Tom Wilson
March 1, 2004
Photos By: Courtesy Of Kenne Bell

Kenne Bell is on a roll. No sooner did we report in depth on the company's supercharger exchange for the '03 Cobra ("Snake-Bite Hit," Mar. '03, p. 71), than we were back looking at its Two-Valve offering for good, old Mustang GTs.

And Jim Bell at Kenne Bell was even more amped up about his GT kit than he was over his Cobra piece. Among other superlatives, he noted his kit on a Two-Valve made more power-about 25 more horsepower-than a stock '03 Cobra with its Roots blower. In fact, Jim went so far as to say his Two-Valve Ford kit is the best one he's ever done.

That's a big claim, so instead of making you wade through this entire article to find the ultimate power figures for the Kenne Bell GT kit, let's note right now that it yielded 433 hp when equipped with optional large air meters, inlets, and throttle bodies. That's with a 271/48-inch blower pulley and 9 pounds of boost. This is the detonation limit with 91-octane pump gasoline, so consider it the hot street setting. The blower can easily double that boost with smaller pulleys, and if you're willing to run racing gasoline for detonation protection and have the piston and connecting rod improvements to withstand such power, then somewhere in the 15-pound boost range and 500 hp makes sense. The brass ring at press time was 24 pounds on a Kenne Bell employee's car. We'd have run it for you, except it had two connecting rods sticking through the block.

Even after half the parts atop the engine were replaced or rearranged, the finished Kenne Bell Two-Valve supercharger installation looks as though it grew there. A single belt is used to drive both the blower and the stock engine accessories. The backward-facing alternator requires a lengthened wiring harness and minor clearancing on its lower face.

The Kit
Given the world is well populated with Mustang Cobras, logically there is little need to attempt huge power from the GT kit. That's what '03 Cobras are for, thanks to their flow-happy Four-Valve heads and stout piston/rod assemblies. The GT kit, rather, is designed to make vertebrae-challenging power at street-able levels, although the nature of the Kenne Bell blower allows high boost levels for those who insist on testing their luck against the GT's cast pistons and pressed metal connecting rods (very weak, these). Therefore, Kenne Bell has packaged its GT blower offerings with a price-conscious, non-intercooled version producing 6 pounds of boost. Kenne Bell says it adds 106 hp.

All remaining versions of the GT kit are intercooled and differ only in the pulley fitted. And there are plenty of pulleys-7-14 pounds in 1-pound increments-so the GT owner definitely has his choice of boost levels. Obviously, the 6-pound kit has clear price appeal that should help it sell, and the 9-pound kit is the limit with 91-octane pump gas, so that seems a logical bestseller as well. It makes an additional 160 hp, says KB.

This Bullitt is wearing "all options." That means it has the cold-air kit, the 90mm mass air meter, the Big Tube kit, and the 75mm throttle body. These parts add around $600 to the tab and 20 hp to the tires.

From there on out, race fuel is the rule. Because the Kenne Bell supercharger allows quick and simple pulley changes, we'd say the 9-pound kit should be the top seller, along with a spare pulley for track days when 100-octane unleaded can go in the tank. Fit the 14-pound pulley and enjoy an additional 262 hp, says Kenne Bell.

In general terms, fitting the Lysholm screw supercharger Kenne Bell uses to the 4.6 GT engine follows Cobra practice. The stock GT intake manifold is removed, and the Kenne Bell supercharger and intake manifold assembly are fitted in its place. This nestles the supercharger in the 4.6's valley, with the intercooler (on versions so equipped) in the valley at the bottom of the intake manifold.

Intake air thus passes through the air filter, the mass air meter, and the throttle body, and then enters the supercharger. The supercharger discharges the air downward and through the intercooler core (if so equipped). Then the air turns 180 degrees at the bottom of the intake casting. Flowing straight up, it goes to the top of the intake where it makes a second 180-degree turn to flow down into the cylinder head. The S-bends aren't the last word in airflow, but they don't seem to hurt power production, and they keep the blower from pro-truding through the hood by lowering the packaging height.

