# Why can't ramjet diffusers be stacked to achieve a good overall pressure ratio?

The problem with ramjets is that they only gain efficiency similiar to turbojets at speeds above mach 2. However, ramjets can operate at speeds of a few hundred km/h. Why therefore cant ramjet diffusers be stacked, analogously to compressor stages in a turbojets to achive a high compression ratio and efficiency at low speeds?