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MG MGB Technical - driving with no air filter...

I am not sure if this topic has been discussed previously: what damage am I doing to my engine by driving without an air filter? It is a B series with twin HS 4's fed by a single (oil-bath) air filter. I don't make a habit of this... just the last 150 miles.

The induction noise is great! And perhaps I am imagining things, but the performance seems remarkably improved also... but I have a nagging feeling that I am ruining my engine.

This may seem barbaric: is it possible for me to put something instead of oil in the air filter: i.e. remove the innards and repace with something that acts as a good filter, without choking the engine as much?

Any other tips, warnings, etc? Ta.

Hi Tom

Without any filter you risk quite large pieces of debris getting in and causing immediate damage.

With only a coarse filter (eg big holes) you risk rapid bore wear (and other things).

Yes, you will see an improvement in power (and increased induction noise), although you would need to change the carb needles to maximise the power.

I wouldn't drive a car without a proper air filter in place unless I was driving it a short distance to the scrap yard.

There are lots of relatively free-flowing air filters on the market.


Do a search on "K & N" who manufacture excellent free-flowing performance air filters. They may well do one that will fit inside your existing air filter case.

Guy Weller


As Guy says, K&N is the generally recognised performance air filter replacement. As you have standard HS4's you will have no problem fitting two separate K&N filters in place of the oil bath type. Air volume into the carbs will be much improved so you will need to change to richer needles (AAA assuming your engine is standard).

Oil bath filters were fitted for export markets where fine dust could be a problem but I presume in NZ that this is not a problem you encounter.

Laurie Webb
77 BGT 1950cc (with K&N filters)
L Webb

Hi Tom

When I got my midget it was fitted with non-orginal air filters. The non-originality did not bother me, but the astounding ugliness of the air filters did.

My car does not do high mileage, so I opted for the following:
Shiny aluminium "trumpets" (which do not do any filtering, but look terrific and give a nice induction noise), with Piper-Cross foam filters which slide over the trumpets, and which I keep in place with zip ties. For showing the car I cut the zip ties and remove the filters. For driving I leave the filters in place. Best of both worlds.

The Midget has 1 1/4" SU's but I am sure you could find the same set up for the B's 1 1/2s.

James Reinhardt

You risk sucking in both small particles that will girnd down your cylider walls, rings, bearings and they do this quickly. Also risk sucking into the engine large particles even loose nuts which will bombard the piston tops and cause immediate failure. Use a minimum of a screen or a foam air cleaner, Keep your filter clean. An piston engine is only a large air pump using the gas as a catalyst to make it move. Why do you think the top fuel dragsters don't use air cleaners?
John Agosti

2 weeks ago, I was forcibly directed to detour along
side a traffic crew who were sandblasting the paint
off of the roadway asphalt. As my luck would have
it - I had to endure two cycles of traffic signal lights
as my car (having just been washed) was covered
in foul abrasive dust.

If you run continuously without an aircleaner - or
with a poorly designed setup:

On rebuild, you will most likely find that the bores
will have achieved a slight taper. ie: the upper
area of the bores will have a measurable greater
diameter than the bottom of the bores (by a few thou).

Also, the bores will have been slightly ovaled from
the pistons as they thrust against the walls of the
bores when they travel up & down them while
bearing abrasive dirt particles with them.

This is nothing to say about what the pistons,
rings, valves, and carb dashpots and needles
will be subjected to.

Of course, this all depends on how often you plan
to drive, tear down and rebuild your engine.
Do keep in mind that there is a decreasing stock
of usable B engine blocks out there on this planet.

Anyway, none of this is any good for your wallet
or the continuous longevity of your driving fun.

If it's increased air flow you're looking for - pop on
a pair of properly-oiled K & N's and ease your
thoughts of much of the worry.
Daniel Wong

I don't like to be the one to disagree but Morgan didn't use air filters on their +4, there wasn't room. I didn't use air filters on a B that I drove back in the early eighties (FLOGGED woulld be a better word), it didn't suffer anything more than normal wear after a 180,000 thousand miles or so.
Most of the cr*p that gets sucked into the engine goes out with the exhaust gasses.
If you had to drive everyday down dirt roads you need to pay alot more attention to filtration than you do in the city or on the highway.
Get my point ?

This article, although focussed on the cleanliness of oil shows the kind of wear problems and particle size issues related to ring wear and ultimately fuel consumption:

Whilst Dauntless in Tejas(?) has a point and that racing cars used to seldom use air filters to maximise airflow and improve power, they accepted short engine life.

Think about the ratio of air and fuel entering the engine, and then you realise why up to 75% of the potential engine life can be lost by running without air intake filters.

On a wet day, of course, there is little particulate in the air, and the moist air will improve things even further in the power stakes. But for the most part, whilst much will get sent packing down the exhaust valve, the troublesome size of particle is <10Ám, and there are many times more of these small 'uns compared to the bigger 'uns! Additionally, once past the rings, then there is the risk that the solid particles of silica from the environment (with a hardness almost as bad as diamond) will cause further damage to the bearings and cams, since most automotive oil filters are unlikely to remove particles much smaller than 30-40Ám.

Research by Wearcheck in South Africa showed increasing oil consumption and increasing wear rates when dirt was entering the engines through faulty breathers. OK, this was in a mining environment, but the fact remains for many owners living in drier, dusty environments. I have seen paint removed from number plates from where a car has driven through a sand storm in Namibia.

As said above, the use of K&Ns will be the ultimate solution offering better filtration of finer particles with better air flow.

However, on the B there is a real need for a good depth of filter unit to allow the air to flow smoothly through the side of the filter, turn through 90░ and go into the carb. Avoid the pancake type with a cheap foam as these can restrict air flow and reduce performance.

The alternative to K&Ns is as Jame's mentioned, a sub-stack or trumpet, but with a foam filter lightly oiled to minimise dirt ingress.

James, just one question when you remove the foam socks on your trumpets during a display, how do you stop morons sticking things in there, or flies and such like crawling in?

Martin ZT

Answer: Dragsters don't use airfilters to maximize the air intake and yes are willing to sacrifice engine life for the performance. Engines are torn down and rebuilt after approximately six 1/4 mile runs. More air in more air out equals more horsepower. It's only an air pump using gasoline as the catalyst. The race tracks are kept meticulously clean of all debri.
John Agosti

This thread was discussed between 09/11/2004 and 15/11/2004

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