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

Kenne Bell's charge cooler is built around the efficient Mustang Cobra heat exchangers. The plumbing and reservoir tank are KB parts; this one features a reservoir with a large opening for adding ice at the dragstrip.

Being careful to add mechanical strength while not changing the Three-Valve's power potential, the engine was specifically set up to retain stock compression, valve sizes, cams, and oiling. The SHM short-block was built using an iron block, Manley rods and pistons, a Cobra crank, ARP bolts, and Inconel exhaust valves from Manley to round out the tough stuff. Stock Three-Valve heads with a stock valvetrain, except the valve material, were retained. The combination is what SHM offers those requiring 850 hp worth of longevity. As an interesting aside, Adam reports the iron-blocked engine is mechanically quieter than he expected.

Externally, the engine remains stock. The exhaust for all testing was stock cast-iron manifolds, gutted stock cats, and a quiet aftermarket axle-back muffler section. We were disappointed by the gutted cats, and the Cobra we are comparing it to used stock cats, putting it at an immediate disadvantage. On the other hand, we admit the low-restriction cat section is more realistic when reaching the 600-rwhp horizon. To be even more realistic, a proper set of long-tube headers and dump tubes would make things interesting when ST Motorsports inevitably tries for all-out power production in the future

Other nonstock items included a Cobra flywheel and a custom ceramic clutch from Clutchmasters. Adam was positive on the clutch, saying it bites hard with no slip. We don't think these parts influenced the Dynojet readings much.

While it has no affect on the dyno tests, it is interesting to consider the weight ramifications of the blower and iron short-block. Adam figures the blower added 100 pounds, the block 80 pounds, and his rollbar 20 pounds to his already 3,850-pound GT, making it heavier than the Cobra.

Getting air into the naturally aspirated side of the supercharger is a major part of building big power with any blower system. The KB Cool-Air Intake is used, which fits a 9-inch conical air filter, 90mm mass air meter, required hoses, and tubing-to-duct air to the throttle body.

Reviewing our March '03 Cobra test, we see the basic engine was mechanically stout due to its factory-installed forged pistons and rods, excellent crankshaft, and free-flowing Four-Valve cylinder heads. As the engine and fuel system were built by Ford for blower power, Kenne Bell was able to substitute a supercharger atop the Cobra manifold and charge cooler, with the rest of the job orbiting around tuning issues and supporting hardware. KB's in-house tuner Ken Christely concentrated on sorting out Ford's fuel pump control strategy and tuning tables, along with getting air into the supercharger. In the end, the major hardware changes were a large conical air filter and plumbing-KB calls this a Cool-Air Intake-a fuel controller, and Boost-A-Pump to manage the stock fuel pumps.

It's amazing how stock the Cobra engine was. Even when reaching well over 600 rwhp, the stock 90mm mass air meter, throttle body, intake and exhaust manifolds, engine, and functioning catalytic converters were in place. All testing was done with 103-octane unleaded race gas, except for the Hail Mary run when two gallons of 116-octane leaded were added for extra insurance. The air filter was removed, but the Cobra was still easily ready for 600 rwhp.

Important steps in the Cobra's power build are listed below.

RWHP Boost
489 13 3.55 Standard Kenne
Bell kit, includes
KB chip, stock air
filter, and mufflers
560 17 3.25 Conical air filter,
stock mufflers
617 20 2.66 Splash of 116-
octane fuel, after-
cat disconnected
Adam and Kenne Bell electronics expert Ken Christley may look like two goats staring at a watch here, but don't think they've been sliding by on good looks alone. The pair did a great job demonstrating what the current Mustang GT is capable of.

How did the new Mustang GT fare? After weeks of dynoing, electronic tuning, fabrication, and brain-bleeding head-banging, the GT tire smoke blew away to reveal three meaningful dyno sheets at 16, 18, and 20 pounds of peak boost using the 2.6H supercharger

Why just three? For one, lesser boost levels had already been explored in previous testing and articles. Also, the new, large, 2.6-liter Kenne Bell superchargers are best suited for high-boost applications. The standard 2.6 does fine at lesser boosts, but the 2.6H has the ability to blow where other blowers hadn't blown before. Finally, while many dyno pulls were made during a two-month period, it took a while for the supporting hardware-mass air meters and throttle bodies-to catch up with the 2.6 blower and Three-Valve engine combination potential. In the end, they made all the difference

Not only did the stock Three-Valve's lack of mechanical strength become an issue at high boost, but the throttle body and mass air meter are must-haves for huge power. The standard Kenne Bell S197 kit includes a Cool-Air Kit built from a 9-inch conical air filter, matching large-diameter air ducting, and a 90mm mass air meter from the Lightning truck, but uses the stock twin-bore throttle body. This combination will push you into the seat, as the stock throttle body is good well past 500 rwhp, maybe up to 550 rwhp. For the all-out power Adam was spooling up, the stock throttle body proved the choke in the system.

