The methodology of engine operating parameters assessment that could be used for comparative tests has been presented in the paper. The basic assumption is that the engine is tested during free acceleration. Another assumption of the presented method is that the relative change of parameter could be assessed on the basis of values which are close to exact parameter values. All these parameters are both measured with the same accuracy and under the same conditions. The accuracy of the model has been identified for S-4002 engine. The sample test results have been presented.