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MG MGB Technical - K&N filter questions

Hi all!
I'm thinking of getting K&N filters for my 1970 B GT.

From earlier threads I've learnt that I will need to also replace needles in my SU's, can I just by a pair of new needles that will do the job or do I have to test between different needles and see which works best?

Can I reuse the two black filtercontainers (not sure about the correct terminology) that holds the filter elements or do I have to fit new, different looking filtercontainers as well?
Jesper

You can fit K&N filters inside the original filter cans. That is what I did.

I used the needles recommended in Peter Burgess's book. Can't remember what they were and I don't have the book to hand at the moment. I will try and email you with the information later.

David
David Witham

Jesper-
K&N makes more than one airfilter that can be used on the MGB. The one that can be used as a replacement for the original filter will fit neatly into your airfilter cannisters but will not give a worthwhile increase in airflow , thus you will not get an increase in power output. The ones that will flow more air and make a power increase possible is the 3 1/4" X 6" filter. To understand how to fabricate custom mountings for them, click on this: http://www.hotelling.com/mgb6.htm

As a rule, you should only need to go up one stage on the carburetor needle to attain the correct fuel/air mixture ratio.
Steve S.

Thanks for your fast anwers!

Do any of you know what happens with gas consumption after an operation like this?

Steve>> nice pictures, I've bookmarked your site.
Jesper

Jesper,
It really does not alter if at all, BUT any possibility of increased power is usually accompanied by a heavier foot on the pedal to take advantage of that power. That will lead to heavier consumption!
Peter Thomas

Jesper,

I think your car came with AAE's, at least the US spec cars of that year did. Peter B recommends in his book going to ABD's or possibly AAA's. I used AAA's on my 71, as the AAL's were a bit leanish even before I put the K & N's on. Car runs great. The gas mileage may or may not change. Since you are removing a restriction, the efficiency is improved, so you get the same power at a smaller throttle opening. But as Peter T notes, more power begs to be used, so your fuel consumption may increase commensurate with smile factor.
Paul K

Jesper,
I have a 71 MGB/GT and just replaced the two air filters with 2 K & N filters, E-2400. Now the RPM's drop while idleing? Why? Replace neddles? What neddles?
Also, Paul, what brand name produces 'AAA'? Do you think my idleing problem has anything to do with the new filters? It ran just fine prior to the replacements.
William Powell

William if you replace your old filters with K&Ns the mixture will weaken. Your old filters may have been dirty making the mixture richer. SU or BURLEN make the needles when having replaced the needles take it to a rolling road for maximum power. Peter
peter bramble

Are the 3 1/4 x 6" filters significantly better than the thinner models? I like the looks of the thinner ones better!
Steve Simmons

If you own, (or can obtain the use of) a pair of aviation snips or tin-snips, you can cut the surrounding "shroud" of the outer filter can away, thus exposing the K/N element to the under-hood air. Use the original stub stacks and the nut plates with integral choke cable holder, as well as the original bolts etc. That combination is plenty good and will out-flow virtually anything you can put "downstream" of it.

If instead you use the size filter Steve recommends, be absolutely sure to install some kind of stub stack, or you will be taking a step backward rather than forwards.

The needles PB suggests are good choices.

Sean

Sean Brown

Peter Bramble-
Thanks for the response, but how do I remedy the situation? How do I enrich the mixture? And I still don't know what neddles are???
I'm a 'sub-rookie' with motors-
My wife say's I need help, I'm wonderng if she's just refering to the MG??

Bill
William Powell

William
A needle is an important part in the SU carburettor, its job is to let in a proper amount if gas to the stream of air. Above we are referring to needles in plural as MGB's often has two carburettors. Concerning your mixture I suggest you to find a good mechanic to help you with the sorting of the carburettors, especially if you have twin SU's.
Jesper

Sean

The moification discussed using the 3 1/4 thick K&N's and linked via Steve S, Virginia, retains the original filter bases which have the bell shaped inlets. I have just got a pair of "door protectors" and am about to make this mod - first try can be with the original paper filters.

Barry
B.J. Quartermaine

Just interesting information: when I switched to the K&N setup I outlined on my web site (http://www.hotelling.com/mgb6.htm)which Steve was so kind to mention, there was enough adjustment left in my needles that I did not have to change them. Be aware that you MAY not have to change your needles at all. After you install the K&Ns, try adjusting your carbs - you may be surprised and you may save a lot of time by not having to change needles at all!

Roger
Roger Hotelling

There are many paths to the same result, as Sean and others have pointed out. Removing the air filter outer cans is not a new idea, and probably precedes whoever came up with that oval Aluminum plate setup that Moss sells. My original motivation for doing this had less to do with performance than with eliminating some of the physical clutter in the carb area which generally made "Real Life" tuning of the carbs (with filters in place) a PITA.

