The anti-vibration system of this chainsaw ensures comfortable operation and handling, as well as reduced fatigue. The pre-seperation air filtration technology extends the cleaning life of the air filter so you do not have to be constantly maintaining it. The side-access chain tensioner allows you to get in and adjust the chain without putting your hands near it. The fuel and oil caps require no tools to be opened, and have retainers on them that prevent them from being lost. This chainsaw differs from the lower cost MS 311 in that it has more power (4.4 HP as opposed to the 311's 4.2 HP), and has the lntelliCarb compensating carburetor (which measures air on the clean side of the air filter and adjusts its metering diaphragm accordingly, a procedure that keeps RPM levels even when the filter begins to clog), though it lacks the 311's decompression valve (which makes for easier starting). For the MS 391, Stihl recommends using the 33 RSC3 72 saw chain with a 20 inch guide bar.