At half the thickness (if paper pleated) they would probably have half of the surface area as the taller ones = more restriction to air flow . But as the thinner 'K&N' type are practically wide open pore they will flow more air I guess.
We all know this is an 'emotive' subject so expect more comments below
Yes totally agree it’s an emotive subject and also get why many owners would choose to stay with the standard set up and paper OEM filters. When I ran both or my previous CLS55’s I saw no benefit at the time of switching away from OEM, as the cost of these filters was at times 2 for £10, so was happy to remove them every 6 months and replace.
I’ve tried to answer both replies (ALFAitalia also) below so as not to have two separate posts.
It can be an emotive subject through miss information and in many cases interpretation. That’s for sure. I am very comfortable that switching out to the filter I have done will be of no detriment to the engine or the way it runs. This is based on quite a lot of in depth experience with the M157 and also the M113k. I also helped develop a full induction system for another brand of car nearly 20 years ago and this included designing a new filter.
Paper filters have to have more surface area compared to filters of another media, simply because they have a greater PPI (pores per inch).
Many “cotton” type gauze filters (although not always actual cotton), still have a pleated surface area, albeit not as much as paper due to having a lower PPI.
You then have Foam Filters. These are mainly not pleated and have a flat surface area, this offers the lowest surface area, but they also tend to have the lowest PPI rating, thus allowing slightly larger micro particles through the media.
The theory is quite straight forward that the more restrictive the media is to allow air to flow through it, the more surface area you need to match the flow rate of a media, which allows more air to flow through per equal area.
Oil is often added to foam filters to offset the higher porosity of the media. This can also in some cases have a negative impact as it clogs and can transfer oil to certain sensors etc. Oil can be used on some of the cotton type filters.
Does switching away from OEM paper filters offer a performance gain on a standard map? Marginal I’d say. Does it increase the risk of micro particles getting into the engine then theoretically yes. More so on foam filters that require oil and are not oiled once cleaned. However, are these particles of detriment to the internals of an engine? Then this is contentious as depending on each application it could vary. Certainly in the world of M157 engined cars I have not read about engine failure due to long term use of K&N, Pipercross, Green Cotton, Weistec, BMC etc. There have been issues with fitment of the odd aftermarket filter but this is a different issue, where the rubber structure around the filter has disintegrated, this can happen on any type of filter.
With regard to the increase in volumetric area inside the air box created as a result of fitting a shallower type filter, then in my opinion it offers theoretical increased flow due to less restriction. This is as standard or as mapped. Again the improvement is negligible and for many owners not necessary, but as the ECU will adapt as it detects increased airflow, it will not result in a negative impact or running issues. This was proven when I did a lot of upgrade work on my last CLS. Plus is why many of the aftermarket Tuners offer larger capacity intake systems for the M157. Like I said in my initial post, any gains just by adding a shallower filter on a standard engine (Ceteris Parabis - all
Other things being equal) are absolutely marginal. The benefit for me is the ability to flow more air with less restriction, which the ECU compensates for and in my opinion has to be a benefit.
Owners can choose whatever they like and that it absolutely fine.
For me I’m will run the filters for a while and see how they perform. I’ll check and see how clean they are after a couple of months. The OEM ones were quite dirty and have only been installed for 1500 miles.
Jules
