I have them on all of my saws (66's), along with an after market third port with the spark arrester cut out inside the muffler and all holes ported out. The stock face plate is way too restrictive. There is a welded guard over the ported faceplate, it does not last forever, nor does it hinder the performance if it comes off. An after market filter system along with a ported muffler face does wonder to a stock saw. Common sense, more air - less restriction equals more power.
Tree Sling'r