I'm sure this question could apply to many other plane engines, but specifically I'm looking at the Piper Cherokee PA-28-140.
Surprisingly, it appears that the engine used, the Lycoming O-320, has ~320 cubic inches of displacement but only puts out 150hp.
When comparing the horsepower per cubic inch of displacement (CID), the ratio is quite low. We have the technology (and already used it in vehicles in the 50's, 60's etc.) to achieve a better efficiency than that.
Why is this engine (and perhaps other models) so inefficient?