Kenne Bell recently upgraded the blower drives on its short-nose superchargers as used on Two Valves. The new drivesfeature more internal support for the blower driveshaft, along with a distinc-tive pulley shape.

The Kenne Bell assembly includes a new intake manifold, a supercharger, a blower bypass valve, 36-lb/hr fuel injectors, fuel rails, a water neck, a 160-degree thermostat, and-on intercooled models-the air-to-water intercooler's heat exchangers. All this comes assembled by Kenne Bell, so installation of the main attraction is little more than swapping the intake manifold and fitting the stock fuel rails that come off the donor car.

There are other parts to install, however. The necessary electronic updates are handled by a Kenne Bell Mono Chip, which plugs into the EEC V computer. Fuel pressure is raised with a Kenne Bell Boost-a-Pump in the trunk, which takes a bit of wiring and mounting. And intercooled kits require mounting a water reservoir and water-to-air heat exchanger below the radiator, along with the expected hoses and fittings.

In the background is a cold-air kit showing off its big 12-inch open-element air filter. It fits vertically in the right front fender. In front is a Big Tube kit. Its molded plastic construction is much more free-flowing than the stock corrugated rubber bellows. Both kits happen to have 90mm mass air meters attached.

Options include pulleys, of course, and a cold-air kit, a 90mm mass air meter, a Big Tube kit, and a 75mm throttle body. A two-program Switch Chip is a $99 option as well.

With the exception of the Switch Chip, all these options address the naturally aspirated-or inlet-side of the supercharger. This part of the airflow path is often overlooked, but it is a major player in horsepower building.As the power levels rise, some of these options can therefore make significant power gains.

The cold-air kit follows common naturally aspirated practice, placing a conical air filter inside the right front fender where it picks up cool air and avoids fan wash. Connecting the air filter to the mass air meter is a free-flowing fabric tube with a spiral wire stiffener. Theoretically this tubing shouldn't be the last word in airflow, but numerous dyno tests have proven there's a power gain from it.

The 90mm air meter obviously supports more airflow than the stock meter, but even with high boost pressures, the engine doesn't require anywhere near the airflow a 90 is capable of. In fact, the stock meter does a surprisingly good job of flowing air, but it runs out of metering range at the 8-9-pound boost level. In tuning-speak, the voluminous airflow "pegs the meter," meaning its output voltage flatlines at 5 volts. When this happens, the computer has to guess at the airflow entering the engine because the pegged air meter can't read any higher. In practice, the computer then loses control of the air/fuel ratio and bad things happen, among them a loss of power. The larger 90mm meter can meter the higher volume airflow, however, so it allows making high horsepower while retaining closed-loop capability. We recommend it when opting for the 9-pound kit. Naturally, the 90mm meter requires a matching calibration in the Mono or Switch Chip.

All but the entry-level Two Valve Kenne Bell kits are water-to-air intercooled. The intercooler radiator mounts below the radiator and breathes through the lower grille opening. On the road, the water temp stabilizes at about 10 degrees above ambient; on the dyno, a large fan does the same.

The Big Tube kit is a molded plastic tube running from the mass air meter to the throttle body. The stock, corrugated rubber hose is highly restrictive, so it's a smart idea to replace it with something better flowing whenever you have the money. Furthermore, the stock rubber hose doesn't stretch far, and if a larger throttle body (75mm) is used, then the Big Tube kit is required just to fit the throttle body.

That neatly brings us to the throttle body. GTs use a 65mm throttle body, while Bullitts use an oval-bore unit that, unfortunately, doesn't mate with the Kenne Bell intake. At the lower boost levels, a GT-style 70mm throttle body is sufficient (it gains about 11 hp over the 65mm throttle body), but again, around the 8-9-pound boost range, the stock meter begins to cost a bit of power. Furthermore, if you want the Big Tube kit, then you'll want the 75mm throttle body to mate with it, and if the 75mm throttle body is what gets your attention, you'll really want the Big Tube to go with it as the stock inlet tube starves the bigger throttle body. Finally, the Kenne Bell inlet casting (where the throttle body bolts up) is sized for a 75mm meter, so there is no grinding required to fit the larger meter-just bolt it on.

