i wouldnt belive the 450 series would be 4.5hp I think at one point they changed the designation to a torque figure

http://www.briggsandstratton.com/engines/push-mower/detail/?series=450+Series&id=%7B68BC1FA5-2AC4-4F13-84BB-D9B82E6D22CD%7D

4.50 ftlb of torque....