Here's classic Group B techtnology applied to the transversely mounted 1.8T 20V with small port head. Note that the throttle is on the left side (like Audi TT 225 Quattro). Material is cast aluminum. OEM-spec throttle, fuel injectors, and fuel rail attach directly.
The concept behind a dual plenum manifold design is to equalize intake runner velocity, regardless of each runner’s distance from the throttle body, thereby ensuring equal air/fuel ratios in each cylinder. Complex engine dynamics such as intake pulse reflection and rpm resonance are also much less prone to interference from incoming intake charge simply due to the fact that the upper plenum is ‘separated’ from the throttle body.
The lower plenum serves as an air distributor which tapers to increase intake velocity towards the aft side of the plenum. It feeds air into an even gap that runs the full length of the plenum causing the air to "distribute" evenly into the upper capacitance plenum. The cross-sectional area of this gap is roughly 110% of the cross-sectional area of the throttle body opening which allows for an equal distribution of intake air with a safety buffer to insure absolutely no intake restriction.
