First, "dirty" needs to be defined! Is it "cosmetic" or "restrictive"??? Highly significant differences!!!
Each air filter element is designed to support a particular service life in its intended use, which can relate to some highly skewed filter sizes versus engine air flow requirements. In the GM side of things, by observation, a garden variety car which will see a lot of fleet (rent car company) sales, can have a huge air filter element for a 3.1L V-6 whereas a 4.6L Northstar Cadillac will have a filter that's less than
1/2 that size.
There has been some informative discussions on engine air filters in the
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message board website. Engine oil analysis (one thing many participants on that board like to do) tends to support higher silica levels in the oil of engines equipped with the beloved K&N air filter elements. Another test reveals that the factory GM Duramax air filter element flows better and has higher particle retention (in standardized tests) than a K&N replacement element.
GM TSBs also mention the oil in the K&N fouling the computer sensors downstream in the air flow from the K&N air filter too. A "fouled" sensor that will throw a code too!
In general, a fresh air filter will always flow better than a "used" one, the degree of longevity of the filter's higher flow life will depend on the operational environment of the vehicle. It can also depend on the filter media too. I was short on funds and bought a less expensive filter (for the popular 13x3" open element air cleaner) and it turned "dirty" quickly, yet the Fram I got to replace it is still "white".
IF the filter element size/flow situation is on the smaller side of things, a small amount of "wear" will put things into the "decreased performance" area quickly (kind of like the 3.1L V-6 and Northstar V-8 illustration), but if the filter is oversized for the application (for longer service life of the element), so how much of an affect the "dirty" filter has on performance is highly variable and NOT particularly related to brand or type (other than the oil issue previously mentioned).
IF the filter element size/flow is on the "too small" side of things, adding a higher flow element (i.e., K&N or Fram AirHog) might help performance, BUT only if the orig setup is so far undercapacity that it really makes a difference with the higher flow element.
"Dirty" air filters and performance and mpg? If the additionally restricted air filter element will not flow enough air for the particular throttle input, then more throttle input will be desired to maintain a particular performance level of the vehicle. More throttle input = more delta TPS signal = more fuel for a given result, even with feedback loop operation of the fuel system. Just like in the "old days", when lower manifold vacuum levels would kick in the "power circuit" sooner than normal.
Fuel econony? We need to talk about cruise economy or city driving economy, I suspect. At highway cruise, I suspect a dirty air filter element would impact fuel economy very much, unless it was really clogged up. In the city environemnt, where throttle position will change much more frequently and accelerative bursts might be needed, I suspect THAT's where you'll notice the difference in power and economy.
It's a given that the less throttle input it takes to achieve a desired level of performance, fuel ecomomy will typically be better.
Many of the radio talk show hosts have to talk in generalitics more than specifics. Giving the general listening public some general information they can deal with in their daily lives is important, so what they say in response to a question might not be applicable to all vehicles in all situaitons per se. And the television car show operatives can also "sell up" a particular product, which might well be an advertiser on their show.
To me, the key thing is to consider their statements as "general" statements rather than statements that would specifically apply to YOUR vehicle only. End result, somebody somewhere will let a "service professional" make a determination as to whether or not their air filter element is "dirty" and needs changing. OR they'll check the restriction gauge on their air filter housing (if equipped) to see if it's still "green" or "red".
(Let the engine speed now return to idle!)
Enjoy!
C-BODY