Every intake port has to bend. Air enters roughly horizontal from the manifold, then turns downward toward the valve seat. The short-side radius is the inside of that turn — the floor of the port where it curves down into the bowl. It's the single most flow-limiting feature in a production cylinder head, and the place where amateur porters destroy more heads than anywhere else.

Here's the physics: air has mass, and mass wants to travel in straight lines. When the port bends, air on the outside of the turn (the long-side, or roof) stays attached to the surface because centrifugal force pushes it there. Air on the inside — the short-side — wants to separate from the wall and continue straight. When it separates, you get a recirculation bubble that chokes flow and destroys velocity right where you need it most: entering the valve curtain.

What determines short-side behavior:

• Radius of curvature — sharper turns separate sooner. A tight short-side is a flow killer.
• Surface finish — the short-side is one place where smoother is better, unlike the port walls. A polished short-side helps the boundary layer stay attached.
• Approach angle — how the port floor blends into the bowl area under the valve seat.
• Cross-section transition — abrupt changes in area near the turn trigger separation.

Real-world example: Small-block Chevy 23-degree heads have a notoriously tight short-side because the pushrod pinch forces the port to bend hard. Serious porters spend more time on the short-side than any other area — often laying in weld to raise the floor slightly, creating a gentler radius. Done right, this can add 15–25 CFM at high lift. Done wrong (grinding the short-side down to make the port "bigger"), it tightens the radius, kills flow above 0.400" lift, and ruins the head. Once you cut past the parent metal, you can't put it back.

Rule of thumb: The short-side radius should be roughly 25–35% of the port height at the bend. Below 20%, separation is almost guaranteed at high lift. Above 40%, you're probably losing velocity by making the turn too gradual and stealing floor material that supported the valve guide boss.

Flow bench tells the story fast: if CFM stops climbing (or drops) between 0.400" and 0.500" lift, the short-side is separating. High-lift flow lives or dies at that turn.