In terms of function, the best plenum design would have the air duct feed the center of the plenum. Unfortunately, due to space limitations and production costs, manufacturers tend to build plenums that are fed from one end, with the plenum blocked off at the other end. This results in air rushing to the far end of the plenum and creates a slight imbalance of air flow to the individual cylinders are the air will tend to flow past the first cylinder and collect at the far end of the cylinder, which is usually at the last cylinder. Consequently, the first cylinder will run slightly lean while the last cylinder will run slightly rich.