Porsche working on more boxer fours

Porsche has indicated it is developing a higher-output version of its new turbocharged boxer four to power the next Boxster GTS.

Speaking at an engineering workshop in Marseilles, Porsche executives suggested the upcoming 982-series Boxster GTS may employ a more powerful version of the 257kW/420Nm 2.5-litre turbocharged flat four seen in the latest Boxster S.

However, despite the heavily upgraded Boxster and Cayman wearing 718 badging to draw a link with Porsche's mid-engined four-cylinder racers in the 1960s, Porsche has not ruled six-cylinder boxer power for the Cayman GT4 replacement.

The new fours produce more power yet use less fuel than the six-cylinder engines they replace in the outgoing Boxster and Boxster. But Porsche execs said there is even more performance to come from their respective 2.0 and 2.5-litre turbo boxer fours.

"This is just the beginning of engine development for the [four-cylinder] B4 engine, so of course we have more room for improvement," said boxer engine mechanics manager, Dr Bruno Kitsner.

Porsche remains coy about the engine that will power replacements for the more powerful Boxster and Cayman GTS, Kitsner saying only: "I have no idea what kind of engine the GTS will have. I can only tell you it will have the right one. Time will tell."

Porsche engineers admit the B4 boxer engine in the Boxster S is already at its maximum capacity. The 2.5-litre shares its 76.4mm cylinder stroke dimensions with the Boxster's smaller 2.0-litre engine, but has a larger 102mm bore.

The new four-cylinder engine was developed alongside the 911's new 3.0-litre twin-turbo six-cylinder. The modular B4 and B6 boxer engines share the same 118mm bore centres, the same 76.4mm stroke and, in the case of the 2.0-litre four, the same 102mm bore dimensions.

Therefore, because 2.5 litres is the maximum capacity of the B4, the engine of the next-generation 911 Turbo, which is already under development, cannot be larger than 3746cc – although Porsche is almost certain to call it a 3.8-litre rather than a 3.7.

Porsche indicated it won't add another turbo to the single-turbo B4 engine to increase power, because it already had difficulty plumbing its top-mounted intercooler with the Boxster's side air intakes.

"Two turbochargers could not work efficiently together," said Kitsner.

"[And] Two catalytic converters would be impossible to package."

While the 911's 3.0-litre twin-turbo boxer six could be used to power a replacement for the current Boxster Spyder and Cayman GT4 (which employ 276kW/420Nm and 283/420Nm versions of the same 3.8-litre flat six), its turbo configuration could also provide clues to the makeup of the Boxster/Cayman GTS engine.

The new 911 engine runs a compression ratio of 10.0:1 and maximum boost pressure of 1.0- bar in the Carrera and 1.1 bar in the Carrera S. Meantime, both four-cylinder Boxster engines run compression of just 9.5:1 and, while the Boxster's 2.0-litre can boost up to 1.4 bar, the 2.5-litre in the Boxster S runs only 1.0 bar of boost.

Of course, the bigger Boxster S engine also incorporates variable turbine geometry (VTG) technology, and a bigger turbo including a larger turbine wheel (up from 50 to 55mm) and a bigger compressor wheel (up from 58 to 64mm).

So the new Boxster and Cayman GTS are likely to be powered by a 2.5-litre boxer four with higher compression and greater boost to eclipse not only the outgoing model's 243kW/370Nm outputs, but the 257kW/420Nm of the latest Boxster S.