Kenne Bell uses Accufab billet throttle bodies because they're well built. It doesn't hurt that Accufab is less than two miles from Kenne Bell either. Both 70 and 75mm throttle bodies are used-the larger unit is there mainly to accept the Big Tube kit's large pipe diameter.

The Bullitt Option
Kenne Bell has developed the necessary extra parts to accommodate the Bullitts. The changes are simply those required to fit the supercharger to the Bullitt, and-considering the KB blower uses its own intake manifold, throttle body mount, and inlet tube-once the KB kit goes on, there is no difference between a GT and Bullitt.

The major pieces required to accommodate the Bullitt include a set of stock fuel rails (the Bullitt rails are ever so slightly different and won't fit the KB blower/intake assembly), an alternator pulley, a GT-style aftermarket 70mm throttle body (the Bullitt throttle body doesn't fit the KB blower inlet), a stock GT intake hose (can be upgraded with the Big Tube kit), and various small brackets and fittings.

Bullitt pricing was not set at press time, but an extra $400 was guesstimated. Given the popularity of Bullitt intake manifolds, owners of these cars ought to be able to sell their stock intakes to recoup some of their investment.

Kenne Bell casts its own intake manifold for its blower installation. It really is the backbone of the kit, as nearly everything bolts to it. The charge-air entry hole in the center and the passages for the intercooler plumbing are plainly visible in these top and bottom views.

If you want to see a real piece of work, check out Kenne Bell's installation instruction booklet for GT Mustangs. One hundred and four pages long with approximately 275 photos, the thickness of this manual illustrates the numerous small steps required to install these superchargers, along with the attention to detail given by KB.

In fact, there's nothing difficult about installing these superchargers, but there are plenty of small steps, and KB likes to hold your hand all the way. For example, the manual devotes six photos to installing the EGR solenoid to the KB-supplied bracket. It would be difficult to get lost with step-by-step instructions such as these.

We've condensed the instructions to a handful of photos and captions so you can make up your own mind of what's required.

How much power the Kenne Bell supercharger makes is, naturally, the heart of the matter. To find out, we went to Kenne Bell to observe a Bullitt installation, do a few dyno tests, and drive the finished product.

In the course of developing its Two-Valve kit, Kenne Bell has run numerous dyno tests on GTs and a Bullitt. These run the gamut of pulley sizes, intercooler fitment, octane rating, electronic tuning, Boost-a-Pump settings, and on and on. To illustrate the Two-Valve kit's abilities, we're presenting some of Kenne Bell's tests run when we were not present, along with those tests performed with us on hand. To cut to the chase, we're not worried KB was cooking the books during their development cycle, and we are confident all the data printed here is accurate. Furthermore, all testing we're showing here was performed on a Mustang Bullitt, which, as previously noted, is mechanically identical to a GT once the supercharger is installed.

We snagged this partially assembled blower/manifold unit off Kenne Bell's assembly bench for photos. It obviously shows the superchargers can be had polished, and, even though this big-pullied unit is a 6-pound, nonintercooled piece, it uses the same intake manifold with the deep "sump" for the intercooler core. The throttle body bolts to the four-bolt flange at the right rear of the supercharger. Its opening is sized for a 75mm throttle body. KB ships the supercharger/intake assembled a little more than shown here so you don't have to bother with it.

Stock Bullitt
Step one is always a baseline. Our naturally aspirated subject Bullitt put out 239.7 hp and 270.7 lb-ft of torque. These are strong numbers, equating to 270 flywheel horsepower and more than 300 lb-ft of torque, due to our test car having JBA short-tube headers, a 2.5-inch MRT high-flow-cat H-pipe, and a SpinTech after-cat exhaust. These parts remained on the car through the following tests; otherwise, the Bullitt was stock.

