So far I've done two mows, no noticeable power difference (don't think i'd notice .5 hp ever) but it sounds much better and I got two mows (mine and my neighbors) of the tank and still have enough left for maybe half a yard. Its hard to compare it to a 5+yr old spark plug though...
Its in an old Toro self-propelled mower with a 6hp Briggs & Stratton. It's the best mulcher under 6.5hp I've ever used
