Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - AirFilters


I am thinking about replacing my current standard airfilters. With either K&N Pancake or K&N base plate airfilters.

Do they differ much is one better than the other?
If I do choose pancakes which depth should I choose.

I have also found speedograph richfield pancake airfilters which are a lot cheaper than the K&N ones,

Changing air filters usually upsets the mixture profile though the throttle range, often causing dead spots in throttle response even though mixture seems correct at idle. You usually need to fit a 'richer' needle (i.e. one that passes progressively more fuel as it rises from the jet than a standard needle) as well as fully rebalance the carbs for air flow and mixture balance.
Paul Hunt 2

i have used both the standard boxes and those cheaper speedograph-type filters in the past week. whilst i duly acknowledge theat these cheap pancakes will affect allsorts, and are universally considered too restrictive by people in a far better position to comment than myself, and must be used with a stub stack etc etc, i have used them with and without stub stacks, and compared them to the standard units on my BGT. other than increased induction noise, there is little to choose from, though if pushed, i would say things are slightly better with the pancakes (though dont ask me to define this in numbers). stub stacks made no noticeable difference whatsoever, and i gave them back to my friend who indluged me in my experiments. Now, this comes with a couple of caveats - the main one being that i am definitely not an expert in anything mechanical (quite the opposite in fact). the other is that my only tests were done on the raod, ie. i cannot measure small bhp changes anyway. but this makes me wonder if sometimes we can get too hung up on small theoretical bhp changes etc etc - if it runs good, is this not enough? would an inexperienced person like me even notice an extra 2 bhp - i doubt it! as a naive mg owner ( only been at it a few months), i am more obsessed with rust, correct timing,vlave clearances, clean air filters, oil, water etc etc than squeezing the last drops of power out of the car - i guess this will start to obsess me in a year or so!
so, advice from someone not qualified to give it, is try them out and see what you reckon (but if, unlike me, you can afford the k and n versions, then i reckon you should go with them. i would
m rae

I have some of the Speedograph pancake filters but have never fitted them as I was under the impression they needed stub stacks, interested to know if they really do or dont.

They didnt come with fittings either, but they were from an old closed mechanics garage, so maybe did once.

Not helpful, I know, but would like to know if they work or not

I replaced my standard ones with K&Ns sitting on the standard MG base plate and with a clear end cap I machined out of thick plastic. I can't remember the exact size but they fit perfectly over that existing base plate and are about 3 inches deep. When I do the first running in of the engine though I will use the standard filters and cans since I only have the standard carb needles. Once I get it running I will swap back to the K&Ns and see if I need to change to the richer needle too.

Simon Jansen

A pair of 6" Diameter x 3 1/4" deep K&N airfilters (APT Part# SD23-319) will permit increased airflow without sacrificing protection. In the smaller Original Equipment size these reusable cotton element filters have a flow capacity of 6.5 Cubic Feet per Minute while some filtering elements made with paper have a flow capacity as little as 3.2 Cubic Feet per Minute. With proper jet adjustment, when installed on an Original Equipment specification engine these larger aircleaners are worth about 3 HP on their own. When attempting to build a deeper-breathing engine, they are a prerequisite. The B Series engine with its Siamesed intake port design causes some very powerful shockwaves within the induction system. The volume and depth of this large filter dissipates these very effectively. Both 5 7/8" Base x 3 1/2" Top x 2" Deep cone and the 6 Diameter x 1 3/4 Deep pancake type filters reflect these shockwaves back into the induction system, causing induction pulse problems which will increasingly disrupt airflow above 3,500 RPM and cause the pistons of the carburetors to move independently of airflow, interfering with their consistency in their metering of fuel. In order for the carburetors to help the engine develop its maximum power output, the fuel/air ratio may vary no more than 6% from the ideal. For this same reason, if you should elect to install ram pipes inside an aircleaner, never use one that has a mouth that is closer to the inside of the cover than the size of the bore of the carburetor (1 1/2 SU HS/HIF 4, 1 3/4 SU HS/HIF 6). Airhorns that have walls with an exponential curve can dampen these shockwaves to a limited degree, but should not be considered to be a substitute for an air filter assembly of adequate clearance and volume.
When installing these deeper aircleaners, one thing that I might suggest would be the fitting between the carburetors and these custom aircleaners of a pair of 1 1/2" deep velocity stacks with a 7 taper and a .250" radius (APT Part# RP-HS4). These are proven to boost airflow by 5.2% by drastically reducing the contraction of the airflow at the mouth of the carburetor. As a side benefit, this reduction in contraction will help to accelerate the velocity of the fuel/air charge, maintaining fuel suspension in the airflow and enhancing volumetric efficiency at high engine speeds. If you feel that you do not desire to include this modification, at the very least install a pair of Advanced Performance Technology's stub stacks (APT Part# SS51). This additional refinement won't create a perceptible increase in power (about a 2-3 HP increase), but they will make both the throttle response and the engine running characteristics slightly smoother by reducing turbulence at the mouth of the intake tract and contraction of the airflow, thus providing more efficient fuel atomization at the jet bridge and allowing the greater flow potential of the larger airfilters to be fully exploited. They might even eventually pay for themselves by thus slightly improving fuel economy (maybe). The Original Equipment aircleaner boxes incorporate stubstacks into the airfilter housing design, so it's obvious that the engineers at the factory saw the value in them.
Steve S.

