ticket comes through--and your luck continues to allow you to locate a set--the best choice of all is the '00 Cobra R heads. These are extremely rare, though, and an easier way to get a set is to find a pair of Ford GT or Shelby GT500 heads, as these are very similar to the Cobra R casting. All of these heads present various issues when it comes to intake manifolds, but that's another story altogether. The Terminator heads are considered the best cost-effective heads, and in the end, we chose to use a set of late-production Terminator heads. Early Terminator heads only had three or four threads for the spark plugs. Ford later went to nine threads to cure a spark-plug blowout issue.
The cylinder heads were sourced from Darrin Burch at BC Automotive, and shipped directly to Kris Starnes of Kris Starnes Racing Heads, in Hastings, Florida. Starnes has been providing some fast Mustang enthusiasts with his ported cylinder heads, and he came highly recommended by Papitto. Starnes worked over the Bullitt's Two-Valve heads that allowed it to run 10.01 seconds in the quarter-mile. The porting job on the Four-Valve heads was beautiful both aesthetically and, as you'll see later, functionally.
Kris first worked the exterior of the upper plenum and removed all the extraneous stuff that wouldn't be used. He then port-matched the throttle body opening to mate up with the BBK twin 65mm unit that we're reusing. We also had to remove an Allen-head plug on the gasket aside of the throttle body to allow routing of the idle air, as the Bullitt intake doesn't utilize this port. Next, the inside of the upper manifold was ported to smooth flow and increase the intake volume as much as possible. After streamlining it, Starnes dropped the upper plenum off to Bruce Birkett of Palatka, Florida, who applied a liquid-like, silver finish.
The lower manifold of our intake setup is a huge departure from the standard Cobra intake manifold. Starnes converted the long-runner manifold to a short-and-straight runner design by applying a lot of machine work and epoxy. The bottom portion of the runners were removed and the areas filled with epoxy. This transforms the intake to allow a straight flow of air from the plenum area directly to the intake runners.
Starnes does not recommend this configuration on all applications, as it is particularly suited to high-rpm, forced-induction motors. The shorter runners will kill port velocity, and both torque and driveability will suffer on a naturally aspirated street car, or even a low-boost-level blower or turbo setup. Keep in mind that the sort of powerband change that a short-runner intake provides may require different gearing or, as in our case, a higher stall speed for our torque converter.
With the removal of the runners in the intake rework, we lost considerable plenum volume. Starnes supplied us with a fix for that in the form of a 1-inch aluminum spacer that he offers on a custom basis.