You've spent hours smoothing a port to a mirror finish, and now I'm telling you to weld a bump back into it. Welcome to anti-reversion, the counterintuitive corner of head porting where a deliberate obstruction makes more power than a clean pipe.

What reversion actually is: During valve overlap — that brief window when both valves are open near TDC — exhaust pulses can travel backward up the intake port. At low RPM, cylinder pressure hasn't dropped fast enough, and hot exhaust gas physically pushes back past the intake valve, contaminating the fresh charge with inert residuals. On the exhaust side, the opposite happens: after blowdown, low pressure in the port can suck combustion gases back into the chamber during overlap.

The result is soft, lumpy idle, poor low-RPM torque, and a fuel-charge that's diluted with hot leftover exhaust. Big-cam engines suffer worst because overlap durations of 60-90° crank angle give reversion plenty of time to travel.

The anti-reversion dam: A step, ledge, or raised ring machined (or TIG-welded) into the port wall — typically on the pushrod side of the intake port, roughly 1.5 to 2 inches upstream of the valve seat. Forward flow (toward the chamber) sees the dam as a mild venturi and accelerates smoothly past it. Reverse flow hits the sharp trailing edge of the step and separates violently, forming turbulent eddies that dissipate the pulse's energy before it can climb further up the runner.

Real-world example: Big-block Chevy oval-port heads with a 268°+ duration cam notoriously lope, load up plugs, and can't hold vacuum below 900 RPM. A 0.060-0.080" tall anti-reversion step ground into the intake floor about 1.75" back from the seat routinely picks up 1.5-2" Hg of manifold vacuum at idle and drops 15-30 lb-ft into the 2000-3500 RPM torque valley — with zero change to peak flow above 4500 RPM.

Rule of thumb: Dam height should be roughly 8-12% of port height at that station. Taller and you choke forward flow; shorter and reverse pulses walk right over it. Position it at approximately 1.5x valve diameter upstream of the seat — far enough that forward flow re-attaches before the bowl, close enough to catch reversion before it escapes into the plenum.

The trailing edge must be sharp — a rounded step is just a lump. The leading edge should be gently ramped so forward flow doesn't see it as a restriction. Asymmetry is the whole point: friendly one direction, hostile the other.