Steve S. Good seeing you back on the site, hope things are well.
Steve C.
Steve C.

Steve S. ~
Good to see you here again. I will have to get a pair of the APT velocity stacks you mention and give them a try. Thanks for helping us all out!
Bob Muenchausen

Thank you all for your advice it has given me a lot to think about & research further befor I purchase any.

I had no idea it was so complicated, I thought it was as simple as bolting on the new filters.


Hi again,
this is complex stuff for me! can someone tell me (be brutally honest as i acknowledge my ignorance!)if i have done something daft/pointless or otherwise. after reading the above, and, as i mentioned, trying out these stub stacks things with my cheapo pancakes, i tried one more experiment last weekend. I put the original baseplates as used with the big black can airfilters (coopers cans i think they are called - the big ugly bits that are original to the car)between the carbs and the pancake filters, with the filters bolted onto the plates (sealed with rubber gasket). so, in effect i have used the original mounting plates as stacks i suppose. My results were/are great (i think/hope) - engine revs freely, runs great, seems better than without baseplates - is this just my wishful imagination or could it help (i noted the nice curvy intake port on the baseplates, is this heping to smooth my air?). ran like this for 150 miles at weekend, then pulled over into layby, pulled spark plugs, which for once are showing the classic biscuit colour that my manual describes as ideal, leading me to believe that i dont need to do anything about needles/mixture/lean running issues (which perhaps indicates its all imagination).
please tell me if its all in my imagination that this set up is a/better than orignal big can things, and b/ possibly better than bolting the pancakes on with nothing between them and the carbs. if its all in my head then this will avoid me making a fool of myself yet again whilst trying to talk cars with more learned friends in the pub.
my best excuse is that i am out there having a go!
m rae

As Steve S. mentioned above, the OE cast base plate for the Cooper's filter housings, has that shallow rolled edge air horn/velocity stack and by itself it does work well for most general purpose work. As he also says, the factory would not have invested in it if they didn't know it would help the engine breathe easier. I think his point, tho, is that there are better units (as he mentions) available if you want to experiment. I use the same cast base plates on my GT as you mention in your last posting with the filters Steve mentions, have for years, and they have delivered what I consider to be good results.

Steve S. knows his stuff, and I have a feeling, has had a better opportunity than most to experiment with improvements and only posts those things he KNOWS will improve performance. I would follow his advice as it has proven to be spot on for as long as any of us has read his work. If you want to read more, you can find plenty to learn at this site:
Bob Muenchausen

What stub stack or ram pipe would you recommend for the standard OEM filter assemblies? In my case, I am looking to improve the breathing on an MGA. On the APT website, the stub stacks that Steve S. mentioned look like pretty rough castings with not much roll at the lip, but the ram pipes are nicely polished with a good roll. Will a ram pipe provide any benefit in a standard assembly, or is the mouth of the ram pipe positioned too high?

'57 MGA coupe
M. D.

