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MG MGB Technical - Changing Air Filters

I have a 72B and while I'm waiting for the head to come back from machine shop I've cleaning and repainting things. While dismantling the airfilter boxes I started to wonder about changing the boxes to something else.

I've searched the archieves and find a couple postings discussing the K&N filters, the washables vs. paper filters, etc. And then there are the Moss MG logo chrome bodies, a oval mesh design etc.

I must admit (time for some of you to get a good laugh) I have played with my air filters a tad. I installed some vent hose (same diameter as horn on filter box) to the rear air filter horn and ran it forward to the front of the engine compartment. I also ran a shorter piece of vent hose to the front point air filter horn. The two hoses enter a metal box attached to the frame for the radiator where a large pre-existing hole was. Sorta a mini-ram air system getting fresh air to both filter boxes!

Yeah, it seemed like a good idea at the time, but it had to come out when I took the head off for a valve job.

So now I'm looking at air filter options. Several say just stick with the originals, a few who have seen my system shake their heads and say stay with original configuaration. But it appears to me that a more open air filter attach to each carb would be better.

Is the $50-100 cost for changing filters to a K&N chrome system worth the money? Will better air flow to carbs just be the first step toward having to also change carb settings, etc.? Or better yet, is my mini-ram air system a good or a bad idea?

Thanks again for all the assistance.

Stillwater, MN
R.W Anderson

A few years ago I ran a B on a dyno to see what it had after running in a reco motor. Without the filters it was down 7 bhp at the wheels & the mixture was far too RICH because of the amount of spit back bacause of the poor air flow into the carbs. Sharp corners are only 70% efficient - hence the use of trumpets on race cars. I tried using the adaptor plates only & it was still down 4 bhp. Paper filters are good enough for a standard motor & last well unless in very dirty conditions, unless you are trying to use max revs all the time. In my opinion don't waste money on K&Ns unless you want to clean them all the time.
Garth Bagnall

I posted a pretty detailed analysis recently under "filtering oil, air, and morons"
FR Millmore

Don't get the chrome mesh units. They do a terrible job of filtering and don't flow air for beans.
Your Original Equipment airfilters have stub stacks incorporated into the design of their backplates. As Garth noted above, the sharp corner at the mouth of the carburetor is inefficient, so if you switch to K&N airfilters, you'll need to purchase a pair of stubstacks. These (APT Part # SS51) can be obtained from Advanced Performance Technology at . Do not bother gettng the airfilter elements that are a direct replacement for the Original Equipment paper element airfilters. In the smaller Original Equipment size these reusable cotton element filters have an air flow capacity of 6.5 Cubic Feet per Minute while some filtering elements made with paper have an air flow 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 will have no effect if you retain the use of any of the variants of the Original Equipment airfilter housings. They are simply too restrictive. Instead, install a set of the pair of 5 7/8 Diameter x 3 deep K&N units (K&N Part # E-3190). These will permit increased flow of the fuel-air charge without sacrificing protection. With proper fuel jet adjustment, when installed on an Original Equipment specification engine these larger air filters are worth about 3 HP on their own. When attempting to build a deeper-breathing engine, they are a prerequisite.
Steve S.

I got this idea from Hap Waldrop, and it works like a champ: First, use the standard sub-stacks as noted by Steve, since these provide just about the best airflow for these carbs. To these you attach K&N cone filters (part # RU-4110 - these have a hose clamp setup and fit the sub-stack perfectly). They are 3" deep, so measure first - brake boosters can be a problem.

On my HIF's I secured the sub-stack to the carb body using 5/16-18 x 1 button head cap screws. Hap's original concept uses counter-sunk hex screws for a perfectly smooth sub-stack surface, which is a better idea - I just don't have the facilities to do the proper counter-sinking right now.

Here's a quick picture of the finished product:

You may be running lean after installing K&N filters, so watch for that.


Rick Stevens

This thread was discussed between 03/04/2007 and 05/04/2007

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