Richard Holdener
August 1, 2005
It was great to get the '05 Three-Valve GT on the chassis dyno, especially since we knew we were going to get to test the new Kenne Bell supercharger kit.

For me, the glass is almost always half full. Here it is 2005, and I have a great job where I get to build all manner of high-performance machines, a wonderful wife who supports my chosen profession, and even a new baby on the way. To top things off, Ford went and introduced what is one of the best-looking Mustangs to ever roll off the assembly line.

In addition to the drop-dead good looks, the '05 was blessed with what is arguably the most powerful motor ever to grace the simple GT. Despite the differences in the gross-versus-net power ratings, it would take a good-running 428 Cobra Jet to keep up with a new 281-inch Three-Valve GT.

As impressive as the new 4.6 Three-Valve GT motors are, the glass-is-half-empty side of me can't resist pointing out the fact that noticeably absent in '05 Mustang offerings is a Cobra, or, more specifically, a supercharged Cobra. A case can certainly be made that the Three-Valve can all but replace the normally aspirated Four-Valve powerplant, but all the extra valves and variable cam timing in the world won't make up for the missing supercharger available on the '03-'04 Four-Valve Cobra 4.6s. Luckily for us, the gang at Kenne Bell specializes in filling that half-empty modular performance glass. Enter the Three-Valve supercharger kit.

The most important part of the new Kenne Bell blower kit for the '05 Three-Valve GT was the Autorotor twin-screw supercharger. Offering immediate boost response and improved efficiency compared to a typical Roots-style blower, the twin-screw was capable of supporting over 500 hp.

Naturally, we all welcomed the power increase from 260 hp to 300 hp offered by the new Three-Valve configuration of the familiar 4.6. The extra 40 hp has been a long time coming, but can the normally aspirated Three-Valve really be expected to take the fight to the 5.7-liter LS1s of the world? Despite the new 300hp rating, the larger 5.7 LS1--to say nothing of the new 400hp 6.0 LS2--will easily dispatch the Three-Valve GT.

With the introduction of the supercharged 5.4 Cobra a year or so away, Mustang enthusiasts are temporarily sans factory supercharging, but the aftermarket is always willing and able. Kenne Bell has long been associated with supercharging Mustangs, way back to the 5.0 days. Its offerings for the 4.6 include the early and late Two-Valve and most models of the Four-Valve Cobra (including Mach 1).

Kenne Bell really came into its own when Ford introduced the supercharged Lightning and then the supercharged Four-Valve Cobra. Factory equipped with positive-displacement Eaton superchargers, enthusiasts soon found that the route to huge power from their Lightning or Cobra motor was with a Kenne Bell blower upgrade. Simply replacing the factory Eaton M112 supercharger with a more efficient and powerful Autorotor twin-screw blower resulted in big-time power gains. Where the supercharged Cobra guys struggled to eclipse 500 wheel horsepower, some '03-'04s equipped with the twin-screw blower were soon besting 700 wheel horsepower and dipping into the nines, all without resorting to nitrous.

The new Kenne Bell kit came equipped with a 2.75-inch (six-rib) blower pulley to work with the stock 6.5-inch crank pulley. The pulley combination provided 9 psi of boost to the 4.6 mod motor.

All that supercharged experience with the previous mod motors has paid off in the form of a new kit for the Three-Valve 4.6 used in the '05 GT. Given the near (normally aspirated) Four-Valve Cobra performance from the Three-Valve 4.6, it's not sur-prising that Kenne Bell was able to not only reach the power output of the '03-'04 super-charged Four-Valve engine, but surpass it.

Installation of the Kenne Bell blower kit on the Three-Valve Ford pushed up the rear-wheel power output from 268 hp to 459 hp. Think about it--a gain of 191 hp with just 9 psi of boost. As we have come to expect, the Three-Valve super-charger kit from Kenne Bell is somewhat more involved than just plopping a blower in place of the factory intake. The days of building a suitable blower mount (for a centrifugal) or an adapter plate (for a Roots-style or twin screw) and combining it with an FMU are long gone. Blower kits (at least complete ones) have become a serious undertaking.

In the case of the new Kenne Bell Three-Valve kit, the serious undertaking included not only the required ECU recalibration (no easy task given the combination of drive-by-wire throttle and variable cam timing), but also a dedicated air-to-water intercooler. Included in the Kenne Bell Three-Valve kit is an air-to-water intercooler located in the lower intake manifold. The pressurized air is fed by the (already efficient) twin-screw blower though the intercooler, the lower intake, and finally to the cylinder head ports. Like the system used on the factory supercharged Four-Valve Cobra, the Kenne Bell air-to-water intercooler system employed a front-mounted heat exchanger to dissipate the unwanted heat to the atmosphere.

The supercharger was attached to the lower intake using a custom machined adapter plate. Note the pair of screws inside the triangular opening on the underside of the unit.

