Can any of you geardos tell me if that's the reason or why some cars see an increase and others don't?
Every car is different, but they will all see small power gains by replacing the OEM filters with less restrictive aftermarket ones. The question is, are the gains worth it?
Also, we need to consider two different modifications, one being a simple filter swap and the intake being a replacement for the entire intake airway.
A simple filter swap will eliminate much of the restriction caused by the stock filter. A filter swap is usually good for about 5-10 hp on a big v8, or roughly a 1-2% power increase. Not much, and when you scale that down to a small 4 cylinder car you're likely talking 1-3 hp which is insignificant and not even possible to validate with a dyno since run-to-run variations usually vary by more than that. Statistically it's "noise".
Changing an intake can make larger gains than a simple filter swap because the restriction in the factory piping (which often includes noise reducing baffles/chambers etc.) is eliminated. It's not uncommon to see 20 hp gains on large engines, but again that is only working out to gains in the 5% arena which works out to 6-8 hp on small engines.
Complicating things are the claims made by intake manufacturers, many of whom will take the worst dyno run before the intake and the best one after and then publish the "peak" power difference between the two. Others will tune the car in addition to adding their intake and then claim gains "up to" a certain amount. As a general rule I usually take a manufacturer's claimed power improvement and divide by 2.
Any air intake also stands to corrupt the air flow through the Mass Air Flow sensor and cause variations in air/fuel ratios in open loop operation (WOT). This may even be done on purpose by the intake manufacturer since leaning out the A/F ratio of an engine tends to increase power. In the case of the intake actually replacing part of the MAF sensor, the intake may or may not have the same flow characteristics and/or cross sectional area of the OEM MAF sensor whether intentional or not.
The main thing to consider with intakes is that they provide power increases by reducing restriction to airflow, which by definition means that they have their greatest effect at the engine's maximum power level and taper off very quickly before that. If your engine makes peak power at 6000 RPM, the effect of the intake is mostly lost below 4500-5000 RPM. So you're paying for a small increase in airflow which only benefits the engine at the uppermost limit of its operating speed. Unless you're racing and looking for that last 10th of a second, it doesn't make too much difference.
Most of the low hanging fruit has long been picked as far as simple power increases are concerned. OEMs are under immense pressure to put advertising numbers up so their cars look better than the competition. The good ol' days of 20 hp intake and exhaust system upgrades are pretty much behind us, and the OEM equipment is generally very well designed and doesn't leave a whole lot on the table.
And the last point is filtration efficiency. As I stated earlier, there is not much public knowledge of how much potential damage can be caused by allowing "dirty" air into an engine as far as damage inside the combustion chamber goes, but there IS evidence that it can cause significant damage to the high speed parts of a turbocharger.
Being a little older and wiser than I once was, I'm no longer chasing every last single hp from my 4 cylinder daily driver… in fact with my Cruze I'm quite happy so far just leaving the engine stock (minus a spark plug upgrade for drivability improvements). The fact the Cruze has a turbocharger is one reason I will not likely "upgrade" the air filter to a free flowing unit. However my other car, a high mileage 2002 ZO6 Corvette, has been wearing an oiled wire/gauze air filter since I bought it in 2004. Two years ago I pulled the heads to install a new cam and lifters and the heads were in terrific condition; intake valve seats were like new and the valve guides were still nice and tight. I lapped the exhaust valve seats (they had some mild pitting) and they were ready to bolt back on. The cylinder bores still show the original machining marks and the engine uses no oil to speak of. It had ~110,000 miles on it at the time and is now over 120,000 miles.
