Tom Wilson
December 14, 2009

Horse Sense:
We were still two blocks away from Westech when we were surprised to hear the thunder of what sounded like a 7,000-rpm locomotive over our '91 Mustang's stereo. Turning down the tunes and opening a window, we discovered the distant roar was just our test engine warming up on the dyno.

So, you bolted a Kenne Bell Twin Screw supercharger to your GT500 and managed to detonate a connecting rod through the block by way of tuning exuberance. What's your next move? Get a bigger blower, of course.

Like any good hot-rodder, Mark Meiering ordered a robust aluminum short-block and bolted the newest, largest Kenne Bell supercharger atop it. This made Mark the first to fiddle with Kenne Bell's just-announced 3.6-liter water-cooled supercharger, and when the combination was tested on one of Westech's Superflow engine dynos we were invited to tag along for the first-ever proof-of-concept and tuning ride.

Considering the heretofore largest-of-the-large Kenne Bell blowers-the 2.8-liter-could slap down over 800 hp to the rear tires of an otherwise stock GT500, just what is the 3.6-liter supposed to do? Make more power, of course, but via increased efficiency rather than increased boost. By the way, there are 2.8 and 2.8H (High Pressure Ratio) Kenne Bell superchargers. In this article we casually refer to the "2.8" blower simply to denote its displacement; in all cases we're referring to the 2.8H version. All 3.6 blowers are considered high-pressure ratio, but because that's the standard 3.6, there's no need for the "H."

When Jim Bell, the big rotor at Kenne Bell, decided to build the 3.6-liter supercharger he was looking for several gains. First, a larger blower would be useful for larger-displacement engines. These are typically big-inch Chrysler and Chevy units, not the pip-squeak 281ci 4.6-liter mills, or even the middling 5.4-liter Four-Valves. They make good power with the 2.8-liter in all but the wildest applications. Of course, many 5.4 owners are a little over the edge, so the larger blower could come in handy for a larger percentage of Shelby owners than we might first imagine.

Second, Jim knew he could increase the supercharger efficiency, on all engines, because the larger blower would turn more slowly. Plus, he could make a few mechanical upgrades. This would mean that for the same boost the supercharger would take less power to drive and thus shuttle more power to the Mustang's wheels rather than waste it driving the supercharger. So, while many Mustangs don't really require a larger blower, they do benefit from a more efficient blower.

Also, high-boost screw blowers suffer from non-linear heat distribution. A screw blower is a compressor and unavoidably heats the air, and because the air enters one end of the supercharger and exits at the other, a screw blower runs cooler at its inlet end and hotter at its outlet end. On moderately boosted street engines this effect isn't enough to bother with, but as the boost goes up, so does the air temperature-it's just a fact of physics and not a sign of poor blower design.

Just how hot is hot? At crazy high boost 100-degree incoming air becomes 350-degree outlet air-that's a huge heat gain for a twin-screw blower. Jim says he horsebacks a 10-degree gain per pound of boost. Thus, when boost arcs into the 25-pound stratosphere, the hot blower rotors have expanded enough to crash into each other, destroying themselves and sending gritty metal bits downstream into the engine. The rotors always touch first at the gear end (discharge end) of the blower because it is considerably hotter than the inlet end, which you can think of as air-cooled.

Kenne Bell's soon-to-be patented cure is to water-cool the front, or discharge end, of the big 3.6-liter compressor. The inlet end remains air-cooled by the incoming air, while the water takes away excessive heat at the discharge or gear end. Kenne Bell denotes the water-cooled blowers by the initials "LC "for liquid cooling.

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