Rob and Ryan are to be commended for their efforts to help us understand, but this old fart has gotten nowhere in two tries, so far. Rob's X.XX is not for me and Ryan's details are great but I do not know what the L and HD start with. To make it simpler for this blockhead, could someone tell me what is the difference between the L and HD powerplants? Is it just pulleys? If so, is it upper and lower that are different? Thnx for this info and your patience but I know nothing about this stuff and own two HD's. Do I want to mess with them? Yes? How? No? Why not? thnx again!