Head flow,vs. cam lift?......

Using a water analogy again - if you have a typical garden hose with a spray handle that starts with a fine mist and goes to a stream as the handle is squeezed - once the water exiting the handle forms a stream - it's at max flow for that volume and pressure and squeezing harder just tires your hand. The smaller the hose, the earlier in the "sqeezing" the water forms a stream.