I installed a Jamex Sports airfilter in my 928s4 '88. The paper filter was at least 20.000 km old so I thought its best to replace it. The new filter definitely made a big difference! The engine feels more responsive and powerful. It picks up a lot quicker. Haven't tested high speeds yet. I can definitely recommend a good filter.

Jamex say that on their spec sheet:
Paper element: 450 cfm
Paper element after 30k km: 400 cfm
Cotton element: 600 cfm
Cotton element after 30k km: 500 cfm
Jamex element 850 cfm
Jamex element after 30k km: 700 cfm




One has to be careful when reading all these claims on air filter flow rates. The "rated capacity," as listed below, is the flow rate at a given flow rate. Unfortunately, there is no universal standard as to the pressure drop to be used - SAE standard J726 leaves it to the manufacturer. Let's assume it is the same for all the filters tested - the pressure drop used is somewhere in the range of 10 inches of water. Whether the flow through the element is turbulent or laminar is unknown without detailed data, but for the benefit of the performance enthusiast let's say it is laminar. That makes the pressure drop directly proportional to air flow. Also for the purposes here let's say that the engine flows 450 CFM at max power. The pressure drops of the various filters described previously would then be approximately:

Paper element: 10"
Paper element after 30k km: 11"
Cotton element: 7.5"
Cotton element after 30k km: 9"
Jamex element 5.2"
Jamex element after 30k km: 6"

I don't have the number handy, but atmospheric pressure is approximately 390 inches of water - let's say that the manifold pressure at max power is about 360 inches. The power loss at max rpm caused by the filters will then be:

Paper element: 2.7%
Paper element after 30k km: 2.9%
Cotton element: 2%
Cotton element after 30k km: 2.4%
Jamex element 1.4%
Jamex element after 30k km: 1.6%

The pressure drop at half the maximum rpm, more related to acceleration performance would be half of those numbers. Also, the flow could be more
turbulent than laminar and in that case the numbers at lower rpms would be 1/4 of those at max rpm. And then I would bet my next paycheck that for
filtration efficiency the standard paper element would be far superior to the others. I doubt if anyone could feel the difference of 1% in power. As to "responsive" and "pick-up" I can't imagine even a measurable change. I think there is virtually nothing to be gained and some engine durability to be lost in using these filters. Not another comment for a lister regarding the K&N filter: " I did notice an accumulation of fines in the intake track on the engine side of the filter." As far as the thought that a filter could be "clogged" after 20K miles, that is very, very unlikely unless the car is operated in a very dusty environment off paved roads. You can just look at the filter and if you can see light through it there is probably little restriction. Also, a standard paper filter can be blown out with compressed air or even washed with soap and water. It was a well-kept secret at Fram that virtually no filter that got replaced was even close to needing it. Except in those vehicles operating close to Mount St. Helen.
When racing one could just remove the filter, but even then the flow-straightening effect that is lost could have some effect on the MAF accuracy. I hope K&N isn't a sponsor of this list..

Gary Casey


even though what you say may all be true, it definitely feels different, and more than 1%. Its possible that the filter was older than I expected and more clogged than you and I imagined. I inspected the paper filter several times ever since I have the car, and cleaned it too. You'd have to be very careful using pressured air on the old paper filters. And I'm hesitating when you say you can wash them. I can imagine that washing may do damage on the cell structure in the paper element, and reduce its capacity very dramatically.

I doubt that my air filter still performed at a reasonable level.
Anyone out there with K&N who wants to respond?



I have done several back to back dyno runs comparing stock Porsche paper air filters to K&N filters on S4s, my GT, & 6.5L 32v engines. The 6.5L showed no
difference at all while the S4s & GT showed a loss of between 4 and 6 hp with the K&N. The Hp curve with the K&N was noticeably rougher than with the
stock Porsche filter. In other experiments I've done with air flow through the airbox, I find better power if the air flow is smooth through the box and down into the MAF. My thought is that with the coarse screen pleats of the K&N oriented at 90 deg to the direction of the air flowing in from the sides of the air box, turbulence is created which upsets the MAF as well as reducing overall mass volume into the engine. The '85/'86 air box is a little deeper than the S4/GT airbox so possibly the K&N creates less turbulence in those boxes. Others have found more power by cutting holes in the air box top which, I believe, helps insure air flowing straight down through the filter rather than in from the sides. I think the 6.5L engine had holes in the top of the air box.

