Can be bumped to an 8 HP but the 8 HP can't be bumped to a 9.9. The 9.9 and 15 share a block as do the 6 & 8 hp. The Tohatsu/Nissan two strokes were 8 and 9.8 using the same block... How tired is your 8 HP Tohatsu? I've been looking for an 8 or 9.8 Nissan or Tohatsu 2 stroke.?