The Tech Behind SpaceX’s New Engine

But if the preburner was given the same fuel and oxidizer as the engine itself, and at the same ratio, it would essentially just be a smaller rocket. But unlike the gas-generator cycle which burns just a small amount of fuel and oxidizer in the preburner, in a full-flow staged combustion engine, all of the fuel and oxidizer is run through their respective preburners. In addition, the fact that the fuel and oxidizer enter the combustion chamber as gasses further improves engine efficiency over conventional designs which spray them in as liquids.

Source: hackaday.com