We obtain attractor and inertial-manifold results for a class of 3D turbulent flow models on a periodic spatial domain in which hyperviscous terms are added spectrally to the standard incompressible Navier-Stokes equations (NSE). Let P m be the projection onto the first m eigenspaces of A = − , let µ and α be positive constants with α 3/2, and let Q m = I − P m , then we add to the NSE operators µA ϕ in a general family such that A ϕ Q m A α in the sense of quadratic forms. The models are motivated by characteristics of spectral eddy-viscosity (SEV) and spectral vanishing viscosity (SVV) models. A distinguished class of our models adds extra hyperviscosity terms only to high wavenumbers past a cutoff λ m 0 where m 0 m, so that for large enough m 0 the inertial-range wavenumbers see only standard NSE viscosity. We first obtain estimates on the Hausdorff and fractal dimensions of the attractor A (respectively dim H A and dim F A). For a constant K α on the order of unity we show ifrepresents characteristic macroscopic length, and l is the Kolmogorov length scale, i.e. l = (ν 3 / ) where is Kolmogorov's mean rate of dissipation of energy in turbulent flow. All bracketed constants and K α are dimensionless and scale-invariant. The estimate grows in m due to the term λ m /λ 1 but at a rate lower than m 3/5 , and the estimate grows in µ as the relative size of ν to µ. The exponent on l 0 /l is significantly less than the Landau-Lifschitz predicted value of 3. If we impose the condition λ m (1/l ) 2 , the estimates become K α [l 0 /l ] 3 for µ ν and As a corollary, for most of the cases of the operators A ϕ in the distinguished-class case that we expect will be typically used in practice we also obtain an M, now of dimension m 0 for m 0 large enough, though under conditions requiring generally larger m 0 than the m in the special class. In both cases, for large enough m (respectively m 0 ), we have an inertial manifold for a system in which the inertial range essentially behaves according to standard NSE physics, and in particular trajectories on M are controlled by essentially NSE dynamics.KEY WORDS: 3D turbulent flow models; attractor dimension; inertial manifolds; degrees of freedom.