Interesting how the lumps detract flow at low lifts and then kick in as the lift increases. Similar to cam duration trade off in relation to RPM.
When the valve starts to open, piston speed, flow rate and velocity is relatively low, and the charge is largely "glued" on to the floor, the shortest distance between the two points - the carb and the cylinder. So sticking a lump on the floor would make for a longer, circuitous path for the initial charge.
On the other hand, at high piston speeds and high lift the charge is swiftly moving and the inertia of the fast-moving mass packs the charge up against the roof of the port. This is when the lump helps funnel the charge into the cylinder at high velocity with a minimum of turbulence.
So one can infer that a bigger valve - and a higher lift cam - will amplify the effect of lumps.
