I checked my air filter for the first time and found it pretty dirty. I guess they didn't bother checking it when it was last serviced. OK, it is somewhat of a pain getting to it but not that bad. Any opinions on the best filter to use? I am tempted to get a K&N but when someone boasts about more air flow, it makes me wonder how much filtering is going on and how often to they need to be cleaned? Are the inexpensive filters like Fram worth it or inferior to the needs of the engine?

 

From a performance point of view, there's no difference from one brand to another. This includes the K&N piece.

There's been some reports of incorrectly sized (too large) filters being listed in parts books. Maybe try a search on this forum for the recommended part numbers.

 

There is no shortage of opinions on the subject! The most useful information I could find was here:

K&N Air Filter Review - Debunking the Myths (and why OEM is better)

According to this study, OEM paper filters trap more dust but are not quite as efficient in airflow. However, for an S-type filter, the airflow difference is of no real consequence up to about 400 cubic feet/minute (CFM), and most day-to-day S-type engine usage is within this range. I did a rough calculation as follows:

At 5000 rpm a 4.2 liter n/a engine uses (5000x4.2) = 21,000 liters of air/fuel mixture per minute.
With a fuel/air ratio of (say) 13:1, the air in the mixture is (21000x13/14) = 19,500 liters/minute, which is 722 CFM.

A turbo or supercharged engine would obviously use more than 722 CFM at WOT. At 11-12 psi, even after IAT2 temp corrections, might be as high as 1000 CFM.

That supports the above study that says OEM filters are superior most of the time, and give better air filtration all of the time. However, K&N may give a small HP gain (not exceeding 5 HP) at WOT or higher rpm's.

Without being aware of the above at the time, I put a K&N in my STR 2 years ago. No regrets.

 

Thanks Robinb, The attached article is good and reinforced my normal cautions about marketing.

 

Jaguar part numbers FYI:


Up to M45254
XR88237
From M45255
AJ82766

 

Not correct. A four stroke engine takes in air every second revolution, so the calc would be (5000 x 4.2 /2)= 10,500 l/min.

No engine has 100% volumetric efficiency, including supercharged engines at off peak RPM- typically it's 80% or so for a modern design like our engines, so call it 8,400 l/min.

8,400 l/min = 296 cu/ft min.

Air/fuel ratio is done using the weight of each component, not volume. It's also not relevant to air filter capacity as the fuel is injected downstream. Your example above infers that the engine is consuming 1,500 l/min of fuel.

 

Thanks Mikey, you are correct about 10500 liters/minute of mixture, and that the A/F ratio is by weight. And you are probably right about volumetric efficiency, so will go with 8400 liters/min of mixture at 5000 rpm.

But of that 8400 liters/min about 8000 liters/min (296 cu.ft/min) will be air at normal atmospheric pressure, and that air must be drawn in thru the filter. So, for an n/a engine, 296 cu.ft/min is easily handled by an OEM paper filter with a design capacity of 350-400 CFM.

But a supercharger operating at 12 psi will almost double that pressure, making the air requirement nearer 600 cu.ft/min. That validates the conclusion of a small power gain for turbo or supercharged engines if a K&N filter is used.

 

A supercharger does indeed increase the pressure in the combustion chamber by 'pre-compressing' the air, but they don't also significantly increase the flow-through or volumetric efficiency of an engine. A non-supercharged engine might max out at 80% as mentioned above, and a supercharged version might get close to 100% peak. It's essentially slightly more air compressed to a higher degree, not a lot more air also compressed to a higher degree.

The -R Jag enginess are quite conservative in this respect and I doubt achieve 100% V/A.

For sake of argument they may flow 370 cu/ft min at max flow (engine red line).

 

Nice article, and for the NA cars the difference in power is neglectable imo.

Going to the SC engine. it does start to add up though, when looking at the last graph, the difference of almost 2" of restriction is really considerable on the supercharger cars, so that proves the extra power you will get (pending to which paper filter you can compare it to of course). Even more so if they are tuned (i.e. with pulleys or so), and one still uses the stock airbox.

