This spec muffler is for “stock” 4-cycle classes and the BP clone class in WKA, FKA, AKRA, NKA, etc. 

Mufflers have been mandated for many tracks due to noise pollution. Unfortunately, track neighbors don’t enjoy the thundering roar of racing engines. If you are not restricted by the type of muffler and running an open class, we recommend using the less restrictive 4106 mufflers. If you are not required to have a muffler, we always found engines make more power without them. 

B91XL 1 5/16″ EXHAUST SILENCER, FITS 1 5/16″ OD 4 CYCLE PIPE. IT ALSO FITS A 1 1/4″ OD 4 CYCLE PIPE WHEN USED WITH A 4134 SHIM.