Tom Wilson
March 1, 2007
Smokin' the hides is child's play with 650 rwhp, as Adam Montague, owner of Spankin' Time Motorsports, demonstrates in his '06 development car.

Horse Sense: After our test, 31-year-old Adam Montague got aggressive. He dropped a built automatic in his '06 GT and hit the track, resulting in an impressive 10.50 at 126 mph while launching at idle and shutting off at 1,100 feet.

When Mustang modular engines were either Two-Valve or Four-Valve, it was fairly easy to sort out the differences between the two. But with the advent of the Three-Valve Mustang GT engine, the question was if it more resembled a Two- or Four-Valve in its power-making potential

Thanks to a new set of tests performed by supercharging specialist Kenne Bell, the answer seems to be that the Three-Valve is similar to "a Four-Valve, we're fairly sure" when a good-size huffer is installed

A large claim, but one worth arguing, as Kenne Bell has the dyno data to support it. The data begins with the much-visited KB Four-Valve Mustang Cobra test we ran in our March '03 issue ("Snake-Bite Hit," p. 71). During the test, a Cobra with KB's 2.4-liter supercharger and hard-won electronic tuning arrived at a maximum of 617 rwhp when given 20 pounds of boost, a splash of 116-octane leaded racing fuel, and breathing through the stock cats

Here's the ST Motorsports engine in 650-rwhp trim, which from a visual standpoint is an S197 Kenne Bell Mustang GT kit. The 90mm Lightning mass air meter blends in nicely, while most of the experimental 2x60mm Accufab throttle body's polished flash is hidden by hoses.

Three years later, the company has turned to the Three-Valve Mustang GT. While developing his latest blowers for the Kenne Bell S197 kit, Jim Bell noticed the new engine was turning in superb numbers, rivaling the Four-Valve's postings and setting the stage to compare the latest Mustang GT against the vaunted Terminator at higher boost levels. What better way to benchmark the newest Mustang's potential?

One major change between the Three- and Four-Valve tests was the dyno. All Four-Valve tests were run on Kenne Bell's in-house Dynojet three years ago while the Three-Valve tests were conducted on a similar Dynojet at Spankin' Time Motorsports. Why the different dynos? It probably had to do with another high-profile development test using the in-house Kenne Bell dyno-something to do with a 5.4 Four-Valve mod motor in a Mustang. Spankin' Time Motor-sports is operated by Adam Montague. He has a good street cred for S197 tuning and was using his own GT during this test. With monitoring by the KB crew to ensure the usual standard-not our preferred SAE-correction factors were used, along with careful attention to temperatures and pressures, the testing at ST Motorsports proceeded.While it should all wash out in the corrected data, we'll note the vast majority of the Three-Valve tests were run during hot and humid weather. The miserable conditions tended to hover well over 100 degrees with approximately 70 percent humidity.

KB's supercharger kits for the S197 Mustang Three-Valve engine do not include a throttle body, as the stock unit is good up to the mid-500 rwhp level. To reach above that, a large throttle body is mandatory; this 2x60mm prototype unit was loaned to ST Motorsports by Accufab for the test.

Another difference is the Cobra test used KB's then-largest blower, a 2.4 Twin Screw. The S197 used two versions in three years of the company's latest home-grown 2.6-liter screw blower, the standard 2.6 and a high-boost specialist, the 2.6H-the "H" stands for High Pressure Ratio. We're showcasing the data from the more expensive H version because we're concentrating on a 15-psi boost and higher tunes. The newer blowers are more efficient, but as the 20-pound boost level is within both superchargers' reach, the larger blower doesn't have an immense advantage compared to the 2.4

Adam's '06 GT is certainly representative of the breed. It had seen its share of tuning and bolt-ons before its debut as a Kenne Bell guinea pig, but it was returned to 100 percent stock for the KB tests. Jim Bell is a stickler for scientific tests his customers can bank on, which is one reason we enjoy testing with him so much. Adam went back to square one

He didn't stay there long. After tuning the KB to 11.5 pounds of boost, his black fastback went 11.00 seconds at 123 mph at California Speedway while posting a 2.0-second, 60-foot time on street tires. Adam pointed out if a low-11-second Three-Valve ride is the goal, all you need is the standard KB kit, a 15-pound pulley, and the included KB tune. Any more is extra tire smoke with street tires, and you're laying yourself wide open for an expensive, prepped long-block.

The high-400 rwhp range was only a departure point for this test. It didn't take long for Adam to reach the Three-Valve's Achilles' heel: its weak short-block internals. Unlike bulletproof Terminator guts, the new GT's rods and pistons are timid creatures when the big boost blows. In Adam's case, the Grim Reaper arrived at 450 rwhp and a Sean Hyland Motorsports short-block followed. To avoid the expense of a prepped short-block, mid-400 rwhp is the limit. Adam and Jim Bell forged ahead, naturally