Stainless Takes the Heat V-8s force upgrades of exhaust systems

The popularity of husky V-8 engines in pickup trucks and sport/utility vehicles has created a challenge for exhaust system suppliers.

It seems that the higher temperature of exhaust gases flowing from the powerplants creates too much fatigue for the downpipe that connects the exhaust manifold to the catalytic converter.

As a result, suppliers such as Arvin Industries Inc. are upgrading from standard 409 grade stainless steel to 439 grade, which contains more chromium and is more durable at high temperatures.

"When you start to see temperatures of 1,000C to 1,100C, you need to move to 439 stainless," says John Grace, Arvin's vice president of systems and technology. "It's a durability question - 409 just couldn't withstand the heat."

Beyond the catalyst, standard stainless steel is still adequate because exhaust gases cool considerably by then.

Arvin has supplied the upgraded downpipes for Ford pickup trucks for the past few years, and Mr. Grace says he expects to see more applications on trucks equipped not only with V-8s but also high-power V-6s.

The switch does not add weight to the vehicle, but it does add "noticeable" cost, he notes.

Another trend is the increasing use of fabricated stainless steel for exhaust manifolds. Traditionally, the manifolds were made of cast iron, but the same component made out of stainless steel weighs up to 30% less.

Arvin began supplying a stainless steel exhaust manifold for the California version of the Toyota Camry in 1997. Mr. Grace says that in a couple of years all U.S. Camrys will use them.

A key advantage of stainless is that, unlike cast iron, it does not absorb as much heat, allowing the exhaust gas to warm the catalyst more quickly.

The content of stainless steel on a typical North American family vehicle has grown steadily, from 32 lbs. (14.5 kg) in 1987 to 49 lbs. (22.3 kg) this year. Despite the growth, stainless could face competition from lightweight but super-expensive titanium.

Arvin and Tenneco Automotive have been developing titanium exhaust systems for automakers, and some weigh less than half of existing stainless steel systems. Within the last six months, Arvin has delivered titanium systems to Chrysler Corp., which is testing them on minivans, Mr. Grace says.

Automakers are interested in titanium because, even though it costs several times more than stainless steel, the lighter systems prevent redesigned vehicles from advancing to a heavier EPA weight class.

Pittsburgh-based Armco Inc. also is working with an automaker on a high-strength stainless steel modular automobile frame to satisfy the requirements of the 80-mile-per-gallon (3L/100 km) "supercar" project sponsored by the Partnership for a New Generation of Vehicles. A prototype is expected to be built next year.