What Is Cylinder Head Swirl and Tumble, and Why Is It Important?
Hot Rod|January 2021
We think of electronic engine management systems and multiport electronic fuel injection (EFI) as primarily responsible for the improved air/fuel distribution and performance as Detroit clawed its way back in the mid 1980s, but—good as they were for that day—initially the new electronics were just Band-Aids grafted onto carryover engine designs. The big change started in the 1990s, when the OEs started tinkering with cylinder head design to induce swirl and tumble in internal combustion engines.

The changes were first evolutionary, but around the dawn of the 21st century, new ground-up engine designs like the GM Gen III LS small-block, Ford Modular engines, and Chrysler 5.7/6.1 Gen III Hemi appeared. The goal of 21st-century cylinder head engineering is controlling and managing the combustion process by combining radical combustion chambers with new port layouts to improve mixture motion. By equalizing variations in cycle-to-cycle and cylinder-to-cylinder flow, less compromise in timing and fuel delivery are needed to cover the worst cylinder or valve-lift point.

SWIRL IN IC ENGINES

“Swirl”—the rotational motion of the incoming air charge about the cylinder’s axis as it enters the combustion chamber on a two-valve head—is determined by the intake valve’s position relative to the bore axis, the chamber shape around the intake, and any helix in the induction tract. Swirl is maximized by paying careful attention to a wedge head’s valve positioning. Think of swirl in an IC engine like the vortex generated by a flushing toilet. Swirl should always be in one direction; swirl that changes direction at different amounts of valve-lift leads to unstable combustion, requires more ignition lead, and reduces knock-limited power potential. Optimized swirl-port heads sometimes flow less air on a flow bench but still yield measurable power gains on a running engine.

TUMBLE IN IC ENGINES

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