Tom Wilson
November 19, 2006
All of Kenne Bell's new superchargers are the same height and width; the displacement difference comes from their length. Here are the 2.6L, 2.4L, and 2.1L superchargers, which differ mainly in length. The 2.4 wears the standard black anodized finish, while the 2.6 and 2.1 units flanking it sport the polished aluminum finish. Either finish is available on all blowers.

Horse Sense: After meeting Kenne Bell principal Jim Bell, a common question is, "Who is Kenne?" The answer is a long-gone business partner of Jim Bell's. But while Kenne is no longer, the name remains, hence the familiar Kenne Bell.

To tell the story of the new Kenne Bell supercharger, we should begin in Sweden. It's there that Lysholm and Autorotor have been manufacturing Twin-Screw-sometimes called Lysholm-superchargers for quite some time. In fact, all the popular twin-screw superchargers the U.S. hot-rodder is familiar with have, until recently, originated from one of these two companies. So, while Kenne Bell, Whipple, Saleen, and others build their own housings and develop their own installations, the tricky-to-machine twin-screw rotor packs have all come from the land of the midnight sun. In Kenne Bell's case, Autorotor was the supplier.

While Kenne Bell is still impressed with Autorotor and may still occasionally purchase some Autorotor blower packs, Kenne Bell is now building its own superchargers for all standard production kits. Why? More profit and more control over the product are the two reasons a company goes to the trouble of rolling their own. In Kenne Bell's case, the ability to jump the competition with superior superchargers was an irresistible motive. By specifying precisely what he wanted in each supercharger, Jim Bell was able to build bigger, tougher blowers than the other guys.

Twin-screw superchargers use male and female rotors; Kenne Bell blowers use a four-lobe male rotor (red) and a six-lobe female rotor (blue) to arrive at a 1.5:1 ratio. Bell hard-anodizes both of these billet-aluminum rotors for abrasion resistance, as some grit is bound to get through. Teflon is not used because it is soft, washes off with continuous exposure to gasoline (some nitrous people run fuel through their superchargers), and flaking Teflon clogs the intercooler. Besides, the twin-screw rotors do not rub on each other or the case, so there is no friction to reduce with slippery coatings.

As Jim put it, "I wanted to have the biggest, baddest twin-screws. We started it, introduced it to Ford, and have no intentions of relinquishing it." And to go with the new blowers, there is a new name: the Kenne Bell Big-Bore Twin Screw.

We should also say Kenne Bell focuses on our kinds of guys-hardcore hot rodders. Others offer blower packages to give a nice bump in street performance, and those superchargers do fine yielding modest boost for casual performance-so do the Kenne Bell offerings. But Kenne Bell is dedicated to also supporting the hardcore blower fringe, where boost pressures resemble tire-gauge readings and the rpm levels look familiar to turbo tuners. As a result, Kenne Bell builds 'em tough.

Much of the strength comes from simply "bigger" engineering. Shaft sizes and bearings are larger, which is a result of moving away from what has been the Eaton standard. That is, many in the supercharger industry have stuck with the internal shaft and bearing dimensions used by the giant Eaton Corporation and its Roots blowers seen on so many OEM applications. This is fine at low boost pressures, says Jim, but at high-boost rotors deflect and cases deform, so he's gone to larger structural components.

Admittedly, some of this beef is required because Kenne Bell has made its blowers as large and as long as possible. With billet cases and their limited outside dimensions, this has meant hogging out the internal dimensions and running a relatively longer supercharger in the bigger units. This can lead to rotor flex and thermal stability issues, but by stepping up the materials, bearing sizes, and attachment hardware Jim Bell says his units easily deliver the strength and stability needed.

Twin-screw superchargers ingest air at the end opposite the blower drive. Kenne Bell's new blower uses a billet aluminum end plate, which is lightly machined to aid airflow. This plate also serves as the bulkhead supporting the rear rotor bearings.

Bell also builds them big for airflow. Until now, most of Kenne Bell's superchargers have been 1.7L or 2.2L units. That means one rotation of the rotors moves 1.7 liters of air in the case of the smaller supercharger. Kenne Bell's new line of superchargers is larger at all steps in the product lineup. So where he used to sell a 1.7L blower, now he sells a 2.1L, and so on. This is how the old and new compare:

Blower Displacement, in Liters
Previous Current
1.7 2.1
2.2 2.4
2.4 2.6
na 2.8
Unlike a Roots blower, a twin-screw can discharge its air anywhere around the case at the opposite end from the intake. On Mustang V-8 applications this is always downward, so this side of the supercharger is rarely seen. These two Kenne Bell blowers show how the discharge port varies in area and shape.

Obviously, there are no worries about packing enough blower with the new KBs. These larger superchargers are designed to support major-league boost, especially with the unprecedentedly large 2.8L. Kenne Bell has it slated for the 5.4 V-8 on the Ford GT.

Along with some design improvements, this size increase gives an increase in blower efficiency-"lower parasitic losses," says Jim-in each blower's operating range. More practically, it gives each blower greater boost capability at high rpm. And considering the 1.7L blower was good medicine for the typical hot-street 5.0 or 4.6 Mustang, the 2.1L tables any anxiety about having enough supercharger. For the first-time blower buyer thinking he might want to start with a bolt-on blower, then move to a dedicated high-boost engine and blower combination in the future, the bigger KB blowers hold plenty of promise. All he will need is a tougher engine to withstand the higher boost.

