Engine Speed Related Vibrations

First-order engine-speed-related vibrations (E1) are unusual on most engines. Only inline 3 and 5 cylinder engines, and V6 engines without counterbalance shafts, have a normal first-order vibration. Most first-order engine vibrations are caused by an out-of-balance or out-of-round component that is rotating at the same speed as the crankshaft.
1.5-order engine-speed-related vibrations (E1.5) are normal on three-cylinder engines.
Second-order engine-speed-related vibrations (E2 ) are normal on four-cylinder engines and V6 engines.
2.5-order engine-speed-related vibrations (E2.5) are normal on five-cylinder engines.
Third-order engine-speed-related vibrations (E3) are normal on six-cylinder engines.
Fourth-order engine-speed-related vibrations (E4) are normal on eight-cylinder engines.
Fifth-order engine-speed-related vibrations (E5) are normal on ten-cylinder engines.
Sixth-order engine-speed-related vibrations (E6) are normal on twelve-cylinder engines.