Screw This

Politics, religion and Ford vs. Chevy are universally recognized as taboo topics for folks who value lasting relationships.

Now add a fourth forbidden forum of discussion — Roots-type vs. the screw supercharger.

The debate over which blower is best is itself supercharged. One enthusiast's Website begs this indulgence: “You're free to disagree with me, so please don't send me any nasty letters …”

And within the industry, arguments are waged against a backdrop colored by company pride and the prospects of program success — or failure.

The revelation that Ford Motor Co. will build a production version of its GT40 concept brings to seven the number of its vehicle programs that feature Roots-type boosters. The others: Ford's '02 SVT F-150 Lightning, '02 Harley-Davidson F-150 SuperCrew and '03 SVT Mustang Cobra, along with Jaguar's '02 XKR, '02 XJ and '03 S-Type R.

But the auto maker's love affair with Roots doesn't end there. It's also under the hood of the Marauder Convertible concept, suggesting a future Mercury application.

John Coletti is chief engineer of Ford's Special Vehicle Team, which took the lead in developing GT40. Roots-type technology boasts “a really nice, fat torque curve, especially at lower speeds,” he says.

“When we saw the power and torque curve that we could get on a GT40, it made that decision pretty easy. For the kind of numbers we're pulling, it works.”

Key numbers for the GT40 program are 500 hp and 500 lb.-ft. (678 Nm) of torque. “I think if you're going to do a car like this and you weren't sporting 500-500, I think you'd be a fool,” Coletti says, adding Roots-type's reliability is “extremely good.”

Ditto for Nissan Motor Co. Ltd., which uses Eaton's Roots technology in supercharged versions of its Xterra and Frontier.

“When we were looking at the technologies, Roots-type with Eaton was able to deliver all the requirements I set out, which includes cost, obviously. And fuel economy. And the one thing that really helped a lot was the reliability aspect,” says Larry Dominique, Nissan North America's chief product specialist-trucks and SUVs.

Cast aspersions on the reliability and durability of screw-type technology and you can blow it out your ear, Mercedes-Benz says. The Lysholm-inspired supercharger design that powers its high-performance lineup (SLK 32, C32 and soon-to-arrive SLK 55 AMG) satisfies requirements that go “way beyond what everybody used to do only a generation ago,” says a Mercedes spokesman.

But the Stuttgart-based auto maker still has one foot rooted in the other camp.

“Not to say you couldn't use one design with the other type of vehicle,” he says. “But at least for the time being, given the current engine program, we've sort of assigned one to one engine family and one to AMG.”

And he defends AMG's screw matter-of-factly: “Whether that thing is cranking at 20,000 rpm or 150,000 rpm, if you graphed it … it would always be above what the Roots could crank out.”

Then Coletti confides: “Eventually, we're going to evolve out of the Roots-type blower.”

The No.2 auto maker's performance guru says Ford and Eaton — which lays claim to 90% of the global supercharger market — have discussed screw-type superchargers because they “offer some tuning improvements and some heat management improvements.”

Enter California-based Saleen Inc., which believes boost is the real bogey. Consider its 365-hp S281 Mustang that features Roots technology.

“We output a nominal 8.5 lbs. (0.6 bar) of boost,” says John Spruill, program manager for certification and powertrain engineering. “Perfect little package for that.”

With Saleen's S281-E Mustang, however, “we were looking at increasing the output of the blower to add another 75 hp or so.” Predictably, things got a little hot.

Therefore, because screws enable higher boost output at lower temperatures, Saleen goes that route with S281-E — and pulls 425 hp.

As with success in general, Spruill suggests, timing is everything.

While screws offer more capacity and impressive low-end torque, “if you compare one-to-one, same boost output, equivalent blower sizes and whatnot, you'll see the roots supercharger comes on just a little sooner,” he says.

But if you're keeping score, the ratio of production vehicle programs is running about 98:2. Against the screw.

And that, it appears, is the root of the matter.