Performance is back. As a result, today’s car buyers face a choice of engine technologies-even if they aren’t shopping for high performance.
The majority of today’s engines fall into one of two categories: multivalve or pushrod.
Multivalve essentially means high speed. To understand why, think of an engine as a simple pressure vessel. It takes air and fuel in, ignites it, then spits it out. An engine with more valves will draw more air into the combustion chamber and expel more from the exhaust chamber. That translates into greater horsepower per size. An engine that can take in more air can use more gas without violating its pre-set air-fuel ratio.
Most multivalve engines use four valves per cylinder. Two of the valves push air in; two push exhaust out. So a four-cylinder multivalve engine has 16 valves; a 6-cylinder has 24 valves; an 8 has 32 valves.
The extra valves are opened and closed by extra camshafts. The camshafts sit atop the engine-hence, the name “dual overhead camshaft,” or DOHC.
So everyone should shop for multivalve, right? Not necessarily. Multivalve engines do provide greater horsepower, which is good for passing performance. But if you’re standing at a stoplight or pulling into traffic from a driveway, you need torque to get the car rolling. Specifically, you need low-speed, or “low-end,” torque. The way to generate that hasn’t changed much over the years. It is still done at the lowest cost by the “overhead valve,” or “pushrod,” engine. Pushrod engines get their name from a linkage that connects the engine’s camshaft to its valves. Unlike a multivalve engine, in which the camshaft directly contacts the valves, these engines use the camshaft to move a pushrod, which engages a rocker arm that opens and closes the valves.
If you compare apples to apples-say, a 3-liter pushrod engine to a 3-liter multivalve, the torque numbers are roughly equivalent. But if you are more concerned with torque than horsepower, pushrod technology probably can fulfill your needs for a lower cost. It’s less complex, too. A typical pushrod engine may have several hundred fewer parts than a corresponding multivalve.
Which to choose? If you want a sporty car that accelerates quickly from 50 to 100 miles per hour, go for multivalve. If your main concern is strong launch, pushrod technology probably is still more cost-effective.