Sounds like you are learning fast and having fun so good luck to you. The steps you need to include in your tests are getting the car set up perfectly before switching the filters around, then adjusting the carb to suit the new filters. The fact that your plugs look different now is telling you that you have changed the air flow through the carbs. The question in my mind is what did they look like before? If the plugs looked lean, you have restricted the airflow and in doing so corrected the mixture, but will have capped your performance (your original filters would be better), or if they were sooty, some extra airflow has balanced things and you may have got lucky, but maybe the stub stacks flowed best but pushed the mix too far the other way? Unless you check the mixture each time, you can't judge the effect of the filter. A word of caution about reading plugs with modern unleaded fuel - they don't look like they used to: forget browns, you want grey/white.
Steve Postins

Hi again Steve P,
sounds like i got lucky.....the plugs were not too bad before, but were definitely a wee bit darker/perhaps slighlty sooty prior to altering the filter set up, as i say now they are 'rich-tea' biscuit, which until i read your last thread i was happy with. but of course, what you say makes total sense (no pun intended) as regards modern fuel - i presme though that my biscuit colour is not a bad thing?.
having great fun experimenting with the car - experiments are my job, but have always been scred to fiddle with cars, as they seemed fragile, and i required them for the next day. now i have one that isnt needed on a daily basis, and my big discovery is that they are not so fragile, and that there is fun to be had in doing jobs myself. Am limiting myself to suspension and rust mainly, until i need to fix anything on the engine, as at present, its not broke, so dont feel i should fiddle too much (but having managed to set timing etc ith a strobe, one of the great mysteries of my life is over).
cheers for all the advice, invaluable.
m rae

Air filters are on our cars to protect our engines and in providing clean air to give better engine life & therefore greater efficiency. Clean air filters are as essential as clean oil & oil filters.
When I was working on my MGB (bog standard with a rebuilt, blueprinted +.020 bore) to improve racing performance, I spent some hours with the car on a dynomometer, testing under different conditions at different speeds. This produced some very interesting results on brake horsepower at the wheels (BHP). First the car was run with air filters that had done about 10,000 miles giving max power of 65 BHP at 5000 RPM. Next new filters were fitted and with adjusting the timing by changing the dist springs & mixture a figure of 79BHP at the wheels was achieved. Max advance 32 deg at 6,000. The used filters were refitted an immediate drop in power back to 72 bhp. The filters were then removed altogether which resulted in the same 72 BHP. Whilst the engine was running at 5000 I raised the bonnet to see if the carb pistons were fully up - (I have made a damper with a wire poking through the plastic cap) and saw that the area around the carbs was awash with petrol spray from spit back. We then fitted the alloy adaptor plates only which raised power to 75, then fitted the cans without the filters which gave the same result. Then the new filters were fitted again and power went back up to 79. It is generally accepted that the square corners of the carbs are only 70% efficient which is why trumpets are fitted for racing. What my testing proved was that no air filters results in a loss of power as well as wearing out your engine faster. It also showed that even though filters look reasonably clean they should be replaced at the same time as the oil filter if you want to retain maximum power. My GT is a 67 with the straight tube filter cans. The motor was originally a low comp motor but rebuilt using HC pistons. I haven't tested the K & N filters from Moss but they should give the same result as clean paper filters.
Garth Bagnall

Those results are very interesting! Since you don't mention doing so, I presume you did not adjust the mixture to optimum with each test setup?

Rob Edwards

Hi Rob, The mixture showed quite a bit richer when running without the filters as it was obviously suck ing fuel laden air. I screwed up the jets to get a better reading but it didn't help power output. The original No5 needles are over rich at full throttle so I reprofiled a pair of BV needles. If used as is they are too lean at full song & cause minor overheating but are a bit richer at the bottom which helps get rid of the flat spot when coming off closed throttle. No 21 needles are too lean in the middle range. I have made a set of blocks in 1/8 increments to allow measuring of the needle. Incidently I got 44.5 mpg on a Club 150 mile economy run over some back country winding roads & and still drove at 60mph on the highways where used.
Garth Bagnall

This thread was discussed between 23/11/2005 and 06/12/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.