The effects of SiC and Carbon doping on the superconducting properties of MgB2 polycrystalline samples have been analysed by means of specific heat and magnetic relaxation measurements. It is known that the addition of nanometric powders of SiC and C leads to the enhancement of Birr and Jc. However, the underlying physical mechanism is not completely understood. Magnetic relaxation measurements did not show detectable effects of both the additions on the pinning properties of MgB2. It follows that doping acts mainly introducing disorder into the superconductor and thus raising Bc2. In the case of MgB1.9C0.1, specific heat measurements show that the C substitution on the B sites modifies the low temperature shoulder related to the second gap. This effect is not visible in the sample doped with SiC. From the distribution of Tc determined from the deconvolution of the calorimetric data, we argue that SiC leads to an inhomogeneous distribution of C.