An engine has a crankshaft. A compression piston within a compression cylinder is connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke. An expansion piston within an expansion cylinder is connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke. A crossover passage interconnects the compression and expansion cylinders. The crossover passage includes a crossover compression valve and a crossover expansion valve. A runner section is in a downstream portion of the crossover passage, and a helical end section is integrally connected to the runner section. The helical end section has a funnel spiraling about a valve stem of the crossover expansion valve. The funnel forces incoming air to rotate about the valve stem prior to entering the expansion cylinder to promote turbulent kinetic energy in the cylinder air/fuel charge.