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GENERAL MOTORS SPARKS OVERHEAD VALVE VS. OVERHEAD CAM DEBATE

 GENERAL MOTORS SPARKS OVERHEAD VALVE VS. OVERHEAD CAM DEBATE
 INDIANAPOLIS, May 6 /PRNewswire/ -- "GM's strategy is to provide the right engine for the right vehicle at a price that delivers value to the customer," said Tom Stephens, director of engine engineering at GM Powertrain Division. "That means that both overhead valve and overhead cam technology are appropriate in today's market, as long as they are modern, well-executed designs." Stephens will deliver remarks about the technical benefits of both kinds of engine technology in the Indianapolis Motor Speedway Interview Room at 10 a.m. on Friday, May 8.
 GM engine technology has long been a recognized leader at the Indy 500. Buick's stock block Indy V6 overhead valve engine is posting outstanding practice runs. Buick's Indy V6 is a turbocharged version of the V6 available in many 1992 Buick passenger cars and is the only production-based engine used in Indy car racing today.
 Chevrolet's Indy V8/A engine carries a 39-race win streak into the 1992 Indy 500 race and the new Chevy Indy V8/B engine won its first race earlier this year. Both are 90-degree, alloy block, dual overhead cam race engines.
 The Cadillac Allante with the new 4.6-liter Northstar engine will set the pace for Indy 500 racers this year. The stock Northstar engine will not require any modifications in order to effortlessly lead the competitors to race speeds.
 For 1992, GMC Truck is the "official truck" of the Indy 500. GMC Truck has supplied a lineup of support vehicles in 12 of the last 18 Indy 500 races. These vehicles include Safari and Rally vans, Suburban and Jimmy utility vehicles and Sierra and Sonoma pickups.
 GM overhead valve and overhead cam engine technology continues to provide world leadership off the race track, as well. GM is designing brand-new overhead valve and overhead cam engines. "European and Japanese automakers and Ford Motor Company are rapidly replacing overhead valve engines with the goal of utilizing close to 100-percent overhead cam technology by the mid-'90s," Stephens said. "Our engineers believe there is room for improving both overhead valve and overhead cam engines."
 Chevrolet's recent introduction of the 1992 Chevrolet Corvette with the 5.7-liter LT1 engine proves that plenty of life remains in overhead valve engines. The LT1 churns out 300 horsepower at 5,000 rpm and leaps to attention with 330 lb-ft of torque at 4,000 rpm. The LT1 provides responsive launch, which makes the Corvette capable of bolting from 0-60 mph in an impressive 4.77 seconds.
 "The LT1 is a great example of what we mean by well-executed overhead valve design. It has what we call 'range extension' which refers to the fact that it is capable of running at about 500 rpm faster than previous push-rod engines," Stephens said. The LT1 includes an innovative reverse flow cooling strategy, 360X computer-controlled ignition timing, high compression ratio components, high rpm valvetrain components and increased intake volume.
 Refinements found on the LT1 are making their way to other overhead valve engines in GM engine families. The most notable is the smooth- running 3800 engine featured in Buick luxury models such as the Park Avenue. The 3800 is an overhead valve V6 that features on-center cylinder and valvetrain design, low mass components, a counter-rotating balance shaft and full electronic engine control. The 3800, coupled with GM's 4-speed electronic transmission, has won praise for its outstanding power delivery, effortless operation and world-class reliability.
 GM's commitment to world-class overhead valve powertrain development is balanced with excellence in dual overhead cam engines. Cadillac's 1993 Allante is the most powerful front-wheel-drive car in the world. Equipped with the 4.6-liter, 32-valve DOHC Northstar engine, the Allante travels from 0-60 mph in less than 7.0 seconds with a top speed of more than 150 mph. The Northstar engine delivers 295 horsepower at 6,000 rpm and boasts 290 lb-ft of torque at 4,400 rpm. "The Northstar is a blend of the best characteristics of overhead valve and overhead cam designs. It feels responsive at low engine speeds and has plenty of reserve power at high engine speeds," Stephens said. A look inside the Northstar reveals a die-cast aluminum three-piece block assembly generally found in ultra sports and racing engines. The lower crank case, upper cylinder block and oil manifold plate combine for improved clamping loads resulting in greater block rigidity for reduced overall combustion noise. An ultra-fine pitch roller chain drives the eight-sprocket cam assembly further ensuring the Northstar engine's quiet, reliable operation. The Northstar is innovation in motion, with 14 patents either awarded or pending on unique Northstar engine system designs.
