Porsche will not give the Cayman and Boxster the larger 3.8 liter flat-6 obviously to protect the 911. Their way around this may be to put a small four-cylinder in the Cayman and Boxster so 911 owners will still have their perceived superiority with a bigger six-cylinder motor. The boxer four will be based on the flat-6 meaning Porsche can assemble these motors on the same production line.
If PorscheBoost were to guess the bore and stroke of the 911 Carrera will be retained allowing pistons and rods to be shared for lower costs meaning a 97mm bore and 77.5mm stroke. This means a potential displacement of 2290cc's or roughly 2.3 liters. Porsche could use the Carrera S bore of 102mm taking displacement to 2533cc's or 2.5 liters. A range of 2.3-2.5 liters for the new motor is what we are looking at.
Porsche CEO Matthias Muller in an interview stated the output of the motor will be 'up to 395 horsepower' meaning Porsche would have the most powerful production four-cylinder in the world. Audi is going to produce a four-cylinder somewhere in the 375+ horsepower range in order to take that title from Mercedes. Porsche could potentially beat them both with their own design which likely will be a larger four-cylinder than the other German competition.
It looks like the Boxster and Cayman may finally get the power and torque their chassis have been craving for.