Nicola - Although not really the same verbs which you are asking about, http://dlmcn.com/ir_verbs.html contains the results of research I did on vowel changes in certain "-ir" verbs. For example, for me it is helpful to remember that advertir, convertir, invertir, divertir and the others which end in "-vertir", all adopt the e>ie stem-change in [much of] the present tense, as too do all the "-erir" ones [sugerir, diferir, interferir, referir, transferir, preferir, inferir etc.] However, those which terminate in "-ergir" [sumergir, divergir, and convergir] do *not* undergo any vowel change. In the third category, all "-etir" verbs [e.g. repetir, competir, derretir] - as well as "-egir" ones - replace the 'e' by 'i' in the present tense [apart from the nosotros and vosotros forms, of course].