 By offering a full portfolio of engines, GM is capable of responding to customers' preferences for either an overhead valve or overhead cam engine. The difference between overhead valve and overhead cam engine technology can best be described by analyzing the mechanical differences and by observing the differences in engine character, or feel, between these two kinds of valvetrain technologies.
 OHV engine mechanical attributes include one intake and one exhaust valve per cylinder. Each valve has push-rod linkages which pass through the engine. The push-rod is activated by the camshaft, which drives the push-rod up or down thereby opening or closing the proper valve during the intake and exhaust cycles.
 OHV engine character provides strong torque at low to mid-range engine speeds. Strong torque at low rpms results in good vehicle launch. Most drivers define the character of their vehicles based on the way the vehicle responds at low engine rpms. Strong launch translates into the ability to pull away from a stoplight quickly or the security of being able to accelerate to freeway speeds quickly while on the entrance ramp. With strong torque output, overhead valve engines can be matched with axle ratios that allow the engine to meet customers' needs for responsiveness and highway cruising capability while operating with low engine friction, thereby providing very competitive fuel economy.
 Overhead valve engine designs are less complex than their dual overhead cam counterparts, resulting in fewer parts, reduced cost, and simplified manufacturing requirements. Finally, these tightly packaged OHV designs allow vehicle designers greater flexibility because they can be installed in less space in the engine compartment. This enables vehicle designers to improve vehicle aerodynamics and allows for greater flexibility in overall vehicle styling.
 DOHC engine mechanical attributes include two intake and two exhaust valves per cylinder so more air can flow into the combustion chamber and more exhaust gas can flow out of it. When a larger volume of air can be ingested and expelled from the engine, more power can be generated with the same amount of engine displacement. A DOHC engine has cam lobes that activate the intake and exhaust valves directly, eliminating push-rods. Without the restriction of push-rods running through the engine, the intake and exhaust passages can be designed to provide the maximum amount of air through the combustion chamber.
 DOHC character delivers higher levels of horsepower at high engine rpms. Dual overhead cam engines generally operate at 500-1,000 rpms higher than conventional overhead valve engines. High rpm operation gives the driver a wider engine operating range for quicker acceleration at high vehicle speeds, so passing performance is responsive.
 DOHC engines process larger volumes of air, therefore producing more power per liter of engine displacement. This allows the engine designer more flexibility to reduce the engine displacement, thereby obtaining better fuel economy. DOHC engines also have a high-tech, sporty image because they operate at higher engine rpms, revving high and accelerating quickly when the engine reaches peak rpms.
 "Either type of engine when well designed can produce high levels of fuel economy and performance," Stephens said. "Each engine produces a different kind of character. The engine should not be judged on the type of valvetrain it utilizes, but on how well the engine is matched to its particular vehicle so that it best meets the needs of the driver."
 In the next few years, GM engines will combine the best characteristics of both kinds of valvetrain technology in passenger car applications. Recent OHV designs such as the 5.7-liter LT1 OHV engine already operate at a higher rpm than conventional OHV engines, providing strong launch at low rpms and excellent high speed passing power, with all the other benefits inherent in OHV engine design.
 "Other GM engines will have this 'no trade-offs' character in both DOHC and OHV designs," Stephens said. "This allows GM to select the best engine technology for the right vehicle. When coupled with GM's ultra-smooth, electronically controlled 4-speed transmissions, the result is an outstanding powertrain that provides the best of both worlds."
 -0- 5/6/92
 /NOTE: There will be a news conference regarding GM engine technology in the Indianapolis Motor Speedway Interview Room at 10 a.m. on Friday, May 8.
 CONTACT: Steve Gaut of GM Powertrain Division, 313-857-4029/
 (GM) CO: GM Powertrain Division; General Motors Corporation ST: Michigan, Indiana IN: AUT SU:


SM -- DE029 -- 7274 05/06/92 16:36 EDT
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Publication:PR Newswire
Date:May 6, 1992
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