Base Kenne Bell
Next, the entry-level, nonintercooled Kenne Bell kit was installed and the car was rerun. Spinning the standard 3 3/8-inch blower pulley resulted in 6.2 pounds of boost at 6,000 rpm, along with 350.2 hp and 365.6 lb-ft of torque. That's a massive gain of 110.5 hp and 94.8 lb-ft of torque at the peaks. Down low, where you can really feel a positive-displacement blower such as the Kenne Bell, the improvement was even more impressive. At 2,500 rpm, for example, the torque jumped from 247.8 lb-ft naturally aspirated to 350.8 lb-ft supercharged-a gain of 103 lb-ft. This is the sort of muscle that feels like someone slipped a big-block under the hood-not bad for $3,800.

Note that this test is of the standard kit with no frills, i.e., the blower was breathing through the stock air filter, the mass air meter, the inlet tube, and the throttle body.

The next step was to add intercooling, along with a smaller blower pulley. These are two changes, sure, but they're the next logical progression because pumping the charge air through the intercooler consumes about a pound of boost, and, with the intercooler, more boost can be run without fear of detonation. So, just adding the intercooler would result in a slight power loss-not the sort of thing you want after spending around $1,100 for it-and because you could safely go to the smaller 271/48-inch blower pulley, that combination is what these cars will run. With the 271/48-inch pulley huffing through the intercooler, boost measured 9.1 pounds at 6,000 rpm. Peak num-bers were 402.8 hp at 5,600 rpm and 427.4 lb-ft of torque at 3,400 rpm. These are gains of 52.6 hp and 62.0 lb-ft of torque, so intercooling definitely has its benefits.

Cold-Air Kit
From here on out, what's left are the options (cold-air kit, meter, and so on), smaller pulleys for more boost, and higher-octane fuel to suppress detonation when the boost reaches the danger zone.

To avoid trapping and overheating charge air, all Kenne Bell superchargers are equipped with a bypass valve. Vacuum-powered, the bypass opens during deceleration, reducing inlet drag and allowing air cooling of the supercharger as well. The valve can even be adjusted open for a "valet" function (driveable, but no boost).

Our next test adds the cold-air kit-a cone air filter and hose-while keeping everything else the same. This is a valid test to show what the cold-air kit is worth by itself, but in reality, Kenne Bell says you'd want to add the 90mm mass air at this point as well because the stock 80mm mass air meter begins pegging at this point. Under laboratory conditions this was easily monitored, but you don't necessarily want to run like this (stock mass air, cold-air kit, 9 pounds of boost) on the street.

With the cold-air kit, the power peaks rose to 410.7 hp and 434.3 lb-ft of torque. That's 7.9 extra horsepower and 6.9 additional pound-feet of torque. Boost gained just slightly as well, rising 0.3 pounds at 6,000 rpm to 9.4 pounds.

The Pump-Gas Limit
Kenne Bell explains the pump-gas limit as the maximum combination that can safely run on 91-octane pump fuel, so everything after this is going to require race gas.

If the tune-up seems a bit tame, well, Kenne Bell really doesn't want you spitting connecting rods through your block, and neither do we. You're bound to see hot dogs running more boost than this on the street, but we guarantee you won't see them doing it for long.

The 91-octane limit is quite close to our previous test. It uses the same 271/48-inch blower pulley and cold-air kit, but this time we added the 90mm mass air, the Big Tube kit, and the 75mm throttle body. With these breathing aids, boost rose to 10.3 pounds at 6,000 rpm (it was 8.5 pounds at 4,000 rpm).

Horsepower made significant gains, topping at 433.4 hp at 5,800 rpm, while torque essentially remained unchanged at 436.6 lb-ft at 3,600 rpm. These are gains of 22.7 hp and 2.3 lb-ft, so the inlet side options aid horsepower, but they don't do much for torque.

Blower installation begins by stripping the top of the engine all the way down to the lower intake manifold. As with all modular engines, this means plenty of small-parts removal, but it's nothing difficult.

