Complex sentence: This wasteful practice (when intercoolers are not used) nearly eliminated the gain in engine efficiency from supercharging, but was necessitated by the greater need to prevent at all costs the engine damage that pre-detonation engine knocking causes.
Split into simpler sentences: This wasteful practice (when intercoolers are not used) nearly eliminated the gain in engine efficiency from supercharging. But was necessitated by the greater need to prevent at all costs the engine damage that pre-detonation engine knocking causes.