In this paper we present a stabilized finite element method to solve the transient Navier-Stokes equations based on the decomposition of the unknowns into resolvable and sub grid scales. The latter are approximately accounted for, so as to end up with a stable finite element problem which, in particular, allows to deal with convection-dominated flows and the use of equal velocity-pressure interpolations. Three main issues are addressed. The first is a method to estimate the behavior of the stabilization parameters based on a Fourier analysis of the problem for the subscales. Secondly, the way to deal with transient problems discretized using a finite difference scheme is discussed. Finally, the treatment of the nonlinear term is also analyzed. A very important feature of this work is that the sub grid scales are taken as orthogonal to the finite element space. In the transient case, this simplifies considerably the numerical scheme.