Torque wise the diesels will win every time because they have a flatter torque curve than the gas engines. However if it were the 15 and 17 back to back the 17 would win if the remainder was equal, again because it produces more usable torque.
I wonder if someone changed out the idler? That should be a flat idler riding on the back of the belt. If it was swapped for a sheaved one it would also roll the belt over.