Sticking with pushrods

General Motors and Chrysler continue to improve the breed with recent high-profile engines.

Pushrod internal-combustion engines were supposed to have been consigned to the engineering trash bin by now, replaced by overhead cam (OHC) powerplants with multivalve cylinder heads, variable valve lift and timing, active intake systems, and more. However, two new engines, General Motors' Small Block and Chrysler's Hemi, are breathing new life into pushrod engines by developing numbers competitive with most OHCs. “And pushrod costs are at least 30% lower,” said Joseph M. Guertin, Director of the powertrain team for rear-drive vehicles at DaimlerChrysler.

An OHC engine can produce more power per liter because its direct-acting valvetrain permits higher revving and use of multi-valve cylinder heads. After all, an engine operates as an air pump. Reciprocating pushrods adversely affect engine speed and reduce flexibility in cylinder port designs, but pushrod engines are more compact so they can have greater displacement in a physically smaller package. And some of the new pushrod engines are operating in the same rpm range as most OHC units. The new Corvette 7.0-L LS7 two-valve V8, for instance, revs to 7100 rpm-a first for a mass-production pushrod powerplant.