Why am I getting different frequency values in ANSYS 8.0 with this model when
I request a different number of modes or a different frequency range.

Your model has MPC184 elements which are not supported at 8.0 by the
Block Lanczos solver due to their use of LaGrange Multipliers (see Assumptions
and Restrictions for MPC184.) They frequently give acceptable results for modal analysis,
but for this model (which has element shape checking turned off) they apparently do not.

There have been improvements made in the Block Lanczos solver at 8.1,
which allow the use of MPC184 elements. (Ref: 8.1 release notes: Support for Lagrange Multiplier Formulation in Modal Analyses)

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