It is difficult to make a comment not knowing the size of your boat or what furling system you have but, generally speaking, you should be able to furl a 170. Assuming you have a "typical" yacht in the mid 30' range, your J is likely in the range of 15' or so. Going from a 110 to a 170 adds only 60% of the J or about 9' to the furl which is likely only about 3.5-4 turns of the drum by the time you have the first 110% furled. Unless you have a small drum (i.e. a small furler for the size of your yacht) or unnecessarily thick furling line (which is commonly sized for ease of handling rather than strength) that should be doable. In ether case, one can, as previously suggested, strip the cover from at least the length of the furling line between the drum and the retreaving point which will allow quite a bit more line on the drum.
In connection with the foregoing, note that there is a distinction between furling and reefing. Some systems such as the Harken
furlers allow the foil to rotate before the tack and head
of the sail begin to be taken in, which, to some extent, allows one to "reef", with some of the belly of the sail's draft
to be gathered in before the foot and the leach begin to furl. With this, the trim of the sail can be maintained somewhat while the sail is partially furled-i.e. reefed--but the "set" isn't particularly good unless one adds some padding of some type--foam pads, rope-- along the length of the luff. Note, however, that a 170 is typically made of relatively lighter weight cloth, for the light airs for which one typically needs such a large sail. Accordingly, reefing the sail and using it it heavy air bodes ill for the sail as it will be (realtively) heavily loaded and can be stretched beyond its elastic limit, ruining its ability to set properly for the light air it was designed to handle. Frankly, I would not attemp to reef such a large and presumably light weight sail unless the foot, leach and tack areas remaining exposed in the reefed mode were reinforced.