Richard Holdener
December 1, 2008

Step By Step

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Mmfp_0812_17_z Psi_boost 2005_gt_20_psi
KB SC '05 GT at 20 PSI
The real surprise of the group was how well the '05 Three-Valve GT did against its Four-Valve brethren. Despite the handicap of having one less valve per cylinder, the Mustang GT motor responded well to the boost. Apparently, the higher static compression and variable cam timing more than made up for the valve deficiency compared to the Terminator motor. The peak power exceeded 700 at the same 20 psi, but the torque production on the Three-Valve motor topped the 600 lb-ft mark, some 28 lb-ft better than the Cobra motor. Even more impressive was the fact that the Three-Valve motor required less blower speed to achieve this power output. From the looks of this test, the '05-up GT motors can hold their head high at the Ford family reunion.
Mmfp_0812_18_z Psi_boost Pulleys
Changing the blower speed and boost level on each combination required changing the blower (and crank) pulleys. Each combination was adjusted to achieve a maximum of 20 psi.
Mmfp_0812_19_z Psi_boost Injector
While the Terminator and GT500 motors made do with their stock injectors (and plenty of fuel pressure), the GT motor was upgraded with a set of 60-lb/hr injectors.

Given its humble, normally aspirated beginnings, the Three-Valve GT motor was never designed with forced induction in mind. Where the Terminator and GT500 motors would easily withstand 20 psi of boost, we could hardly expect the same from the GT motor. To level the playing field, we augmented the GT motor with a suitable reciprocating assembly. The stock components were removed and replaced with forged components from Sean Hyland Motorsport. This included a Cobra steel crank, forged connecting rods, and forged pistons that duplicated the stock compression ratio. We could have dropped the static compression, but we wanted to run the test in stock configuration with the only addition being the required stronger rotating assembly. Besides, with only three valves per cylinder, the GT motor was already at a deficit compared to the Four-Valve Cobra and bigger GT500 motor. We hoped the higher static compression and variable cam timing would help offset the valve count. As it turned out, the Three-Valve motor more than held its own.

All three of the supercharger kits run on the GT, Cobra, and GT500 featured air-to-water intercooler systems. The Kenne Bell supercharger was installed along with the Mammoth intake manifold, 4.5-inch intake tubing and MAF and the dual 75mm throttle body. The GT also required a Mafi a, FRPP fuel pump kit (with dual BAP) and 60-pound injectors. The GT motor was equipped with a 6.5-inch crank pulley and 3.0-inch blower pulley to produce the requisite 20 psi of boost. Note that the blower speed (drive ratio of 2.17) was lowest on the GT motor.

Fed 20 psi of boost from the Kenne Bell blower, the Sean Hyland GT motor produced 704 hp at 6,500 rpm and 607 lb-ft at 4,700 rpm. Like the Terminator and GT500 motors, the power output was still climbing at 6,500 rpm. Though the peak power outputs were similar between the two 4.6L motors, it was the Three-Valve that produced much more average power. Torque production on the GT motor was up by 28 lb-ft over the Terminator. Is this enough to make up for the weight difference between the '03 Cobra and the new GT? Only a trip to the strip would settle that argument, but for now, know that the GT motor can more than hold its own against the Four-Valve contingent--at least those displacing 4.6 liters, that is.

'07 5.4L GT500
756 HP at 6,200 RPM
684 LB-FT of Torque at 4,800 RPM
Pulley Combination 7.5 Crank/2.75 Blower (2.73 Drive Ratio)
Looking back to the recent past, we see the T-bird Super Coupe as a possible predecessor of the current crop of supercharged Stangs, but credit probably belongs to the Lightning for starting the current mod-motor mania. Though saddled with the more-restrictive Two-Valve cylinder heads, the Lightning was blessed with an abundance of displacement compared to the Mustangs.

With 5.4 liters of displacement (330 ci), the Lightning motor combined displacement with the immediate boost response of the Roots-style blower to produce one exceptionally torquey machine. We know now that Ford soon introduced the Terminator and all but one-upped the valve-challenged Lightning motors, but the Terminator was still down on displacement. It wasn't until Ford introduced the ultra-exotic, all-aluminum GT supercar that we saw the proper combination of displacement with the high-fl owing Four-Valve heads.

Luckily for enthusiasts, Ford saw fit to drop a version of that motor in the Mustang we now know as the GT500. Though missing the aluminum block, the dry-sump oiling, and the twin-screw supercharger of the GT motor, the GT500 still offered plenty of performance. Compared to the previous Terminator motor, the GT500 mill offers more displacement, a larger Roots-style blower, and free-flowing cylinder heads. Time and technology march on, and we enthusiasts get to reap the rewards.