Jimbo

Hi,
I was thinking of getting a K&N filter to replace the stock one. I have a '99 bmw 323i.
I heard that the filter might let in more hot air and the performance would be degraded.
I also heard that the only way to get more performance from a filter change is to add the cold air box.

Any thoughts or opinions would be appreciated.
Jimbo

wrong forum / website!

uh, this is an A4 forum, talking about a4s, not about K&N air filters for 323i - try www.bimmer.org

Cary B

Jimbo,
If you are going to buy one of those K+N cone filters, do yourself a favor and get some sort of cold air induction kit to go along with it. Otherwise you are going to get noise upon excelleration but not to much Hp. Consider a panel filter as another option.

Cary B.
99 1.8T

MikeH

How to build Horsepower, Vol 2: Carburetors and Intake Manifolds by Dave Vizard.

It has an entire chapter on air cleaners. His suggestion is that K&N is better than anything else, but the effect of simply changing the air cleaner element is unpredictable. It can even REDUCE power in certain circumstances, he says, if the fuel system is expecting some pressure drop across the filter. (this probably applies more to carburetors than fuel injection, though)

If power is your concern, cold air induction and improved airflow is the real ticket. It probably makes sense to upgrade to K&N as part of an induction system upgrade, but not as the only step.

On the other hand, according to Vizard, a good enough reason to change to K&N is that it cleans air better than conventional filters.

patrik

I have difficulties believing that an air filter that is less restrictive to the flow (lower pressure drop) actually can clean the air better!

usually, the pressure drop accross a filter is dirctly related to the porosity of the same. This means that if a filter is very efficient, it has smaller "holes" to NOT let dirt/dust through. This means, on the other hand, that an air filter with less pressure drop actually has larger pores and so allows partickels to pass.

Conclusion: If a K & N filter helps the performance, it's gotta filter worse that the stock filter.

Anyone agreeing or disagreeing??

Patrik, 1.8 TQMS+1bar APR (K&N not installed)

RTM

I can think of at least one way in which a less restrictive filter could actually filter better. Just use a better filter media (smaller holes), but use a greater surface area. This could be accomplished by using more folds in the media, or deeper folds, both of which could work in the case of a paper element. (That's the reason for having folds in the first place).

I also have my doubts with the foam filters, and I don't have any statistics on K&N, etc. (Are they really as efficient given the higher flow capability?) The sticky oil helps a lot, of course.

It seems like "everyone" assumes that the designers of the cars are idiots in that they restrict the airflow, where they could get FREE horsepower (which they pay dearly to get in other mechanical ways) with a simple filter change. (Either a bigger filter, different "performance" media, or whatever). (Gee, I think I'll redesign this engine for 5-valves per cylinder, low restriction exhaust, etc. for a cost of $200 per engine, but then lose all that horsepower by putting in a too-small, restrictive filter.) It seems to me that in any decently designed stock automobile, the stock filter or equivalent is going to be about as good as it gets. (Improvements are possible for engines where the owner makes hp-raising modifications, of course).

Francis Y.

A4.org members are often more knowledgeable, helpful, and NICER than found elsewhere.

Jimbo

Thanks for the helpful responses. I tried bimmer.org, no help. Thanks Francis Y. I agree

Rodney

Performance is not the only consideration for auto designers/manufacturers. Cost is VERY important, the K&N filter costs about 10 times as much as the stock filter...that's probably the biggest reason that it's not used. Also power gains from a filter are pretty small, if they are noticeable it's usually in the form of a more responsive throttle, hence little return for a large investment (from the automakers point of view).

The K&N has been shown to flow better than stock (can't remember what car the test was done on), BUT did not clean the air as well, and its airflow degraded (as it got dirtier) more quickly than the paper filter. Once the archives come back up check them, there was a great post in the race forum about the results of flow testing a K&N filter vs. stock (not an A4, but the results should be simmilar).

Regards,
Rodney

'99 A4 1.8tqms

MartinR

A good friend of mine builds Porsche Engines as a business - and runs a Superflow Dyno. he
has tested stock filters, K&N, foam etc etc and found NO appreciable difference in horsepower due to filters. The only time they saw a difference was when they made extra holes in the airbox.