The THERPSICHORE three dimensional linear ideal magnetohydrodynamic stability code has been extended to the full MHD equations. The new code is used to calculate the physical growth rates of non-local low-n modes for l=2 torsatron configurations. A comprehensive investigation of the relation between the Mercier modes and the low-n modes has been performed. The unstable localized low-n modes are found to be correlated with the Mercier criterion. Finite growth rates of the low-n modes correspond to finite values of the Mercier criterion parameter. Near the Mercier marginal stability boundary, the low-n modes tend to be weakly unstable with very small growth rates. However, the stability of global-type low-n modes is found to be decorrelated from that of Mercier modes. The low-n modes with global radial structure can be more stable or more unstable than Mercier modes. (author) 19 figs., 2 tabs., 31 refs.