Mitsubishi spent years of research to determine the most efficient pattern of in-cylinder airflow. The patented "clockwise tumble" flow of the Mitsubishi GDI engine is the result. Tumble vs. Swirl Conventional engines create "swirl" airflow. But this takes the fuel around the outside of the cylinder. It's impossible to concentrate the fuel, so it doesn't always burn completely. Tumble-shaped airflow solves this problem. The tumble shape breaks up as the cylinder compresses, forming small typhoon-like eddies. These enable fuel to be concentrated around the sparkplug, for outstanding combustion efficiency even with extremely lean air-fuel ratios. A tumble pattern was employed on the Mitsubishi Vertical Vortex (MVV) lean-burn engine. However, counter-clockwise tumble was not feasible for direct injection. Why does it have to be clockwise? If the air tumbles counter-clockwise, it will carry a directly injected fuel spray into the sparkplug, creating a soot buildup that leads to misfires. Also, counter-clockwise flow doesn't allow time for a directly injected spray of petrol to vaporise. These problems were solved by the invention of the Upright Straight Intake Ports, which enabled "clockwise tumble."