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MG MGB Technical - Air filter change to K&N

Hello all,

I am interested in changing over the standard air filter setup on my B and have heard some good reviews about what K&N can provide. Has anyone had any experiences that they would like to share in this regard?

I am interested in keeping the same larger size filters rather than going for the flat pancake style units.

The reasoning behind the planned swap over is to increase the breathability of the engine, I would potentially be looking to modify the exhaust side of things also at some stage.

C Brennan

This was done on my V8 because the exhaust manifolds had to be changed for tubular type. Both changes meant the engine breathed better which mucked up the mixture through the range. Adjusted to pass the UK MOT at idle there was a massive flat-spot at mid-throttle which I had to pull the choke to get past. Richened to get over that was far to rich at idle. I fiddled about with needles and got some that were relatively richer through the range than the originals, which certainly improved matters, but I still need to run it a bit richer at idle which means I have to give them a tweak at MOT time then tweak them back again.
Paul Hunt

Are you planning on retaining the Cooper cans along with the new K&Ns? Some have abandoned them due to the fact that the tubes on the cans contribute to reduced flow, myself included. When I first did this, I stuck with the original filters, but created what has become known as "Bob filters" - check the archives, but basically it involves creating a piece for each filter to seal off the ends using the original retaining bolts. I found that by simply removing the cans, the mixture went weak in the mid to higher revs. I switched to AAA needles after reading info in this forum, and the car has run great ever since.
Some feel that the presence of the original retaining bolts produces a disturbance in flow into the carb, so K&N filter no. RU-4410 can be used to eliminate these bolts. What you have to do to use them is to drill out the holes in the original stub stacks and use short bolts to hold them to the carbs. These stub stacks are quite good at improving flow of air to the carbs, do not remove these from the system, whichever way you choose to go! Once you have them bolted to the carbs, the K&N RU-4410 filters clamp around the flange of the stacks, so that there is no disturbance to air flow into the engine. I feel it is a very nice system. I didn't need to change needles again after going to the K&Ns, so it would seem to me the biggest gains came from getting rid of the cans. By getting rid of them, you will likely notice more intake noise. I find its not a big deal around town, in fact I like the sound. However, for longer trips, I have actually switched back to the original system to keep the noise level down, as on one trip I found the excess noise bothersome.
Erick Vesterback

A pair of 5 7/8” Diameter x 3 ¼” deep K&N airfilters (K&N Part # E-3190) will permit increased flow of the fuel / air charge without sacrificing protection. With proper fuel jet adjustment and a pair of SU AAA fuel-metering needles, when installed on an Original Equipment specification engine these larger airfilters are worth about 3 HP on their own. When attempting to build a deeper-breathing engine, they are a prerequisite. A K&N airfilter is made of 4 to 6 layers of cotton gauze sandwiched between two epoxy-coated aluminum wire screens. The cotton is treated with a specially formulated grade of oil causing tackiness throughout the cotton’s microscopic strands. The nature of the cotton allows high volumes of airflow, and when combined with the tackiness of the oil creates an efficient filtering media that ensures engine protection. In contrast, most other filtering media cannot maintain the same balance of airflow and filtration throughout the airfilter's life without sacrificing either one or the other. K&N airfilters have the additional advantage of being almost infinitely reusable so long as reasonable care is taken when cleaning them. K&N makes a cleaning kit for this specific purpose that does an excellent job. Once cleaned, they should be allowed to air dry. Because the filtering medium is made of cotton, you should not use a hair drier to speed up the drying process. If you make this mistake, the filtering medium will shrink.

