They could've had a V8--but more opt for 4

Four is the new six.

Four-cylinders have replaced sixes as America's most popular engine choice, powering 43 percent of U.S. light vehicles sold in the first half of this year, according to IHS Automotive.

In 2005, V6s were in an identical 43 percent of vehicles sold. But over six years, four-cylinder engines rose while six-bangers fell, with fours overtaking sixes in 2009. That American icon, the V8, has lost favor even faster. It now is under one of every six hoods versus almost one of three in 2005.

The reduction in cylinders is even starker if you exclude fleet sales, which include batches of contractor pickups and vans for rug-cleaning franchisees. So far this year, more than half the vehicles sold to retail consumers had four cylinders, up sharply from a third in 2006, says J.D. Power and Associates.

Two factors are driving the shift. Buyers beset by high fuel prices are downsizing vehicles or opting for the smaller of two engine choices. And manufacturers are shrinking vehicles and engines to meet tougher 2016 federal fuel economy rules.

The automaker push and consumer pull are reshaping the American automotive market. Sales of smaller vehicles are growing: Small and mid-sized vehicles account for 44 percent of sales this year versus 36 percent in 2005. And the changes are remaking engines, as technology-laden powertrains provide more punch per piston.

"We're offering a continuous rollout of technologies," said Greg Johnson, Lincoln powertrain integration manager, in part because consumers now expect fuel prices to keep rising over time and "are going to continue to vote with their pocketbooks."

Ford Motor Co. is introducing downsized and turbocharged EcoBoost engines in several vehicles and six-speed transmissions across its entire U.S. lineup.

The added technologies mean consumers aren't giving up power. Ford's current naturally aspirated engines typically deliver 80 hp per liter, while its EcoBoost engines deliver 120 hp per liter. And, Johnson adds, "Ours are not a trade-off on the road. You have the torque for 0-to-60, passing, merging onto freeways."

Or consider the Chevrolet Cruze. The Eco version of the compact is rated at 42 mpg on the highway. But even the standard version includes a variable-displacement oil pump that pumps only what's needed as a way to improve engine efficiency.

For consumers, the obvious upside of lighter, more fuel-efficient vehicles is in the lower operating costs, especially at the fuel pump. The downside: The cost of all that technology used to tweak engines and reduce weight adds up.

Making engines simultaneously smaller, more powerful and more fuel efficient costs a bundle because manufacturers must combine multiple technologies such as turbocharging, gasoline direct injection and variable valve timing, said Michael Omotoso, J.D. Power's senior manager of global powertrain forecasting.

But manufacturers and ultimately consumers are willing to "pay more because conventional gas engines are much cheaper than hybrids and electrical vehicles," Omotoso said. "If you can get 40 mpg from a sub-$20,000 car, why buy a hybrid for a lot more money?"

He estimates that gasoline-electric hybrids are priced $4,000 to $6,000 higher than comparable four-cylinder gasoline vehicles, with pure electrics $10,000 to $15,000 more.

Automakers are even preparing to test demand for three-cylinder engines.

At the 2010 Beijing auto show, Ford unveiled a 1-liter turbocharged three-cylinder engine. It will be used in the Ford Fiesta in the United States. Ford hasn't released the engine's output.

The automaker hasn't identified the turbo supplier, but Continental AG and BorgWarner Inc. both have developed three-cylinder turbochargers.

A BorgWarner turbocharger will be used on a three-cylinder gasoline engine to be installed in BMW's Mini, said BorgWarner spokeswoman Erica Nielsen in an e-mailed statement.

Kregg Wiggins, senior vice president of Continental's North American powertrain division, expects engine sizes to shrink, with the percentage of engines that are under 2 liters doubling over the next six or seven years.

In 2017, Continental forecasts, 3.4 million engines installed in North American-built vehicles will have a displacement of less than 2 liters, up from 1.2 million in 2010. With sales forecast to grow, that would mean one in five vehicles would have a 2.0-liter or smaller engine in 2017, up from one in 10 in 2010.

The U.S. market for three-cylinder engines will remain a niche, Wiggins predicts. Still, it's indicative of the lengths to which automakers will go in altering engines.

"Automakers have to--mileage rules are more and more stringent," J.D. Power's Omotoso said. "The industry needs fleet improvements of 4 percent a year to reach the 2016 rules."