Aerodynamics of a Ram Air Duct | Momentum Loss
On top of pressure loss due to the skin friction of the duct itself, there is momentum loss if the flow is forced to change direction abruptly, which is almost always the case in automobile engine induction with the awkward tubing shapes that are normally present.
It is a simple concept to understand by visualizing an S-shaped tube. The S-shape causes the air to have to change direction twice, this may not seem like much. But at high-fluid speeds, you can lose total pressure to extreme direction changes (you end up losing the dynamic pressure component of total pressure. Momentum changes slow down the air speed, hence you are losing total pressure by means of a reduction in dynamic pressure loss). Below are some examples of different designs by nature, and a good design such as the Velossa Tech Design BIG MOUTH ram air intake snorkel series.