Valves vs. cams? For GM, it’s no contest
General Motors Corp. sees a future for engines with overhead valves, a technology most automakers have abandoned in favor of overhead camshafts.
GM will introduce a family of overhead-valve V-6 engines beginning with a 3.5-liter in the 2004 Chevrolet Malibu and plans to keep other OHV engines in its fleet for years.
Few foreign manufacturers offer OHV car or light-truck engines in the U.S., and Ford Motor Co. and Chrysler Group have reduced their OHV offerings in favor of overhead-cam designs.
OHV engines have a single camshaft in the engine block that operates pushrods that open and close the valves. With OHC engines, camshafts are mounted in the cylinder heads, closer to the valves, where they can open and close the valves faster and allow the engine to operate at higher speeds.
Despite the competition’s plans, overhead-valve technology will account for the majority of GM’s future engines, said Ned McClurg, vice president of engineering for GM Powertrain.
“We’re making major investments in them. A case can be made that overhead-valve engines produce more power per mass and more power per physical space,” McClurg said.
OHV engines are widely considered low-tech compared with overhead-cam engines and incapable of meeting the more stringent emissions requirements facing automakers. McClurg disputes such claims.
“We are doing just fine with overhead-valve engines, whether you’re looking at quality, performance, fuel economy or customer satisfaction,” he said.
“I know we’re arguing against the tide, but we’re going to surprise the industry with innovation,” he said. “We’re not going to give away anything on noise, emissions, quality or reliability.”
Art Spinella, general manager of CNW Marketing/Research, a Bandon, Ore., firm that studies consumer buying decisions for the auto industry, says most customers couldn’t care less what is under the hood.
“The vast majority, and I mean 85% or more, don’t care. There’s a certain percentage who do, but they already drive European or Japanese vehicles,” Spinella said. “As long as it starts ... it doesn’t matter if it’s overhead valve, overhead cam, hybrid or a fuel cell. They don’t care if there are hamsters under the hood as long as it gets them to the office.”
In the 1980s, overhead-valve engines couldn’t match the performance or fuel economy of overhead-cam engines, Spinella said, “but today the engine configuration or design really is not an issue.”
GM says cost is a major factor favoring OHV engines because they have fewer parts and are cheaper to manufacture than OHC engines, most of which have two camshafts per cylinder head and four valves per cylinder (instead of two on the OHV).
“When you have four camshafts instead of one, the difference is pretty straightforward,” McClurg said regarding cost. GM estimates that its OHV truck V-8s cost $700 to $800 less to produce than the OHC V-8s of archrival Ford.
GM’s V-8s have more horsepower, the power used to sustain speed, but Ford’s have greater torque, the power needed to move the vehicle’s mass to achieve speed. OHV engines also tend to be more compact than overhead-cam engines, making them easier to fit into a variety of vehicles.
In fuel economy, Environmental Protection Agency says GM’s 4.8- and 5.3-liter overhead-valve V-8 engines tie Ford’s 4.6- and 5.4-liter overhead-cam V-8s. In combined city and highway driving, the smaller engines are rated at 17 miles per gallon and the larger versions at 16.
Spinella says, though, that in practice he has found GM’s engines to be more fuel-efficient than Ford’s. CNW maintains a fleet of vehicles for use in consumer research clinics, and Spinella says the V-8s in GM’s full-size sport utility vehicles “get much better real-world mileage” than the V-6 in the mid-size Ford Explorer.
Dave Szczupak, Ford’s vice president of powertrain operations, argues that his company’s overhead-cam engines are smoother, quieter and more efficient without a cost penalty.
“We don’t believe there’s a financial disadvantage to our engines,” Szczupak said, adding that any additional cost from the greater complexity of overhead-cam designs is offset by other production efficiencies.
Ford builds overhead-cam V-8s for trucks and cars and a V-10 truck engine from the same design -- a total of 1.3 million engines a year. Ford also is adopting a flexible manufacturing system it says will allow it to revise future engine production at lower cost.
“With the manufacturing efficiencies we will realize, we will produce our engines at very similar cost” to those of GM, Szczupak said.
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Rick Popely writes for the Chicago Tribune, a Times sister paper. E-mail: rpopely@tribune.com.
