A novel approach to the widespread problem of evaluating the noise quality of a car engine is presented. Traditionally some psychoacoustic metrics of the sound are calculated and used to generate a certain Noise Quality Index (NQI) according to a previous jury assessment. In this paper we relate well-known psychoacoustic metrics which usually contribute to generate the NQI to time-frequency characteristics of car noise, in order to find relations between both kinds of parameters. These relations could help engine designers to predict the noise quality just analysing the spectral changes of the noise waveform. Some promising results are showed.