Naturally, the highlight of the Kenne Bell intercooled supercharger kit is the blower itself. In the case of the standard intercooled system, the kit featured a 1.7-liter twin-screw Autorotor supercharger. MM&FF readers should remember the testing performed on both the Lightning and Cobra motors originally equipped with the Eaton M112 Roots-style blowers. Simply replacing the Roots-style blowers with a more efficient twin-screw resulted in significant power gains (even at the same boost level). Compared to a traditional Roots-style blower, the twin-screw design offers improved flow capability (per blower displacement), decreased inlet temperatures, and even a reduction in the parasitic losses associated with driving the blower.

For those who want to take things to the next level, Kenne Bell will also offer the Three-Valve kit with the larger 2.4 blower, capable of exceeding 700 wheel horsepower with the proper engine combination. Check out the supplied graph for a rundown on the power gains offered by the blower upgrade. The only thing holding the motor back from exceeding 535 wheel horsepower was the stock clutch. Apparently the clamping force simply wasn't up to the task of withstanding the onslaught of over 500 lb-ft of torque.

The thing we enjoy about covering a supercharger kit from Kenne Bell is that those guys always seem to have answers, which is a good thing since we always have questions, like, "Did you run it at different timing levels? How about different air/fuel ratios? Did the variable cam timing have any effect on the power once you added the blower?"

Equipped with the new Kenne Bell supercharger, the 4.6 Three-Valve GT produced 459 hp and 422 lb-ft of torque.

A perfect example of the kind of data generated by Kenne Bell's R&D was the sensitivity of the factory mass air meter. Believe it or not, you can no longer just pop on a cone filter in place of the factory airbox assembly, nor can you reorient, reposition, or even rotate the factory mass air meter without dramatically altering the air/fuel curve. This will come as a shock to those of us who cut our performance teeth on the 77mm Pro M meters in the 5.0 Mustangs, but the factory mass air meter is so sensitive that even rotating it from its original inclination will create havoc. Think about that when you order a cold-air intake system for your '05 GT. Unless the new air intake system comes with a revised program for the ECU, don't be surprised if the air/fuel ratio is way out of whack the first time you take it to a dyno.

The MAF sensitivity did not present a problem for Kenne Bell since it had to reprogram the factory computer anyway. This was necessary since the kit included a set of 42-psi injectors and a (larger) 90mm mass air meter to go along with the Boost-a-Pump.

The Boost-a-Pump and larger injectors ensured plenty of flow potential from the fuel system while the revised programming ensured that the fuel would be delivered in the correct amounts and at the correct time.

Ditto for the timing curve, as naturally the factory ignition timing curve was altered to ensure safe operation under boost. Use of the 90mm mass air meter allowed the power to be taken to 500 wheel horsepower without topping out the voltage reading (5 volts). The stock meter was only good for around 400 wheel horsepower before the voltage reached the maximum reading of 5 volts. The twin-screw blower combined with the efficient air-to-water intercooler allowed the Three-Valve motor to be run safely at 9 psi on pump gas without fear of detonation.

As with any good supercharger, additional power is just a pulley change away. This is especially true of the larger 2.4 blower, as running 14 psi through the 4.6 resulted in a peak power number of 535 hp and nearly 500 lb-ft of torque. It's likely the 600hp mark will fall once the factory clutch has been upgraded with the Centerforce dual-friction system. Kenne Bell was in the process of upgrading the stock clutch with the unit from Centerforce, as this story was being completed.

While impressed with the results of the test run with the 1.7 blower at 9 psi, things got serious when Kenne Bell installed the larger 2.4 huffer and upped the boost pressure to 14 psi. The otherwise stock Three-Valve produced 535 hp and right at 500 lb-ft of torque.

Unlike the blower upgrades for the Two-Valve 5.4 Lightning and the '03-'04 Four-Valve Cobras, the kit for the '05 4.6 Three-Valve required a couple of dedicated castings, including a new lower intake manifold. Natur-ally, space constraints prohibited combining the blower system with the factory Three-Valve (long-runner) intake manifold. With space at a premium, the most natural position for the air-to-water intercooler was in the valley between the cylinder heads.

Unfortunately, this location was already being occupied by the lower intake manifold. The only way the two systems were going to fit was to have them cohabitate, thus the intercooler core was positioned inside the lower intake ( la the Four-Valve Cobra). This obviously eliminated the possibility of any appreciable intake runner length, but the immediate boost response offered by the positive-displacement blower more than made up for any loss caused by the new intake configuration.

Incidentally, for those '05 GT owners on a budget, the Kenne Bell blower kit will also be available without an intercooler. The boost supplied by the nonintercooled kit will be lowered to just 6 psi, but the loss of the intercooler core will drop the price of the kit substantially.