91 Octane And Stock Exhaust
Using a second Bullitt with precisely the same engine tune (blower, 271/48-inch pulley, pump gas, and all the inlet side options) as the previous test, but wearing stock exhaust, we were able to approximate what aftermarket exhaust is worth on a street GT supercharged with 9 pounds of boost. In this case, the figures were 411.5 hp and 416.7 lb-ft of torque. This is 21.9 hp and 19.9 lb-ft of torque behind the free-flow exhaust car.

Race Gas
OK, so it's the weekend and you want to take your Kenne Bell-enhanced GT to the strip and let 'er rip. The first thing you'll need is race gas. For this test, we used 100-octane unleaded. This allows moving to a 231/48-inch pulley and 14.9 pounds of boost at 6,000 rpm (and 14.3 pounds at 4,000 rpm).

On the dyno, this resulted in an exciting 503.1 hp at 5,900 rpm and 520.1 lb-ft at 3,300 rpm-and, we might add, you have the pin pulled on your connecting rods at this power level. The slightest bit of detonation and the oil-down crew will be sweeping up your engine.

Kenne Bell superchargers are supplied with big injectors, and on '99-'00 Mustangs, new '01-and-later fuel rails are supplied ('01-and-later GTs reuse their stock fuel rails-the shape of the rail is what's important). You just need to fit the injectors and rails.

Furthermore, more than your connecting rods are maxing out at 500 hp. The 90mm mass air is near pegging (a function of its electronics, not air-flow capacity), and the fuel pump has been flat-lined since 4,400 rpm. Kenne Bell says that's OK because the fuel pressure just barely hangs in there due to the Boost-A-Pump. The company also recommends shifting at 5,500 rpm with this combination to avoid fuel starvation-that's still 499.2 hp-so you can tell Kenne Bell's getting nervous.

And don't think you'll get more fuel by jacking up the fuel pump with even more voltage. Kenne Bell found the stock pump has a 75-psi workable limit (that's as much as its chip tuning allows), because at 80 psi the pump pop-off valve blows, the fuel pressure plummets, and it takes a second or so to build back up to 80 psi again, where it will pop off again, and so on. Obviously, this is going to lead to detonation and emptying of the wallet, so don't be that guy.

But you can certainly be the cool guy with this sort of torque and power in your street GT. Later, when the short-block becomes tired, the aftermarket is offering a growing number of reasonably priced, forged-piston-and-rod, modular short-blocks. With one of those safely under your Two-Valve heads, 14 pounds of boost and 500 hp is yours for the price of a blower pulley. We'll bet that can cure the Cobra blues.

It takes two strong guys to heft the blower assembly atop the engine. Naturally, all new gaskets, hardware, and other small parts are supplied. Aside from hand tools, all you'll need is antiseize compound, Teflon tape, 2 gallons of coolant, eight Denso Iridium sparkplugs (IT20 or T20EPR-U), rags, and the like. KB even supplies the required metric Allen wrenches.

The Price Of Boost
Not all pricing for Kenne Bell's kits had been set at press time. What should prove the most popular example-an intercooled GT kit-will retail for $4,899. That's for everything minus "the options," so it includes the supercharger, the intake manifold, the Mono Chip, the Boost-A-Pump, and so on.

Admittedly, 49 Franklins can be tough to give up. If it's too much, the $3,799, 6-pound, nonintercooled entry-level kit will save 11 pics of ol' Ben. The big savings reflect the cost of intercoolers, and the surprisingly high-priced, hand-fabbed water tank, fittings, hoses, and pumps associated with it.

What premium the Bullitt kit will bring over the GT kit was unknown, but the guess was $400 above the standard GT kit.

Likewise, pricing on the options was also incomplete at press time. The cold-air kit is an existing part at Kenne Bell and brings $119, so that much is known. Also, the combination of the 90mm mass air meter, the Big Tube kit, and the Switch Chip upgrade was $498, but a breakdown of individual prices was not available.

