We bolted the Ford to the dyno with a set of 15⁄8-inch long-tube headers and an 18-inch collector extension to help the low-speed torque, then we fired the small-block up. For the first test, we were reusing the stock, Explorer cam, so the engine didn't need any break-in time. We we set the timing to 34 degrees and rejetted the carburetor to get our desired 12.5:1 air/fuel ratio. With the tuning finalized, the little cam made far more power than we expected, cranking out 328 lb-ft of torque at 4,200 and 294 hp at 5,400 rpm. We probably could have coaxed an even 300 hp out of this stock configuration, but with such good numbers, we decided to jump right into the first Lunati cam.
The hardest part of swapping the cam was just getting the timing cover off and back on again. The rest went very smoothly, and we reused the stock lifters and suffered no ulcers worrying about breaking in the cam. In less than an hour, we were ready to test. Despite adding 30 degrees of intake duration, this cam improved torque throughout the entire test from 3,000 rpm on up. At 4,300 rpm, the torque peaked at 348 lb-ft, a solid 20–lb-ft improvement, and the Lunati continued to outperform the stock cam all the way through with a peak horsepower increase of 60 hp at 354 hp at 5,700 rpm! Perhaps even more surprising was how well this engine idled, calmly purring with 16 inches of manifold vacuum at 980 rpm. This is a function of the somewhat wider lobe-separation angle of 112 degrees.
This test went so well that we were eager to see what the slightly bigger cam would do. This one added another 10 degrees of duration over the smaller Voodoo cam, along with 0.020-inch more intake valve lift. The longer duration created a slight loss of torque below 3,300 rpm compared to both previous cams, but beyond that mild loss of power, the bigger Voodo out-powered its smaller cousin, cranking out a peak torque of 355 lb-ft at 4,300 (a gain of 7 lb-ft over Test 2) and charging to a peak horsepower of 376 at 6,000 rpm. That's an improvement of 22 hp over Test 2. But what we were most impressed with was the solid gain in torque over the smaller Voodoo cam from 3,500 and up. In a lightweight Fox or early Mustang with a five-speed behind it, this would be a great little street engine. As you might expect with more duration, the idle quality dropped significantly but was still able to make 11.5 inches of vacuum at 980 rpm. That's more than enough to feed a power-brake booster.
For the cherry on top of this Blue Oval desert, we bolted Mike Thermos' simple Powerstar plate nitrous system under the carburetor, installed the 140hp jets (55 fuel and 55 nitrous) and used the trigger to retard the timing on the MSD ignition by 8 degrees. We heated the bottle to 950 psi and hit the button at 4,500 rpm. The dyno recorded a peak horsepower reading of 489 hp, but the air/fuel ratio was slightly on the rich side at 11.5:1, so we increased the nitrous jet to a 61 and hit the button again. This pushed that little Ford to a peak horsepower of 503, and that was where we stopped. Tim's junkyard refugee survived the beating and we had a bitchin' story.
Mike Slover at Slover's Porting Service primarily focused on the exhaust side of the GT40P heads. Even with relatively small valve sizes of 1.85-inch intake and 1.46-inch exhaust, the P-heads flow much better than previous production iron heads and even slightly better than the GT40 castings.
Slover machined the heads for screw-in studs and guideplates since the original configuration used a net lash system that was just too weak. We used Comp's adjustable guideplates to ensure the rockers lined up properly over the valvestems.
The stock GT40P springs must be upgraded when going to a longer-duration cam. This photo shows the Lunati dual springs we used to ensure valve control at 6,000-rpm-plus engine speeds. They offer 125 pounds on the seat and 325 pounds at 0.600-inch valve lift. These specs are perhaps a bit light, but work because of the Ford's smaller, lighter valves.
We tried two different Lunati Voodoo hydraulic roller cams. The smaller of the two (221 degrees at 0.050) delivered 30 more degrees of duration on the intake side compared to the stocker. The larger one was still relatively conservative with 231 degrees of duration at 0.050. Both Lunati cams were ground on cast-iron cores, which allowed the use of a stock cast-iron distributor gear.
One way to stay within budget on an upgrade like this is to reuse the stock hydraulic roller lifters. Even with stronger valvespring pressure, these lifters worked flawlessly. We were also able to remove the lifters for the cam change without having to remove the heads. Retro-fit hydraulic roller lifters (in earlier non-roller cam blocks) are taller and can't be removed unless you pull the heads.