Review of K&N Air Filter for Mazda 2.5T engine

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2019 CX-9 GT
Have y'all seen the size of the stock filter. I mean how thick that thing is. It's like having a jacket on as a face mask.

2019 CX-9 GT

So installed k&n. Instantly noticed turbo spools so much quicker off the line. And max torque as soon as turbo spools. It's a big noticable difference. I am using 93 premium before and after the filter.

The power dip at 4500 is hardly noticable. I think this poser dip is related to Timing correction however I haven't logged a run yet to see all the data. I used to tune my 335 and x5 n55s so I have experience with this and timing corrections.

My plan is to reduce plug gap by .005-.008" to create better spark. In high compression engines, you want smaller plug gap to reduce spark blowout. Heat, boost and high compression can easily cause spark blowout.

The filter is definitely a must for any 2.5t. the power to redline is much much better.

K&n 33-3024

K&N Engine Air Filter: High Performance, Premium, Washable, Replacement Filter: Fits 2012-2019 Mazda L4 (CX-5, CX-9, 6, 2, Atenza, Axela, 33-3024

https://www.amazon.com/dp/ (commissions earned)
 
Slight rambling/rant ahead.

Actually, thicker filter is not necessarily a bad thing.
Deeper pleats = more surface area = lower face velocity = lower restriction.
Also, more surface area = more capacity.

I'd be curious to see performance test comparison.

One issue with the K&N media is the low filtering efficiency.
Factory or parts store replacement standard type media are in the 99.9% efficiency range.
K&N is in the 96.9% range over the life of the filter. Lowest efficiency is when they are new/clean.
While that's only 3% difference, you have to look at it the other way in that the K&N lets 30x more debris into the engine.

The K&N typically has lower initial restriction, but plugs up quicker, so you have to clean it more....which drops the efficiency down again.

After learning what I have in the filtration industry, I'll stick with OE type replacement filters for my daily driver vehicles mainly due to the extra engine protection.
 
Slight rambling/rant ahead.

Actually, thicker filter is not necessarily a bad thing.
Deeper pleats = more surface area = lower face velocity = lower restriction.
Also, more surface area = more capacity.

I'd be curious to see performance test comparison.

One issue with the K&N media is the low filtering efficiency.
Factory or parts store replacement standard type media are in the 99.9% efficiency range.
K&N is in the 96.9% range over the life of the filter. Lowest efficiency is when they are new/clean.
While that's only 3% difference, you have to look at it the other way in that the K&N lets 30x more debris into the engine.

The K&N typically has lower initial restriction, but plugs up quicker, so you have to clean it more....which drops the efficiency down again.

After learning what I have in the filtration industry, I'll stick with OE type replacement filters for my daily driver vehicles mainly due to the extra engine protection.


Jason from Engineering Explained, a YouTube channel, tested this with a Subaru Crosstrek. He compared a used OEM air filter, a brand new OEM air filter, an aftermarket OEM-replacement air filter, and a K&N panel filter. The K&N filter does make the most power and it does make the car faster, but not by much.

More detailed, independent ISO 5011 testing using an expensive machine and engine air filters for a GM Duramax Diesel can be found here.


When going with an aftermarket filter, generally you're trading filtering efficiency for a very slight performance increase. Where you live and how you drive should influence the decision you make. With regard to the K&N filter, if you want it to flow as well as it does when new (which means that it doesn't filter as well as when it's "dirty"), you have to clean and oil it regularly. Otherwise it will gunk up faster than an OEM filter and you'll end up with a more restrictive filter than the OEM one.
 
And...the resistance in any filter element only comes into play at wide open throttle and high rpms. At less than wide open throttle the throttle is doing its job of limiting air flow, so resistance in the filter and other parts of the induction system are moot. The resistance in the system varies approximately as the square of the velocity of the air flow. Low rpms, with low velocity air flow, will not show much resistance no matter how restrictive a point in the induction system is.
 
I have that link bookmarked and usually include it in these types of replies. We is ISO5011 testing for most of the tests at my company.

I agree, depending on your application and location, K&N could be just fine. Only paved roads in developed cities for example. I bet most people wouldn't actually experience any engine issues with one (paved roads in cities).

And I can attest to the clogged filter may not be noticed under part throttle. My motorcycle got to the point where the bike wouldn't run at all. It was odd because it ran fine on a Friday, but wouldn't start the next Monday. I didn't notice any performance issue until it wouldn't run. Of course, when I put a new filter in, it had to pick up 30hp at WOT. I forgot it could lift the front tire winding out 1st gear.....it was a bit of a surprise to say the least.
 
Slight rambling/rant ahead.

Actually, thicker filter is not necessarily a bad thing.
Deeper pleats = more surface area = lower face velocity = lower restriction.
Also, more surface area = more capacity.

I'd be curious to see performance test comparison.

One issue with the K&N media is the low filtering efficiency.
Factory or parts store replacement standard type media are in the 99.9% efficiency range.
K&N is in the 96.9% range over the life of the filter. Lowest efficiency is when they are new/clean.
While that's only 3% difference, you have to look at it the other way in that the K&N lets 30x more debris into the engine.

The K&N typically has lower initial restriction, but plugs up quicker, so you have to clean it more....which drops the efficiency down again.

After learning what I have in the filtration industry, I'll stick with OE type replacement filters for my daily driver vehicles mainly due to the extra engine protection.
Odraphnew is correct. The best way to test a filter/restriction is with a magnahelic? (very precise) vacuum gauge. But in general a production engine will never tax a stock filter. Don`t be fooled by a K&N display, it`s there to convince you your stock filter is poor with skewed data etc.

As for vacuum etc, a stock engine will pull a vacuum through the entire intake system, but it is so small in reality that a stock filter (even a very dirty one) is unnoticed by the engine.
 
And not just K&N. The same applies for every "performance" filter. The choice is yours. A $38 Mazda air filter, or a $32 WIX or equal top after market brand, or a $65 K&N with questionable filtration. (I'm not sure I'd get the $15 Fram. I don't think so.)
 
I used K&N filters in my Hemi Charger and V-6 Chrysler 300 and they improved performance and gas mileage. However, I just purchase a 2023 CX-5 with the turbo and living in Florida which has nothing but sand, on and off paved roads, I don't think I'll take a chance of getting sand in the turbo. Not worth it IMO.
 
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