[oil pan 4]

Originally Posted by Honda

I disagree, the main entrance (air filter) is still restricted, you are sucking PCV pressure and possibly some engine oil as well with dirty air filter which leads to carbon build up in the intake manifold causing even more restrictions. I understand that the computer in todays cars are programmed to compensate for our laziness on maintenance and will gather information from various sensors to come up with the best air/fuel ratio. The computer will try but there is no guaranty since air (O2) is limited and requires additional energy to get it.
let's assume we have 4 cylinder engine with 1-3-4-2 firing order, if #4 cylinder is on power stroke it only needs to move 4000lbs metal with clean air filter, while with dirty air filter it also needs to push #2 cylinder harder to suck air. Take a syringe and try to pull it down with the end clogged, you will need more power to do so compared to if the end is not restricted.
I think this can apply also to blower motors. you go to the repair shop because the heater doesn't work, the mechanic found that the resistor is burned, if he is good enough he will recommend to replace the blower motor and the cabin air filter even if the motor is running, why? the resistor burned because the blower motor demanding more amps to run. it turned out clogged Mr cabin filter was behind all this mess.

You are absoluty correct with the srynge anolgy. The only problem is you fail to include the biggest intake restriction of them all, the throttle plate. The srynge tests doesnt factor in
isobaric losses found on the intake process.
Your biggest restriction on a gas engine by far is the throttle. Throttling losses (isobaric expansion) acounts for up to 2 horsepower loss on a small 4 cylinder engine cruising down the road at high way speed. The 2 horses woth of isobaric loss you get while cruising down the road added to the tiny restriction offered by the air filter is immeasurable.

Now in a diesel, a restricted air filter tracts directly with fuel economy. Diesels dont have a pesky isobaric process to argue with, on a diesel its all about craming as much air mass into the cylinder as possible.
With that said the diesel world largely dislikes K&N air filters, they let more dirt in.