FRANKFURT – If Porsche’s embrace of 4-door luxury cars and cross/utility vehicles was hard for brand purists to swallow, here’s another development sure to cause controversy: a 4-cyl. engine.

Stuttgart’s storied producer of purebred sports cars is considering a 4-cyl. engine for the next-generation Boxster and Cayman 2-seaters, expected next year, Wolfgang Hatz, Porsche’s executive vice president-research and development, tells WardAuto during Volkswagen’s Group Night at the auto show here.

“We are thinking about a 4-cyl., but we have not made a final decision,” Hatz says. “For fuel economy and engine downsizing in the Boxster and Cayman, I think it’s a possible opportunity.”

Without revealing details, Hatz tells WardAuto a Porsche 4-cyl. would be horizontally opposed, rather than an inline architecture, making it compact and ideally suited for mid-engine sports cars such as the Boxster and Cayman.

He is confident Porsche can produce a 4-cyl. engine that is good enough to uphold the brand’s high-performance brand traits. “It has to be a Porsche. If we bring it, it will be a real Porsche.”

Porsche unveils the seventh-generation ’12 911 Carrera here, which arrives in U.S. showrooms in February with a 350-hp 3.4L boxer engine placed in the back of the vehicle.

Sales of the current-generation 911 remain brisk in the U.S. Through the first eight months, the auto maker sold 4,253 units, up 9% from like-2010, according to WardsAuto data.

The 911 finished 2010 as the second-best seller in WardsAuto Luxury Sport segment of 11 vehicles in the U.S., including the No.1 Chevrolet Corvette.

Hatz quickly dismisses the notion of a 4-cyl. 911. “I see that as quite difficult,” he says.

A 4-cyl. engine would return Porsche to its roots: The legendary 356, which was the auto maker’s sole product line from 1948 until the late 1950s, was powered by a 4-cyl. boxer engine. The first 911 arrived in 1963.

Porsche’s consideration of a 4-cyl. engine illustrates the drastic strategic changes necessary to meet pending fuel-economy rules.

For instance, all versions of the new 911 will come standard with stop/start technology. The technology will help the car boost its fuel economy about 16% compared with the current 911.

Hatz says the car’s U.S. fuel-economy rating, based on Environmental Protection Agency testing protocol, is yet to be determined.

Never before offered on the 911, stop/start is filtering throughout the Porsche lineup worldwide. The Panamera and Cayenne both come equipped with the technology, and the next-generation Cayman and Boxster also will have it.

Hatz says customers like the system on the Cayenne and Panamera. “Now it will be interesting to see how it will be on a sports car.”

He says he has driven the new 911 extensively and likes the stop/start performance. The system is very similar to that on the Panamera, although the engine kicks back on faster with the new 911.

In the European driving cycle, the new 911 with an improved 7-speed dual-clutch transmission will achieve carbon-dioxide emissions below 200 g/km, making it the first sports car in the world to do so, Hatz says.

Corporate average fuel-economy requirements in the U.S. will make it difficult for Porsche to maintain its independence, especially after 2016, when auto makers are expected to achieve a 35.5-mpg (6.6 L/100 km) fleet average.

Including Porsche within the Volkswagen Group dramatically improves the luxury brand’s fleet average because of high-volume sales of the VW Golf and Jetta.

VW earlier this week announced its planned acquisition of a controlling stake in Porsche will not conclude by year’s end, raising questions about the viability of the deal.

“At the moment, we are not part of the (VW) group,” Hatz says. “But maybe we will be, most likely.”

Likewise, VW Chairman Martin Winterkorn shows little concern that the Porsche acquisition will fall through. “Wait and see,” he tells WardAuto at the VW Group Night event.

tmurphy@wardsauto.com