It's no secret that Porsche is working on a four-cylinder boxer engine; it's already admitted it's under development. The questions that haven't been answered yet are, when is it coming, and which vehicles will use it?

In December we reported that a new four-cylinder boxer engine could make its debut in a Porscheas soon as the 2013 Frankfurt Auto Show. Now it seems that we won't see the flat four from Porsche for another three years.

A Zuffenhausen-based Porsche official told Autocar that the new powerplant is still early in its development, and it's not due before 2016. It is expected to make its debut in the refreshed 2016 Boxster and Cayman sports cars.

The new engine is said to share its design with the boxer six in the current Boxster, Cayman, and 911. It will feature direct-injection technology, and likely, start-stop functionality. We expect it will displace 2.5 liters and will be offered both in naturally aspirated and turbocharged forms, with the latter producing close to 350 horsepower.

It's not the only four-cylinder coming to Porsche, however. The upcoming 2014 Porsche Macan compact crossover actually will be the first new Porsche to receive a four-cylinder engine. That engine will be a Volkswagen AG 2.0-liter in-line four-cylinder, and will be developed by the VW Group in gasoline and diesel forms.