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

[racermp]

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)

 

[sm1ke]

Thanks for the review! I posted a link to it in our Resources section.

 

[Odrapnew]

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.

 

[sm1ke]

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.

 

[PTguy]

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.

 

[Odrapnew]

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.

 

[Phillshz]

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.

 

[theblooms]

After watching how much a K&N "filter" doesn't actually filter, yeah, nope.

Which Car Air Filter is Best? Let's find out! Fram, K&N, Wix, Purolator, & AC Delco showdown

Best car air filter showdown: Fram Extra Guard, K&N Performance, Purolator, Wix, AC Delco, and EP Auto. Performance testing: filtering particles 5 microns an...

youtu.be

 

[PTguy]

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.)

 

[N7turbo]

I like AEM dry flow filters and think their design is a cut above.

 

[John B]

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.

 

[AL Cx5]

I added a "filter minder", air filter box differential vacuum gauge to our 24 CX 5 turbo. At WOT the turbo pulls -15" WC on the filter box, according to this vacuum gauge.

Under normal driving the gauge rarely moved.

I bought a spare lower part of the air filter box. This is where the dust laden air enters.

First I added a 2.5" inlet. The low end torque went away. It was a DOG at the low end and my FE suffered. At higher RPM the vacuum gauge didn't move.

Next, I made an inexpensive variable flow CAI device and attached to the 2.5" inlet stub. Low torque was restored. WOT the filter box did not move the vacuum gauge. The spring loaded damper does not start to open until the air filter box is under 0.04 to 0.08" WC. That is not much vacuum. I like that it only allows enough air in to maintain 0.04 to 0.08"wc vacuum. Or variable flow.

I ran some WOT tests and a couple 0 to 60 MPH tests with and without the variable flow CAI. Of course letting the engine breath made more power. It made about 10 HP at peak HP and moved the peak rpm higher. It gained the most power in lower, mid range, from 2500 to 3500 RPM. Right in the sweet spot. I have about $70 in parts I ordered from Amazon.

Sprinkled through out this thread is a progression of various tests and test results using Mazda Edit to generate HP and Torque curves. I used Torque Pro connected to the ECU to generate 0 to 60 tests.

Here is a teaser, much more info in the link below...

https://mazdas247.com/forum/t/2024-cx-5-turbo-air-filter-box-test-yikes.123881544/page-2

 

[N7turbo]

^ another option might to be meet half-way and add a chamber. It would work as an air reservoir, so when you stomp the throttle wide open at low speeds, the engine has something to work with until you get going fast enough to fill the air intake runners and the engine won't run out of breath.

For me, I'm just going to focus on smooth piping with minimal bends, with a good flowing dry filter. I feel velocity of air has to be hampered by the stock air box.

 

[AL Cx5]

^ another option might to be meet half-way and add a chamber. It would work as an air reservoir, so when you stomp the throttle wide open at low speeds, the engine has something to work with until you get going fast enough to fill the air intake runners and the engine won't run out of breath.

For me, I'm just going to focus on smooth piping with minimal bends, with a good flowing dry filter. I feel velocity of air has to be hampered by the stock air box.

You have described the above varriable flow CAI.

A varriable flow maintains low end torque, protects the engine from ingesting water, superior filtration over CAI filters. It meters in air starting when the stock air box starts to restrict air. It starts to open at as little as 0.04" wc. And it opens quickly if one were to "stomp it"! You need to try one to see what I mean.

My tests showed the above mod never allowed the air box to go negative using a delta P gauge mounted on the clean air side of the filter. This means it provided all the air the engine asked for.

It's inexpensive and easy to fabricate. Cost $70.

It made the same peak HP and more mid range HP as any of the avaible CAIs AND maintained low end torque.

I'm all for innovation, I look forward to a better solution that doesnt loose low end torque, higher peak HP and protects the engine as well better than the varriable flow CAI.

I will be the first to congratulate this innovation.

Please provide tangible test results.

 

[PTguy]

??? Never go negative? It has to go negative for any air to flow. Let's call atmospheric pressure zero psig...zero gauge pressure. Air will only flow downhill unless it is pumped, i.e., flow from a higher pressure area to a lower pressure area. If the higher pressure area is at zero, the place it flows to must be below zero, must be negative. And the outlet side of the air filter must be lower pressure than the inlet side...or the air isn't flowing.

Air flow tuning is a complex project. The speed of air flow, the length and diameter of the passages, all play a part. Add the variability of a turbocharger which speed (pumping capacity) depends on the volume and heat of the exhaust gases and things get more interesting. (Recalling that a turbocharger is a centrifugal air compressor attached to a gas turbine.)

 

[Odrapnew]

I'm sure it does go negative, but the gauge might not be sensitive enough to measure.

Also, the position of the gauge is important. Putting it in a 'dead space' area of the intake will give false readings.

Ideally the gauge would be at the turbo inlet as that would be the furthest from all the changes being made at the very start of the intake tract.

 

[AL Cx5]

??? Never go negative? It has to go negative for any air to flow. Let's call atmospheric pressure zero psig...zero gauge pressure. Air will only flow downhill unless it is pumped, i.e., flow from a higher pressure area to a lower pressure area. If the higher pressure area is at zero, the place it flows to must be below zero, must be negative. And the outlet side of the air filter must be lower pressure than the inlet side...or the air isn't flowing.

Air flow tuning is a complex project. The speed of air flow, the length and diameter of the passages, all play a part. Add the variability of a turbocharger which speed (pumping capacity) depends on the volume and heat of the exhaust gases and things get more interesting. (Recalling that a turbocharger is a centrifugal air compressor attached to a gas turbine.)

Thank you for clearifying my post.

It would have been more accurate to.have said:

The delta P gauge never shows the filter box goes negative. It does not register below -1" WC.

And the varriable flow damper only allows the DAP, dirty air plenum to go between -0.04 and -0.08" WC before openings.

I agree that inlet air to a turbo engine is a complex subject and don't pretend to know everything.

I do know my tests resulted in a nice HP increase and maintained low end torque. The dyno curves confirmed my butt dyno. 0 to 60 also confirmed an increase as well.

One could wear out a few engines testing to optimize a design. My mod is a step up from an oem and maybe others will continue to refine.

I challenge others to do this mod and post their results.

$70 for parts is a bargain...