If you were to go to my site, http://hometown.aol.com/bobmunch/index1.html and look at the photo of the engine, you would note the "Use {of} the original stub stacks and the nut plates with integral choke cable holder...". Very little other than the tin cans were removed or changed, making the modification pretty easily done for those of us on a budget and/or with limited fabrication tools or skills. One reason why, I suspect, that it has become as popular as it has.
Bob Muenchausen

Of note is that with the HIF carbs, the choke holder is part of the heatshield rather than the filter cans.
B.J. Quartermaine

Well guys seen as we having a discussion re flow and mixture thro SU carbs on the spridget threads I thought I might draw in the big guns to widen the net.
Can someone please explain why installing a more efficient air filter should require a change to the needle? given that the volumetric efficiency of the engine hasn't changed and no alteration has been made to the flow thro' the carb eg removing the bridge, modifying the piston etc.
As I understand it the flow thro' the card is control by creating a low pressure area across the bridge which induces the fuel to flow thro' the jet, pressure difference between one side of the piston and the othercausing it to rise and fall varying the volume of air flowing. As this also lifts the needle it also allows more fuel to flow the keeping the mixture correct. So if we make the filter more efficient it flows more air the piston rises it flows more fuel. Where am I going wrong?

Graham
Graham Robson

Graham,

My guess would be the filter change introduces more air, enough to affect the mixture but not enough to affect the differential pressure which lifts the piston. Not a design engineer, but I think any restriction removal in the intake tract would slightly bump up the VE.

You are right about the basic operation, the low pressure area created by the airflow is called "venturi effect", which takes place when a fluid runs through a restriction in the pipe. The shape of the restriction affects the amount of vacuum created. We use this principle in steam plants with air ejectors (removes air from condensers) and eductors (pumps water from bilges).
Paul K

This is fascinating! I have often thought about a pair of bob-cleaners but have wondered about changes in fuel efficiency and so on.

Is it also possible to change the resulting mixture by changing the back-pressure in the exhaust? I recently Kevinned my exhaust (replaced my middle silencer with a straight pipe) and have created a bunch of leaks at either end of the pipe. This seems to have messed up my gas-flowing (I understand about losing more mixture due to valve overlap when there's less back-pressure) but does it also change the mixture?

Admittedly, the plugs I just took out look the same as they ever did. Lovely light brown.
--
Olly
Oliver Stephenson

Steve-
Yes, the 6" X 3 1/4" K&N airfilters are better than the thinner ones. The more shallow the airfilter, the greater the problems with induction pulses screwing up the flow of the induction system. Because of their greater surface area they flow more air and don't have to be cleaned as often.

Oliver-
Yes, removing the middle silencer will effect backpressure, thus allowing the engine to exhale a bit more easily. This creates a better scavenging effect, resulting in an increased pressure differential between the atmosphere in the cylinder and the atmosphere in the outside world, resulting in the fuel/air mixture coming into the cylinder faster. This in turn increases both the interia and fill rate of the fuel/air charge and causes the cylinder to be filled with a larger fuel/air charge, hence the need for more fuel. As a rule, the increase in fuel need resulting from simply replacing the front silencer with a straight pipe on an otherwise stock engine is minor and can be met by adjusting the jet height setting to a richer position.
Steve S.

Thanks Steve!

--
Olly
Oliver Stephenson

Steve Simmons,

Also, the thinner air filters cause the air to have to make a sharp turn as it enters the carb. Any sharp turns are detrimental to flow.
Paul K

Too many Steve S's around here!
Steve Simmons

I have just bought and am about to fit a K&N coupled pair - and I was thinking of doing as Oliver has done and replacing the centre box with straight pipe. Did you have any difficulty in getting the slightly larger dia. pipe to sleeve over the extg? (Mine is stainless.) Having done this, is there a) any improvement in performance, and b) much increase in noise levels, or improvement/deterioration in sound quality?

Thanks Guys
Jonathan D

Thanks, Steve, that tutorial on the center silencer explains why folks like Roger and I have not really had to change needles. Apparently backpressure has kept the flow down to within range of the needles we use, and I would bet that your Peter Burgess head simply increases the need for a needle change and perhaps a more dramatic one.
Bob Muenchausen

That sounds logical Bob, and it's supported by the fact that I have a completely stock exhaust system.
Roger
Roger Hotelling

I didn't have any problem getting the new pipe over the top, 'cos it was quite a bit too big. There was a decrease in performance and a fair raise in sound level as I bodged it up and it's blowing like something quite blowy. Mind, it was only 4 quid's worth of parts and even with the blow it sounds better than my broken silencer. I'll do it properly soon and let you know...

If you ignore the sound from the blow then the real exhaust note sounds great, and not too much louder. No-one's going to think you're a kevin...

ttfn,
--
Olly
Oliver Stephenson

This thread was discussed between 06/06/2002 and 13/06/2002

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