What does plenum volume have to do with power?
Lots! There are all kinds of theories on this but as a general rule of plenum volume it has been found that, for a four cylinder engine, a plenum volume (for maximum performance) should be about 50% to 60% of the total engine displacement. So for a 2.5 liter (2500cc) Subaru boxer engine, you should have a plenum volume of around 1250cc to 1500cc. The stock Subaru manifold is about 725cc. This represents about 29% of the total engine displacement, a far cry from the recommended 50 to 60%. Why is the stock Subaru manifold plenum so small? These stock plenums are geared more for fuel economy and emissions. Also, the Subaru engineers were constrained by all the other parts of the engine as far as available room for a larger manifold.