In the smaller Original Equipment size, these reusable cotton element airfilters have an airflow capacity of 6.5 Cubic Feet per Minute while some airfiltering elements that are made with paper have an airflow capacity as little as 3.2 Cubic Feet per Minute. However, switching from the Original Equipment airfilter elements to the same size K&N airfilter elements (K&N Part # E-2400) will have no effect if you retain the use of any of the variations of the Original Equipment airfilter housings. The purpose of their inlet tubes on the airfilter housings is first to accelerate the airflow beyond supersonic levels so that intake noise is literally sucked into the engine. This occurs because the air goes in faster than the sound goes out (sound only travels in air). This is a big issue since the advent of noise regulations, and it is illegal under US Federal law to alter or modify the air intakes on modern vehicles for this reason. In the case of the airfilter housing with offset air inlet tubes for the 1½” SU HIF4 Series carburetors, their secondary purpose is to cause large particles of contaminants to be centrifuged out of the airflow and fall to the bottom of the airfilter housings. That leaves the airfilter element clean to filter out finer particles, plus the airflow slows with a small pressure drop, thus allowing the airfilter element to flow well and clean the air adequately. While successive versions of this basic airfilter housing became progressively quieter, they also became progressively more restrictive. The initial version (Front, BMC Part # AHH 7354; Rear, BMC Part # BHH 154) were originally intended for use with 1½” SU HS4 Series carburetors and are characterized by their large, straight inlet tubes that direct the incoming air directly at the airfilter, while those of the later version that was intended for the UK/European market with the 1½” SU HIF4 Series were characterized by offset inlet tubes being curved downwards on their ends (Front, BMC Part # 548; Rear, BMC Part # 549), while the North American Market counterpart for these were similar (Front, BMC Part # 665; Rear, BMC Part # 666), but more restrictive. In all cases, including Non-Original Equipment items, the base plates of the airfilter housings have holes that align with those of the gaskets that fit between the airfilter housings and the carburetors. Make sure that these holes are properly aligned, otherwise the vacuum pistons inside of the vacuum chambers (dashpots) of the carburetors will not assume their appropriate positions relative to the jets in order to cause the fuel-metering needles to meter fuel appropriately.
Steve S.


Have you got a typo or the wrong units on those filter flow specs? It seems awfully low to me.

C R Huff


Thanks very much for the fantastic coments, very informative! I will certanly be retaining the stub stacks as I have heard only positive comments about them. At this stage I am looking at K&N filters with the same size specs as original coopers filters however utilising a front plate available in the Moss catalogue that esentially leaves the filter elements completely open around the entire surface area.

Appreciate your help guys, thanks!
C Brennan

If you switch over K&N filters you may well need to change the needles (AAA) is a good starting point..
K Harris

This is less technical, and more subjective, but:

I found the K&Ns to be fairly awful on my tuned 1850cc B. I was having all sorts of trouble getting it to run right, then I fitted 2" trumpets along with Pipercross 'socks'.

This was a much cheaper option, and the difference was unbelievable: the engine revved far more freely. It also sounded pretty good.

I got mine from Burton, but they're widely available.
D Ainsworth


What needles with the socks? We usually find socks are very restrictive and tend to make an engine run rich, which, if you had weak needles would make it 'better' with socks. We usually gain bhp removing the socks, especially with Webers.

peter burgess

Hi Peter,

I can't remember exactly which needles they were, but they were certainly a fair bit richer than standard. The carbs were standard HIFs, but with plain butterflies.

The trumpets had a very wide throat, I don't know how much difference that made, but curiously, it didn't seem to make much difference if the socks were removed!
D Ainsworth

I have the large size K&Ns open with the aluminium backplates as sold by many companies. In my case, I was replacing aweful wire gauze pancake filters with no curved baseplates so the difference was unsurprisingly big! I had to change from number 5 to number 6 needles (slightly richer - AUD 135 carbs). Points to note are that although many claim the filters don't need cleaning for interstellar mileages I found a simply unbelievable difference when I did mine after about 20k miles. I used a proper K&N cleaning kit which, as previously stated, does a very good job quite easily. The other note is that mine are very difficult to tighten up enough to stop the nuts vibrating loose whilst avoiding crushing the filters. I plan to put rigid tubing around the through bolts inside the filters so I can tighten the bolt up against the tube instead of the filter - my kit was secondhand so I don't know if such tubing is normally standard or not but it's definitely needed.

T Jenner

It sounds like most are using the K&N E-2400's ( which are a direct replacement for the stock filters. It surprises me that more aren't using the RU-4410's ( Is there a disadvantage to these that I am not aware of? It would seem to me that the 4410's would be better, because of the fact that the filter fits on via a clamp around the stub stack as opposed to bolts going through the filter cavity (thereby interupting air flow and also possibly crushing the filter as Tim mentions without tubes to tighten up against). The only disadvantage I can think of is that they require drilling out the stub stacks to counter sink the heads of the (shorter) attachment bolts. Also, being that the 4410 is tapered, maybe there is a little less airflow? Anyone with experience each way that could comment?
Erick Vesterback

This thread was discussed between 21/11/2008 and 28/11/2008

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