Aug
28
Heavyweight horsepower! the Hemi V8 returns to battle in a truck, but also will see use in future Chrysler passenger cars. Light, compact and simple, the 5.7-liter overhead valve engine is surprisingly modern, and plenty potent - Engineer - DaimlerChrysler’s production design and development
Filed Under Chrysler News
“The assignment to ready a replacement for the 5.9-liter engine came in 1996,” says Bob Lee, director of Rear Wheel Drive Engine Engineering for the Chrysler Group, “but we didn’t start in earnest until late 1997, as the 3.7-liter (V6) and 4.7-liter (V8) were being designed and launched.” This meant a lot of 18-hour days for Lee and his team, punctuated by a short delay in the program as Daimler-Benz and Chrysler merged. The merger, however, also gave the group the chance to compare notes with their Daimler colleagues, specifically about the use of two plugs per cylinder. “We discovered that we knew some things they didn’t, and they knew some things we didn’t,” says Lee.
The initial design studies for the Hemi covered a lot of ground, and investigated a number of design and production possibilities. One concept, in particular, would have delivered the entire valvetrain to the line as a cartridge, where it would be bolted to the block. “There were a few problems with this idea, despite the fact that it would have simplified the production process by having a single supplier deliver the completed unit on a just-in-time basis to the assembly plant,” says Lee. Specifically, the tolerances between the cartridge and the block would have necessitated complex machining operations, and the design of an engine block very different from that found in production today. “One of the benefits of this design,” says Lee, “was that it would have made it simple to add variable valve timing, but the benefits didn’t justify the costs.” In the end, Lee and his team settled on a simple design, roughly the same size as the company s 4.7-liter V8, that could be modified to include both V6 and V10 var iants.
The cast iron block is a deep-skirt design with five cross-bolted main bearing caps, and cylinder bore spacing that is the same as the 5.9-liter V8. A forged crank was considered, but rejected due to cost. “When you design an engine like this,” says Lee, “you look at things like a forged crank, but you prepare everything such that an expensive, difficult-to-machine piece like this isn’t necessary.” Once the aftermarket gets its hands on the engine, Lee expects a forged crank to become available, just not from the factory. The aluminum cylinder heads have hemispherical combustion chambers, two valves per cylinder, hydraulic lifters with roller followers two spark plugs per cylinder activated by separate coil-on-plug ignition systems, and are cast using semi-permanent molds. The single’ camshaft is located high in the block to optimize the pushrod angle–something the legendary 426 Hemi, with its severely canted intake valve, did without. “That engine trapped a lot of fuel in the cylinder,” says Lee. “This one doesn’t.” The new Hemi has an included valve angle of 34.5, a compression ratio of 9.6:1 (it was 10.25:1 on the 426 motor), and dual wideband knock sensors.
