We present a general study of pulse amplification in silicon Raman amplifiers, with special emphasis on the effects of spectral compression and group delay due to the Raman gain dispersion. We use the undepleted-pump approximation to analytically calculate the dynamics of an arbitrary pulse spectra and find the temporal profile of the pulse at the amplifier's output. We show that cw-pumped silicon waveguides are extremely inefficient in amplification of subpicosecond optical pulses but provide large net gains and controllable group delays for pulses with widths of ∼10 ps.