Richard Holdener
April 1, 2007
If our blower looked this cool, we don't think we'd ever want to install it on a car. While this cutaway of the new Kenne Bell Big Bore blower looks impressive, it is actually the performance of the new design that really shines.

When It comes to positive-displacement superchargers, bigger is definitely better. The reason for this oversimplification is that the flow rate of any positive-displacement supercharger (Roots-style or twin screw) is a function of the displacement. For every blower revolution, a positive-displacement supercharger provides a given amount of airflow. Increasing the airflow is a simple matter of increasing either the number of revolutions (relative to engine speed) or the amount of air supplied per revolu-tion. Blower speed is increased by installing a smaller blower or larger crank pulley. Additionally, increasing the amount of air supplied per revolution is accomplished by increasing the rotor size, which is brought about by increased rotor length and/or diameter.

Obviously, it is also possible to improve the blower's flow rate, response rate, and overall efficiency by altering the inlet into the rotors or screws and by reshaping the discharge area. Not surprisingly, the rotor design itself also has an effect on the output and efficiency of the supercharger, but by and large, the major determining factor remains the displace-ment of the blower itself. When you are looking to make big power, there is no better weapon than a big blower.

With this philosophy, the twin-screw blower experts at Kenne Bell went to work to further improve the already impressive arsenal of superchargers currently available for the Ford contingent. While the company did not introduce the twin-screw blower to the Mustang market (that honor is held by the now-defunct Sprintex kit), Kenne Bell has been at the twin-screw game longer than anyone else in the Mustang industry. Twin-screw blowers are now available from a variety of sources, including Whipple, Ford Racing Performance Parts, and Saleen to name a few, but none of these sources offer the wide variety of blowers designed specifically for the intended application currently employed by Kenne Bell.

Just like cams, cylinder heads, and intake manifolds, one blower size does not work on all applications. Despite our bigger-is-better theme, not every Mustang owner is looking for maximum power production. It is for these applications that Kenne Bell offers a wide range of blower sizes.

Through extensive engine, chassis, and blower dyno testing, Kenne Bell was able to determine that the one-size-fits-all philosophy was entirely inaccurate. You don't see a centrifugal supercharger (or even turbochargers) of just one size offered by those manufacturers. By the same token, a stock '96 (non-PI) Two-Valve motor doesn't need nearly as much supercharger as a modified Four-Valve Cobra motor. While bigger blowers will indeed offer more airflow and power potential, even positive-displacement superchargers have optimum efficiency ranges, meaning smaller blowers are usually more responsive than larger blowers.

The key is to size the blower properly for the intended application. Why put a 1,000hp blower on a motor that will never see the high side of 400 hp? You'd be much better off running a blower capable of just 500-600 hp to ensure immediate boost response. After all, having boost available as soon as you stick your foot in the throttle is the reason you opted for the positive-displacement blower in the first place, right?

For years, Kenne Bell has supplied its Mustang kits with Autorotor twin-screw superchargers ranging in size from 1.7 liters (1.5 liters on the early 5-liter kits) up to the most recent 2.4L monster. You may remember the 2.4L blower was employed by John Mihovetz to exceed 900 hp from an '03 Cobra motor. While the 2.4L KB blower offered more power potential than the smaller 2.3L blowers currently used by the other manufacturers in twin screw kits-to say nothing of the less-efficient Roots-style blowers-Kenne Bell wanted to further distance itself from the competition by offering even larger blowers capable of what might best be described as insane power levels.

While bigger is certainly better, the new Big Bore blowers also offer improved efficiency thanks to a revised inlet design. This port timing is critical for maximum efficiency.

While making more power is a simple matter of making the blower bigger, underhood space is almost always at a premium, especially near the firewall. Since most twin-screw blowers are fed through the rear of the housing (because air for the twin screw must be processed from back to front and not top to bottom like the Roots-style blowers), the area adjacent to the firewall is usually taken up by the blower inlet tubing and bypass valve assembly (on street applications). These space constraints all but control the length of the blower, thus limiting the potential displacement for a given application.

