Here's a counterintuitive truth that trips up nearly every first-time porter: a mirror-polished intake port flows worse than a rough one. This isn't folklore — it's boundary layer physics, and understanding it will change how you finish every port you touch.

When air flows through a port, the molecules touching the wall stick to it (zero velocity), and velocity ramps up as you move toward the port centerline. This velocity gradient is the boundary layer. It comes in two flavors:

• Laminar boundary layer — smooth, orderly flow in thin sheets. Low friction, but thick — it eats into the effective port cross-section.
• Turbulent boundary layer — chaotic mixing at the wall. Higher wall friction per unit area, but much thinner, so the effective flow area is larger.

On the intake side, the turbulent boundary layer wins. It's thinner, resists separation better around the short-side radius, and keeps fuel droplets suspended instead of letting them puddle. A slightly rough surface — around 60–120 grit finish, roughly 80–150 microinches Ra — trips the boundary layer turbulent almost immediately after the port entrance.

Real-world example: Smokey Yunick famously ran Chevy small-block heads with intake ports finished with a coarse cartridge roll (roughly 80 grit). His heads consistently outflowed polished competition heads on the flow bench at high lifts and — more importantly — made more power in the car because the fuel stayed atomized. Modern OEM CFD studies at Ford and GM confirm the same result: production intake ports are cast rough (Ra ~250 microinches) and left that way. Nobody polishes intake runners at the factory.

The exhaust side plays by different rules. Exhaust gas is already turbulent, dry, and hot — there's no fuel to keep suspended. A smoother finish (400 grit, ~25 microinches Ra) reduces friction losses and helps prevent carbon adhesion. Many porters polish the exhaust bowl and short-side to a near-mirror while leaving the intake deliberately textured.

Rule of thumb for finish selection:

• Intake port walls: 60–120 grit cartridge roll, leave the tooling marks visible
• Intake bowl and valve throat: 120–180 grit, slightly smoother but not polished
• Exhaust port and bowl: 240–400 grit, approaching a satin finish
• Combustion chamber: Polish smooth (reduces carbon retention and hot spots)

The mistake beginners make: they sand the intake port to a mirror because it looks fast. What they've actually done is thicken the boundary layer, promoted flow separation at the short-side radius, and encouraged fuel to condense on the walls at low RPM. The dyno tells the truth — mirror-polished intakes routinely lose 5–15 hp versus the same port left rough.