Louie Ott


>Anyone out there with K&N who wants to respond?

I had one for a short while. I got rid of it after dyno testing compared to a stock filter showed a power loss with the K&N, I found a film of oil in my MAF with the K&N, and I also found more traces of fine dirt/dust after the filter with the K&N. Before anyone even says that the oil film was because I over oiled the filter, that was when the filter was brand new, and had been oiled by K&N at the factory.

One theory is that the screen of the K&N creates more turbulence in the air stream. That could explain the lower dyno numbers that have been seen by me and some others. Increased turbulence would also increase noise, which is often perceived as the car being faster.

If anyone is looking at airflow figures for different kinds of air filters, keep in mind the size of the filters tested. I've seen tests where the air flow when testing paper vs foam vs oiled gauze filters stated the results based on the flow per square inch of the different filter materials. The surface area of different filters for the same car application are not all the same though. The stock paper 928 air filter has over three times the filtering surface area as the K&N I had. In other words, even if the stock filter's paper material was three times as restrictive as the oiled gauze of the K&N, the stock filter would still flow more air.

Just my personal experience with the K&N.

Mike Schmidt
'88 928S4 Auto Black/Black "PORSCHE" cloth
PCA Chicago Region



Not sure I have read all this thread and understood it but all I can say is that K&N's seems to work OK but whether they work any better than the standard filter is another matter. I beleive Louie [Ott] feels they offer no real benefit and indeed maybe restrictive.

My own observation is that first of all a key factor is how much dust is in your environment. Paper filter elements are probably more prone to blocking hence higher pressure drop in the fouled condition.

My K&N is due a seriously good clean as I cannot get any oil. However I have noticed from the dirt stains that the majority of the air passes through the centre of the filter in a [circular] path the same size as the mouth of the MAF. This tells me that the pressure drop over the K&N filter must be minimal other wise the air would flow more evenly across the entire surface area.

If anyone washes a paper element and reuses it then that would be a rather strange idea in my opinion. Similarly, blowing a paper element with compressed air would also be dodgy [and with a K&N element]. At least with the K&N unit you can wash it with a gentle stream of water after using the
cleaner fluid [or soak it in a bowl]. It does seem to withstand our hot dusty environment with seemingly no problem and no obvious signs of dust getting past it.


Fred R


Hi Todd,

> I cant find any data about how much air volume a 16 valve 928 actually  needs to flow at wide open throttle.
> Anyone know offhand?

Here's how to calculate the CFM an engine consumes:

CFM = (cid x maxRPM x 0.50 x VE) / 1728


cid = displacement in cubic inches
maxRPM = engine's red-line
VE = Volumetric Efficiency
0.50 = 1/2 the revolutions are power strokes
1728 = conversion from cubic inches to cubic feet

So using a 4.5L engine we get:

CFM = (cid x maxRPM x 0.50 x VE) / 1728
CFM = (272ci x 6500 x 0.50 x .85) / 1728 = 434.84 CFM

And a 4.7L engine:

CFM = (284ci x 6500 x 0.50 x .85) / 1728 = 454.02 CFM

As you can see, it's a very simple calc which is easy to substitute cid, maxRPM, and VE for whatever engine combo one is working with.

> 600cfm?

Well, we now know that none of the 16v engines come close to consuming 600 CFM, so how big of an engine will it take to consume 600 CFM?

Using the above calc and solving for cid we get:

cid = (CFM x 1728) / (maxRPM x 0.50 x VE)
cid = (600 x 1728) / (6500 x 0.50 x .85) = 375ci

Convert ci to Liters:
1 ci = approx 0.0166 Liters
375 x 1ci = 375 x 0.0166L
375 ci = 6.21L

It will take a 6.2L engine to consume 600 CFM using the above parameters, which is quite a bit larger than any of the stock 16v engines.

Alright, since we're here, how many CFM does the hybrid 5.0L 16v consume?

CFM = (302ci x 6500 x 0.50 x .85) / 1728 = 482.80 CFM

513 rwhp