With limited room available for increasing the length of the blower, Kenne Bell took a look at increasing the rotor's diameter-thus the Big Bore blowers were born.

Working with one of the leading twin-screw engineers, Kenne Bell designed its own superchargers. By combining the larger-bore rotors in a billet case (for superior strength compared to a cast case) with improved rotor design and specific inlet and discharge patterns, Kenne Bell was able to produce bigger and more-efficient blowers. The new Big Bore blowers not only provide more airflow and power potential, but the airflow comes at a lower charge temperature with a drop in parasitic loss (power required to drive the blower). According to Kenne Bell, the Big Bore blowers are better in every way.

While improved blower efficiency can increase the power potential offered by the twin screw, the really big news is the increased displacement. KB's Big Bores are now available in displacements ranging from 2.1 liters all the way up to a whopping 2.8 liters. That is over a full liter more than the original 1.7 used on our own Project RSC and nearly half a liter more than the 2.4L blower used to produce 960 hp on the modified '03 Cobra motor. That's easily 1,000-plus horse-power with a twin screw.

Every bit as impressive is that the new Big Bore blowers fit where the original blower once stood. We have physical proof that the 2.1L blower bolts right in place of the original 1.7L; we tested it on Project RSC (see graph). According to Kenne Bell, the Two-Valve motors will receive the 2.1L blower, while the '05-'06 Three-Valve and 5.4L Lightning motors come with the 2.6L blower. The 2.8L blower is reserved for the Four-Valve Cobra motors and the 5.4L 4V used in the Ford GT. The 2.8L will also be installed on the new 5.4L GT500. Just imagine what a GT500 would feel like with a 2.8L twin-screw blower under the hood. We don't think we'd even bother driving it with the factory blower-just drop it off at Kenne Bell for the massive upgrade. That would be one kick-ass Shelby Mustang.

While shooting the fancy new blower on a sheet of colored paper is all well and good, we could hardly expect to drive out to Kenne Bell's for just a photo shoot (no matter how cool the new product). No sir, it's dyno time or nothing for us. Recog-nizing our addiction, the guys at Kenne Bell agreed to provide us the necessary fix by allowing us to test one of its new Big Bore blowers on project RSC.

Though we didn't plan for this test as part of the project, it is nice to have a project car when opportunities like this arrive. We hope to run the new 2.8L blower in the near future on a Ford GT or GT500 motor, but for now we'll whet your appetite with the power gains offered by the 2.1L blower.

We installed the intercooled super-charger kit from Kenne Bell on the modified 4.6L Two-Valve motor. The kit included the standard 1.7L blower, the air-to-water intercooler, and everything needed to produce as much as 9 psi of boost. For this test, the 1.7L blower was equipped with a 3.00-inch blower pulley and run with the timing and air/fuel curves locked in place to make the motor as repeatable as possible. With the 1.7L blower making 7.5-8 psi of boost, the 4.6L Two-Valve motor produced just over 400 hp and 395 lb-ft of torque. After swapping on the new 2.1L Big Bore blower (with the same 3.00-inch blower pulley), the boost pressure jumped to nearly 12 psi, while the power numbers followed suit to the tune of 452 hp and 453 lb-ft of torque. With an effortless 50 hp from the 2.1L, we can't wait to get our hands on one of the Big Bore big boys and really put the spurs to it.

Kenne Bell twin screw Blower Test:1.7L vs.Big Bore 2.2L
In a direct comparison test between the twin-screw blower designs, the new 2.1L Big Bore Kenne Bell blower offered significantly more power than the older 1.7L Autorotor. Equipped with the same drive pulleys (3.00 blower/6.5-inch crank), the 2.1L Big Bore increased the boost pressure supplied to the modified '96 Two-Valve motor by nearly 4 psi, from 7.75 psi to 11.50 psi. The increased boost and efficiency offered by the 2.1L improved the power output from a tad over 400 hp to 450 hp. The peak torque rose a similar amount, from 395 lb-ft to 453 lb- ft, with significant gains offered throughout the rev range. With the proper motor combination, this 2.1L would have no trouble exceeding 600 rwhp. Now, just imagine what the 2.8L blower is like.