Intercooled installations require mounting the water pump, seen here, along with a reservoir tank in the engine compartment and the heat exchanger below theradiator. A few holes must be drilled for some of the mounting bolts.

Screwy Blower
What's the big deal with the Kenne Bell supercharger anyway? While it looks like a Roots-style blower, it is not. A Roots uses two similar rotors to fan air into the intake manifold. The Lysholm screw that Kenne Bell uses employs a pair of dissimilar mating rotors-they mesh together to squeeze the air between them-to compress the air and discharge it in smooth, steady streams instead of a series of pulsations.

There are many advantages to the Lysholm screw, the main one being the air is heated much less than in a Roots blower. The cooler air allows more ignition timing, which makes more horsepower. This is why the Kenne Bell blower makes more power on an '03 Cobra than the Eaton-built Roots.

The only meaningful disadvantage to the Lysholm screw in passenger-car applications is its somewhat higher cost. Even so, you'll see more Lysholm screws from the OEMs (such as on the Ford GT) as efficiency continues to rise in importance.

{{{GT}}} and Cobra Summary
 Temp. In
at 6,000 Rpm
Temp Out
at 6,000 Rpm
Peak Hp
at 6,000 Rpm
at 4,000 Rpm
at 6,000 Rpm
Stock Bullitt
Naturally Aspir.
GT, Base KB,
No Intercooler
Kenne Bell GT,
Kenne Bell GT,
KB w/Cold Air
Kenne Bell GT,
All Opt.
Kenne Bell GT,
All Opt. 2 5/8 Pulley
Kenne Bell GT,
All Opt. 2 3/8 Pulley
Kenne Bell GT,
All Opt., Stock Exhaust
Stock Cobra199 863758.37.8Eaton 3.500
3.00 Pulley26812541713.010.5Eaton 3.000
KB Cobra20510348912.213.13.550
3.25 Pulley24611850813.514.73.250
All Kenne Bell Two-Valve kits use a Boost-A-Pump, shown in its home on the driver-side trunk floor. It wires into the fuel pump and rollover switch-the rest is simply mounting the box.

Temp In and Temp Out are charge-air temperatures into and out of the intercooler. In other words, they are blower discharge air temperature and intercooler discharge air temperature. Boost is given at 4,000 and 6,000 rpm to illustrate any blower drop-off with rpm. Such drops indicate inefficiency. The pulley diameter is in inches. All GT figures are from the current test. All Cobra figures are from a previous test published in our March '03 issue. GT tests conducted in 95-103-degree ambient temperatures, Cobra tests were 68-75-degree temperatures; this affects water temps. All Bullitt/GT tests (same car) ran with short-tube headers, a 2.5-inch high-flow H-pipe, and aftermarket mufflers. All Cobra tests ran with a stock exhaust. Other tests suggest good exhaust is worth 20 hp on GTs, but nothing on Cobras. This area is open to interpretation, so more testing is needed, but some reduction in GT power figures is likely necessary when comparing GT to Cobra power output. Such a reduction will vary with boost, rpm, and power level.

Estimated boost. This run was conducted without measuring probes installed on a second Bullitt. It also shows a 21hp drop from the comparable "GT, All Opt." run above, where the only meaningful difference is modified versus stock exhaust. This agrees with other exhaust tests.

Naturally Aspirated Bullitt Exhaust
As a bonus, Kenne Bell turned us on to some naturally aspirated exhaust figures from its Bullitt work. Wanting to know how much the aftermarket exhaust was worth on its test Bullitt, KB tested the car with both the goodies and stock exhaust. Here are the results.


The modifications were JBA short-tube headers, an MRT two-cat H-pipe, and a SpinTech after-cat.

Dyno Results
 Stock BullittBase Kenne BellIntercooledCold-Air Kit
9-lb Intercooled
1,70071.5220.9 99.4307.0118.5366.0120.7373.0
Dyno Results
 9-lb Intercooled,
All Options
Stock ExhaustRace Gas
14-lb Intercooled,
All Options
1,800127.1370.9 99.5290.1149.0434.9