The Ford flathead is now venerated for the way it introduced V8 power to affordable cars, but there were some issues with its design that can pretty much be boiled down to its name.
Flathead engines put the valves in the block, alongside the pistons. That’s great for simplicity’s sake, and could be part of what allowed Ford to offer the engine to regular buyers, but also limits airflow.
Since the air has to go into the block, up around the valves, then back down into the cylinder, then back up into the block and around and out through the manifold, air flows worse than this sentence.
The design also hampers compression, because there’s a (relatively) large area where the valves operate that can’t be compressed by the cylinder. So you constantly have to balance getting air into the engines against its impact on compression.
Still, the engine’s advantages outweighed its disadvantages for years, powering countless trucks and hot rods before stepping aside to let overhead valve engines take over.