The concept in my head is the air has to be physically moved, the effort for that moving is what I'd count as pumping losses. If the intake is very restrictive, then it's harder to move the air, so more work is required to move the air. Basically the vacuum past the throttle plate is pumping losses that could be avoided. A plugged air filter vs a new one would have similar effect and if I recall correctly most owner's manuals says lower mpg can be a result from not changing the air filter.
Change air for engine oil, for the same flow of oil, a smooth pipe vs a pipe that's 100ft long will take vastly different amounts of effort to move. Air acts like a fluid, it's just very thin. Testing how much efficiency is lost purely by a restrictive intake would be an interesting test. I suspect it's measurable but not a massive effect.
The same concept is on the exhaust side, it's a well known fact on diesels if you delete the muffler (and cat if it has one) that mpg will increase for a diesel. That should be effectively pumping losses as well. If intake didn't make no difference, then there wouldn't be dual length intakes for low vs high rpm. Long tube headers vs short tube effects where the peak torque is a bit.
I'm not an engine engineer, so very possible I could be wrong, but nothing is free, every little design change should help efficiency.
The only real pumping losses I'd think of in the lawnmower setup is the wild size difference from the carb size to the intake size. If that is a smooth transition, or I'd think ideally smaller intake tubes, it would have less pumping losses with the same lawnmower intake.
Another point, individual throttle bodies when no air filter is ran has a bell mouth to the intake. Of my understanding it's to reduce pumping losses to achieve higher max air flows. Now I wonder if the mpg effect could be measured for stock intake vs the ITB setup just for a proof of concept. The same areo effects are happening inside the intake/engine as the outer body of the car. Clearly more air = more fuel needed to be dumped. Defo an interesting thought experiment though.