The vacuum you pull is directly affecting the power consumption of the supercharger (so more vacuum = more power consumed = less power to the wheels), so that is on top of the lower volume of air due to a restriction in an airfilter.

Edit: If you use a standard power correction like DIN or SAE, than just 1,5" inch of mercury lower atmosphere pressure already makes a difference of 20 hp on the 4.2 SC car, just to illustrate...


I don't mind processing a little more dirt, and I don't have to clean them often, maybe 20 or 30 k or so, but I don't drive in the desert either.

But even then for a SC car, If your not power minded and don't enjoy maintenance (cleaning K&N filters does take a little time), then better to stick with oem.

 

Based on that article, you gotta hand it to K&N's marketing people! Many people, including myself, always thought they were one of the top brands available.

I've been using K&N, but my primary reason for it for the jag was because it fits, unless some other brands I tried before.

 

the restriction isn't really in the element. it is in the piping. both before and after the maf.

autospeed.com is a very large Australian e-mag type site run by one guy. he has written a lot of articles and done lots of experiments in this exact area.

with obd-ii you can get some interesting data points, those being pounds of air per second with and without an air filter installed.

 

I do not agree. Let's use your argument...

Even at 90% volumetric efficiency, 296 cu.ft/min becomes 333 cu.ft./min and at redline (say 5500 rpm) that becomes 333x5500/5000 = 366 CFM.
Boyle's Law states that, at any fixed temperature, (pressure x volume) is always constant. For an n/a engine, intake pressure is atmospheric, or 14.7 psi. With a supercharger running at 12 psi, the intake air pressure is (14.7+12) = 26.7 psi.

The IAT2 is about the same in both cases, so the volume of air at atmospheric pressure required by a s/c engine running at 26.7 psi would be (366x26.7/14.7) = 665 CFM.

When Avos' twin-screw S/C becomes available, running at maybe 20 psi, the required volume of air at atmospheric pressure will be 864 CFM.

 

Robin-

Compression ratio of an engine has nothing to do with volumetric efficiency or flow of an engine. Irrespective if an engine has 3:1, 8:1 or 15:1 compression, the same amount of air will flow though it.

 

Who said anything about compression ratio? I thought we were discussing pressures in the inlet manifold.

 

I thought we were talking about airflow. If you're attempting to calculate pressure downstream of the supercharger to determine flow numbers, take into account the heat added by compressing the air, factoring the inherent inefficiency of the blower itself (approx. 60%). Outlet temps are generally in the range of 250-270*F at 12 psi boost.

 

OK, why don't we re-calculate the airflow using an iat downstream of the S/C of 260 F at 12 psi boost.

According to Boyle's Law, 1000 ml of gas at atmospheric pressure (14.7 psi) and 73 F would, if heated to 260 F and pressure-boosted to 26.7 psi, occupy a volume of 734 ml.

Thus, the original figure of 665 CFM would now become 488 CFM. And that's for a basic STR, no extra pulleys or other performance mods. I understand the Kenne Belle S/C is more efficient than the Eaton, so an STR with a twin S/C would maybe flow over 600 CFM. Avos will know. in any event, both figures exceed the rated 350-400 CFM of the OEM paper filter.

So, even after your helpful correction and advice, I cannot agree with your original blanket statement:
"From a performance point of view, there's no difference from one brand to another. This includes the K&N".

 

Someone in all seriousness estimated the K&N maybe adds 1HP for the STR. I think no-one has managed to measure any gain at all from it. But there'll be a psychological gain of at least 10HP for those who believe it's better...

 

I like the K&N because I can clean it and will own it for the life of the car. Unfortunately, I did the airflap delete along with installing the K&N, so its hard to correlate which was better. From the super scientific seat of the pants feel, I would say <5hp while doing both, but the sound is worth it beyond the power.

 

Nothing more accurate than a butt dyno in some people's minds.

There's a post somewhere in the archives demonstrating that the stock S-type set up leaves 1-2 HP 'on the table'. Pretty hard to squeeze out 10 HP under those circumstances.

 

Agree. When I had my S-Type, I asked a local indy Jag parts shop about the K & N filter gains and I got the same response...1 maybe 2 hp gain.