The BK Synchrometer has a larger airhorn adapter and is designed to flow more air than the STE SK. It works unaided in airhorns from 2" (51mm) to 2 5/8" (67mm) in diameter. The STE118 adapter (below) allows the STE BK synchrometer to fit into the same airhorns as the smaller STE SK (1 5/8" to 2 1/8".) The STE BK measures flow mass from 1-35 Kg/Hour (or to 50 Kg/Hour if you open the bypass port.) Select the STE BK over the STE SK if you are measuring airflow on engines 3 liters or larger and not more than 2 carburetors.