mickster

In the stir the $*** up again column, just switched out the K&N filter that's came with my car for a new Mahle I won a few weeks back at a GTG.

Wow-what a noticeable difference. Besides a better fit, and it being new and clean, the car idles a little lower and much, much smoother. I would assume the LH remapped itself down the line (drove about 30 minutes).

Sounds a little better as well.

dprantl

If you want to compare something, you need to do it with new to new. Comparing a K&N filter of unknown age to a brand new Mahle filter is pointless.

Dan
'91 928GT S/C 475hp/460lb.ft

mickster

Good point. Still amazed at how it settled.

tv

Everytime you drive, when you shut down the engine, the MAF wire heats itself up to a very high temp to clean itself. The theory about these filters has been the oil in them, see here http://www.knfilters.com/filtercharger.htm.

Mrmerlin

not to mention that the knn filters dont really offer a good seal like the factory filters do,
thus hot air and dirt get into the engine with the knn filters
Stay Factory its just a better filter

mickster

I believe part of it is the rounded top of the Mahle/Porsche design vs. flat k&n. certainly changes the airflow path and I'm theorizing would minimize the disruption of air above the MAF from 2 sides flowing into each other and makes a wide "V" of airflow into the bottom of the box and into the MAF. That certainly was not designed that way by accident!

GlenL

Good science!

This is why there are reports that going to a K&N was a big improvement. I'm sure it would be compared to a 5-year old filter of any kind.

On the topic of good and bad science:

I recently rebuilt the engine in my '73 Riviera. It's ovebored, higher CR pistons, better heads, bigger cam, better intake manifold. When gunning it, it pulls much harder. Could be 100 more ponies under the hood. But! My visceral reaction is that it's "not as powerful" as I fixed some noisy exhaust issues so the car is much quieter. How many guys have sworn an exhaust mod, like RMB, made their car faster? It's that emotional response in the absence of any hard data.

Bill Ball

Right - Louder always seems faster.

It could be bad science, but I'm just terribly suspicious of a filter that has almost no surface area. For some vehicles, the K&N may match up well to the factory filter, but in our case, the surface area is markedly less. It would have to be some form of magic for a filter with such small surface area to provide as much or more airflow while still providing as complete filtration. I think the K&N fails on the latter. A while back there was a BMW club publication that quoted some testing done by Jim Conforti during development of an intake he was manufacturing that found considerably more grit got past the K&N. Hard to find this any more after a lot of back and forth with K&N. But in general, it's a decent assumption that for any given surface area, filtration ability is inversely related to flow. And with reduced surface area, like the K&N has, maintaining the same filtration and flow is not going to happen.

Note: this pic is just for visual comparison of the pleats and surface area. The stock filter is old and dirty.

chrisjbell

I have used K&N a few times, even tried a knockoff once. The last time was when I decided the stock filter in my truck was too expensive... The K&N would pay for itself within 2 years. I'm religious about filter and fluid changes and when doing maintenance I try to be observant. I noticed the K&N was leaving dirt on the sides of my intake so I tried cleaning, using less oil, using more oil with little difference except with more oil the dirt got greasy. Went back to stock. Again.

Niles

Regarding the shape of the stock air filter, it appears to me that it causes a better distribution of air through the filter. I had previously installed my filter upside down (by mistake), you could see that the dirt was all concentrated over the MAF indicating that all of the air was flowing in through that small part of the filter. This would cause a restriction in airflow and a tendency to run lean.

Jfrahm

Or maybe most of the dirt and air tends to take the path of least resistance, which starts out being right above the MAF and then over time more air is drawn from farther away.

Mrmerlin

from the design standpoint I would say that they the factory took time to figure out a few things.
The big thing is how the filter has a hump this does a few things,
It increases the airbox area under the filter this adds some stability to the air being used.
Next the stepped design also will have more of the incoming dirt being grabbed by the edges of the filter leaving the center most section to have the least dirt.

Try tapping the used factory filter on a piece of white cardboard,
you will see that more dirt ( that is bigger particles) comes from the edges of the filter than the from the middle.

mickster

I had to take off the airbox cover to clean up something after the Mahle install. True to what you say-there were small leaves on the left and right side tops of the filter.

Niles

Joel, you are correct. When the filter is installed upside down (as in my mistake), the path of least resistance is at the MAF. The filter is almost in contact and may be in contact with the MAF. When installed correctly the air flow is more evenly distributed. If you look at the shape of the filter and the filter box, it is obvious the air path becomes more restricted toward the center of the filter plus there is a larger gap between the bottom of the filter and the top of the MAF. I think it is a great design giving a large area of filtration and as a result increased air flow.

GregBBRD

K&N has pretty good marketing.

They do not work well, for our application.

You can't get them out of the vehicles fast enough, in my humble opinion.