You do not "need" the stock lp for the RASP it will work fine without one. The stock lp is only used when pressure drops below 8psi (I think that is the pressure the Hobbs switch is set for) which would be at startup, or if you wanted to prime the system say after a filter change or as a backup if the the Rasp belt broke. At least that is what Bill K told me before I purchased mine. And I can say it will definitely start fine without the lp as part of Bills install instructions were to run the rasp a couple days with the lp electrically disconnected.