You've spent hours smoothing the bowl, radiusing the seat, and blending the transition. Now you're staring at the port floor — the bottom surface of the runner from the manifold face to the bowl. Every instinct says polish it. That instinct will cost you power on a port-injected engine, and here's why: the floor is where fuel lives.

In a dry-flow world (direct injection, or a flow bench with no fuel), the floor is just another wall. Air doesn't care. But in port injection, the injector sprays toward the back of the valve, and a meaningful fraction of that fuel — anywhere from 10% to 40% depending on temperature, pulse width, and port geometry — condenses on the port walls and runs along the floor as a liquid film. This is wet flow, and it obeys different rules than air.

A mirror-polished floor is hydrophobic in the wrong way: fuel beads up, forms unpredictable puddles, and dumps into the cylinder in slugs during transients. Throttle tip-in gets a rich spike, then a lean stumble as the puddle clears. A lightly textured floor — think 120-grit cartridge roll finish, roughly 60–80 microinch Ra — holds a thin, continuous fuel film that meters into the cylinder consistently.

• Floor shape matters more than floor finish. A slight downhill grade from the manifold face toward the bowl (2–4 degrees) uses gravity to keep fuel moving. Flat floors puddle; uphill floors puddle worse.
• Don't raise the floor at the pushrod pinch. On pushrod heads, the floor often dips around the pushrod tube. Filling that dip with epoxy sounds smart until you realize you just created a fuel dam.
• Match the floor to the manifold gasket. A step-up from manifold to head floor catches fuel and creates a shelf. The head floor should sit 0.010–0.020" below the manifold floor at the joint.

Real-world example: Small-block Chevy Vortec heads (906/062 castings) responded famously well to mild floor work — knocking down the pushrod pinch bump and maintaining a slight downhill grade. Racers who mirror-polished everything reported worse tip-in response than guys who left the floor at a 220-grit finish. The rough-floor engines made 8–12 lb-ft more from 2,500–3,500 rpm on the same dyno.

Rule of thumb: On port-injected engines, finish the floor two grit levels rougher than the roof. If the roof gets 320-grit, the floor gets 120-grit. On direct injection, ignore this entirely — polish freely, because no fuel ever touches that surface.