The bulk of the truck throttle body doesn't make it easy to fit up against the KB supercharger. ST Motorsports cobbled this sheet-aluminum adapter together to test the V-10 throttle body on the Three-Valve. Accufab's prototype 2x60mm throttle body made nine more horsepower than the V-10 Ford part, but until Accufab hits the streets, the over-the-counter V-10 Ford unit is a good way to go.

What a choke it was. Adam hit the wall at 550-565 rwhp, and nothing would get the GT past that point until a Ford 2x60mm V-10 throttle body was experimentally fitted. It helped tremendously, but was hopeless in the real world; it was never going to fit the KB kit. The company had longtime collaborator Accufab spool up a 2x60mm throttle body that would work, and it came out nine rwhp ahead of the Ford V-10 part. How much more is that than the stock Mustang GT throttle body when stuffing 18 or so pounds of boost down the Three-Valve's throat? More than 80 rwhp, believe it or not. In this case, Accufab made 652 rwhp with 103-octane unleaded and just under 20 pounds of boost. The best the stock throttle body ever did was 568 rwhp under similar conditions

Because the KB blower allows rapid, four minute pulley changes as long as you have the KB pulley wrench ($25) and the necessary pulleys ($69/each), Adam was able to run the Three-Valve back-to-back with no changes, save for pulley swaps. The results for the 2.6H are as follows.

RWHP Boost
569 16 3.00 Nominal 15.5
pounds of boost;
peak at 16
621 18 2.75 Nominal 17.5
pounds of boost;
peak at 18 pounds
652 20 2.50 Nominal 19.5
pounds of boost;
peak just under
20 pounds

The Three-Valve is looking fine compared to prior Cobra numbers. In fact, after goofing around with our Casio, it seems the Three-Valve is 35 hp ahead of the Cobra at 17 and 20 pounds of boost. Don't forget the Cobra was huffing through stock cats while the Three-Valve benefited from gutted units, possibly accounting for the increases seen here

Looking at it another way, what Kenne Bell has essentially done, given the Sean Hyland short-block and the fact that the KB blower kit uses the Cobra charge cooler in a KB intake manifold, is essentially build a Cobra Terminator engine around a set of Three-Valve cylinder heads. That's smart, and it happens to be a good way of testing them. What we found is the Three-Valve head runs with the Four-Valve variety. Our comparison has enough variables in it that we can't authoritatively say who wins the battle of the cylinder heads-the Three-Valve numbers were posted with KB's latest-style, larger-displacement blower, and we're not sure how much those gutted cats are worth-but there's no doubt it's a close contest. Savvy racers might find the Three-Valve is more torque-friendly than the Four-Valve, which is something to explore. In any case, the newest Kenne Bell blower is ready to make huge power when your Three-Valve is ready

Way cool at the track, today's huge horsepower is too much on the street. It will turn your tires into smoking balls of rubber. Adam scooped up these handfuls after our burnout photo shoot.

Although it took weeks of frustrating tuning, fabrication, and experimen-tation, we can sum up the Kenne Bell S197 Mustang GT 2.6H blower's high boost capabilities with the three dyno runs

The runs are labeled 16, 18, and 20 pounds of boost, which are the peak boost numbers. Most of the duration of each run, however, was made at half a pound of boost less than the peak figure. These "official" runs were made on the same day in rapid succession after all electronic tuning was finalized. Careful attention to maintaining constant temperatures and no adjustments other than changing blower pulleys allows comparing these runs to each other in detail. All data is from the ST Dynojet inertia dyno, using the "standard" correction factor. We prefer to use the SAE correction, but many shops use standard, as it reads a higher number. KB used the standard correction during the '03 Mustang Cobra test as well

Don't forget to eyeball the torque figures; they're fantastic

Kenne Bell recently introduced its own line of Twin-Screw superchargers, which we detailed in our Nov. '06 issue ("Home Screwed," p.48) and are seeing again in this article. Additionally, KB has just released an "H" version of the new 2.6 liter, which is the blower featured in this article and the first time it's been seen anywhere. Testing on the H-blower costs $100 more than the standard.

The H stands for "High Pressure Ratio," which is the internal pressure ratio (the compression ratio) of the Twin-Screw compressor. The only physical difference between Kenne Bell's standard 2.6 and 2.6H blowers is the 2.6H employs a smaller, shorter outlet. This effectively makes the rotors longer, allowing the supercharger to compress air for a longer distance. The blower's displacement, and most confusingly, the boost, does not change with the H blower, but the supercharger's efficiency increases. In other words, the H blower takes less horsepower to turn at high boost. This makes the 2.6H and 2.8H blowers desirable for boost pressures exceeding 15 pounds. Below 15 pounds of boost, the standard 2.6 and 2.8 KBs are preferred

To put some carefully obtained numbers to it, at 19.5 pounds (20-pound peak) of boost, the standard 2.6 blower made 622 rwhp and the 2.6H made 652 rwhp, a gain of 30 rwhp for the H blower. At 15.5 pounds of boost, the H was ahead by only 10 rwhp-569 versus 559. Below 15 pounds, the standard blower makes more horsepower, the more so as boost lowers into the more common 6 to 12 pound range.