Self-propagating high-temperature synthesis is a very easy and low-cost method to synthesize Si3N4. The nitriding of silicon powder takes place in a self-sustained regime under high pressures of nitrogen with dilution of silicon by Si3N4. In this work effects of dilution and green-mixture porosity on combustion velocity and phase content of reaction products are studied. Results are compared with previous work of other authors and different behaviors are found. An explanation of these behaviors is given.