I'm going to chime in here....
As taken from my shop manual:
"Cutting blades on a rotary lawn mower should be sharp enough to cut grass without tearing, tearing causes tops of the grass blades to become brown, it also makes grasses more susceptible to diseases, fungus and blight"
And from another part of the same manual:
"A sharp blade or cutting disc reduces drag on the engine,saves fuel,and extends life on both the engine and blade"
Hope that answers your questions.
Kori
