Many cars for fuel efficiency or performance.

In modern fuel injected cars, secondary intake and fuel injection systems get more complex with rpm; typically its about 3500 rpm or 50% throttle position cause a different fuel map, an intake runner flapper, and for the last 30 years, some automatic cam timing adjustment.

yeah, I'm familiar with the modern fuel injection engines - this conversation was about carburretor engines

They're not all that fancy. My dad's 1967 Volvo 122s had two carbs for a four cylinder engine. It always failed the emissions test and I can't say that it ever purred like a cat.

Hard to imagine a 67 on an emissions test, I'd guess that would be grandfathered.