Let me explain. The principal advantage of full flow system over regular staged combustion is higher Isp of the engine. Higher Isp of the engine is achieved be

Let me explain. The principal advantage of full flow system over regular staged combustion is higher Isp of the engine. Higher Isp of the engine is achieved because the pressure in the chamber of full flow engine is higher, than chamber pressure of staged combustion engine. The pressure in the chamber is higher because pumps produce higher pressure. Pumps can produce higher pressure, because with full flow pumps have more power driving them than with staged combustion. Power here is roughly equal to volume flow (m^3 / s) multiplied by pressure drop (N / m^2). Full flow scheme gives you more volume flow, since all the propellant is used to turn turbines. So even though all the propellant in both cases is burned in the chamber, Isp is higher with full flow scheme. Rocket engine maximizes momentum of the exhaust, not the energy, which is conserved.