Increased boost pressures mean higher temperatures and stresses on the supercharger. This leads to rotor flexing and a need for tougher rotor supports in superchargers designed for high output. This cutaway of the Kenne Bell rotor illustrates how the rotor is located by a stout bolt and twin rows of roller bearing. Because this assembly is at the cooler, less-stressed intake end of the supercharger, these parts are smaller than at the drive end, but the twin row bearing speaks volumes about the critical need to maintain rotor alignment.

Can Kenne Bell go any larger at each step? Not without coming up with an entirely new supercharger. Jim Bell says cast rotor housings are not strong or stable enough at high boost pressures, so he insists on a billet case. The cost-effective way of producing billet cases in large numbers is an extrusion, and the new blower represents the upper limit of available extrusions. Such a limitation hardly bothers us, as the new line of Kenne Bell blowers seems to fill the logical range of supercharger applications.

Even more practical is fitting the superchargers into the existing Kenne Bell kits. The new superchargers package into the company's earlier kits, meaning there is no need to burn the time or money redesigning them. Furthermore, owners of existing KB superchargers can retrofit their cars with the new huffers should they feel the need.

More good news is the pricing, which remains as before. So, you score a larger blower and pay the old price. That's progress.

At press time, Jim Bell said he had switched over to the new superchargers, so availability of the new units is instantaneous. As you are perusing the KB offerings, you might see some new or larger units, such as a 2.6L for Lightning pickups. We've already covered all the KB Mustang blower kits made to date; otherwise, enjoy the details found in the photos and captions.

Although they run a tight 0.003 to 0.004-inch rotor-to-rotor and rotor-to-case clearance, the KB rotors never touch each other or the case. This is a tough trick; uneven heating along the length of the supercharger makes rotor alignment difficult, yet critical, to maintain. In fact, the higher cost of twin-screw superchargers is directly attributable to the difficult job of machining these complex rotors. It takes a $1 million cutting machine to do it, so the opportunities to explore the latest in rotor shapes is limited either to those with the machinery or to those who have connections to it. Kenne Bell falls into the latter category, as its rotors are all machined in Sweden.

Increased boost pressures mean higher temperatures and stresses on the supercharger. This leads to rotor flexing and a need for tougher rotor supports in superchargers designed for high output. This cutaway of the Kenne Bell rotor illustrates how the rotor is located by a stout bolt and twin rows of roller bearing. Because this assembly is at the cooler, less-stressed intake end of the supercharger, these parts are smaller than at the drive end, but the twin row bearing speaks volumes about the critical need to maintain rotor alignment.

Moving to the front of the supercharger, the mounting hardware, and especially the ball bearings, is much larger. This end of the supercharger also absorbs the drive loads from the helically cut pair of drive bearings.

Inside the blower-drive snout, there's nothing new in the latest Kenne Bell superchargers. That's because KB has been making its own drives for years without issues, so there was no need to change this section for the new supercharger. It's interesting to see how thrust loads are cushioned by a coil spring, and how the drive's axial forces are damped by surprisingly hard plastic pieces between the gear and driveshaft. This end of the case is stoutly built to absorb both drive gear and belt loads, in addition to the internal supercharger forces.

Closely inspecting the blower's drive pulley shows the small hole centered in the Allen-headed attachment bolt. That hole vents the blower drive; the two unthreaded holes flanking the Allen bolt are locators for the pulley replacement tool sold by KB. They are part of Kenne Bell's fast and easy pulley changing design. Almost all of KB's Ford blowers use six-rib drive pulleys to retain compatibility with the remainder of the front engine dress (alternator, water pump, and such). KB notes that a large-diameter six-rib pulley with its inherently generous belt wrap is effectively an eight-rib system when it comes to transmitting force.

Kenne Bell blowers are all self-contained oilers. A dipstick is provided to keep tabs on the blower's oil level and color; the oil goes black when overheated. Otherwise, it has a honey-color dye added to it to raise visibility on the dipstick. Maintenance is simple: change the oil every 10,000 miles with Redline 50W synthetic or 50W Mobil 1. There is no drain, so you'll have to swipe your wife's turkey baster to suck the oil out through the dipstick hole.

Kenne Bell's strategy to make a larger blower was to increase the rotor bore and keep the exterior size the same. Holding the new end plate (bare aluminum) over an older end plate (black) shows the increase in bore diameter. The plates are CNC-machined and doweled to the case for positive location.

The two cooling pods on the front cover of the new superchargers are the easiest way to spot the new Kenne Bell blowers. These pods give a little extra oil volume and cooling at this warmest part of the supercharger. The new blower in front has them (they look like blisters for gear clearance but aren't); the older unit in back does not have them.

The size and shape of the blower's entry and discharge apertures vary depending on the displacement, design rpm, and output of the supercharger. The important point is the inlet and exit are carefully sized to maintain an intake/discharge ratio. Jim Bell reports these new superchargers will take 15 hp less to drive and run 20 degrees cooler than his previous twin screws; the more efficient inlet and discharge ratios and improved rotor profiles are why.