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MG MGB Technical - K&N Stub Stacks

Hi all - I have some questions about stub stacks for K&N filters, as I have had conflicting advice.

I've got a '69 B Roadster, engine unmodified apart from unleaded fuel head. It is running a tubular steel manifold and free flow exhaust (with bomb type centre box). The filters are K&Ns on the standard SU carbs.

Are stub stacks really going to help with the airflow and performance, or are they just a gimmick when used on a road use, unmodified engine?

If they are a good thing, which are better - the cast or billet items?

Thanks for any advice.
Resh

If you don't have the aluminium stub stacks that are used with the normal air filters in place, then you are probably losing some airflow. They have curved edges to smooth the flow as it enters the carbs. Not sure if they original stacks will fit the K&N boxes; if not then maybe consider K&N stub stacks.

ttfn,
--
Olly S
Oliver Stephenson

Resh-
Oliver is correct when he says that stub stacks will smooth the flow of air into the carburetors. Without them you'll have unnecessary turbulence, paritially defeating the advantage of your nice Maniflow intake manifold (it is a Maniflow, isn't it?). As for the subject of Cast Vs Billet stub stacks, cast stub stacks usually have a rougher finish which certainly doesn't help reduce turbulence, while billet stub stacks almost always have a superior finish.
Steve S.

Resh,
FYI, Some time ago, I read a carb tuning book that discusses some airflow tests on SUs that determined that the 2 far and away best types and profiles of air horns/stub stacks/trumpets/whatever you want to call them were the OE ones that formed the baseplates on the standard Coopers filters and the full radius TWM(?) type sold by Moss. I don't have the book anymore, but both types gave near identical airflow improvement over a bare carb. they measured airflow of everything tested by a percent improvement over a bare carb opening.
Many of the horns out there are just gimmicks, but the ones that work really work. Of course you have to think of the airflow as one trip - from aircleaner to inside the cylinder. You should also remember that the Coopers baseplates are free (if you still have the stock aircleaners) while the TWM (or is it TRW? I should know, I have a pair) cost $45 each. If you have all kinds of good airflow going into a bonestock cylinder head, a trick set of stacks may not actually help in the end.
Have fun, Wade
Wade Keene

If I'm not mistaken, and I could well be, K&N makes elements which are direct replacements for the original elements and therefore use the stock backplates. And the ones which are paired under a common aluminum cover plate with elements exposed use the stock back plates. The billet stub stacks are little jewels; I haven't seen the cast pieces but although Steve is probably right, I would bet they are pretty smooth too.
Marvin Deupree

If you bought the cast "twin cover" style conversion which uses a one piece outer cap for the filters, then you must be using the stock, factory stub stacks. These conversions usually have either an octagon "MG" or "MGB" cast in the surface with some raised lines. They also have the incorrect bolt spacing and don't fit 100%, but you can usually get them on and sealing.

If on the other hand you are using the replacement chromed canister K&N piece, (with the chromed sheetmetal backing plate) you will likely be experiencing little gain in performance over the original Coopers style air box.

This is one area that will definitely hurt performance, even on a stock engine, so get something with a good radius entry. Do not buy anything trumpet like unless it has at least a 1/4" radius at the mouth and clears the cover plate of the filter box by a good 2"

The stock items are investment cast and are basically as smooth as you can get. They flow very well.

Sean

Sean Brown

My 76B has a marina manifold with a HIF6. The air cleaner assembly is a long-flo round with the flat metal 90 degree sharp bend to the carb opening.

I am looking at getting the APT billet stub stack to improve flow into the carb. My clearance is limited by the brake booster, so a taller filter and a ram pipe don't seem possible.

Will I see any benefit installing the stub inside my filter assembly?
Any recommendations?
Carl Holm

As has already been stated, you will need some sort of "stub stack" to improve the airflow into stock Su or Zenith carb/s, The stock ones are very good and can be hard to improve on. It really depends on what type of air filter arrangement that you have. Some use the stock backing plates others don't. Two good Web sites, aptfast.com (maniflow distributor in CA) and mgamgb.com or mgbmga.com (Doug Jackson) discuss the benifits of correct "stub stacks" as well as airflow rates for the different carbs and airfilter setups.

Safety Fast

Ron
Ron Smith

Ron,
Thanks for the leads.
I have no stock air cleaner assembly to go back to and that is why I am looking at getting a stub stack. The cast stack looks kind of ifffy so I am leaning towards the billet.

Anybody know how well these work or don't work?
Carl Holm

When installing the K&N filters and stub stacks, note that there should be some spacers used for the mounting bolts which will run through the Filter housing AND thru the mounting holes for the stub stacks.

The K&N kit I bought some years ago for my old MGA DID NOT come with these tubular spacers and it was possible to pull the outer filter cover in too tight and bend up the cover and when using the stubstacks there would be nothing to hold the stacks in place. Using some tubular bushings cut to the correct length to span the distance from the inside of the outer cover to the lip of the stubstacks where they mount against the back filter cover and the carb flange, will prevent crushing the outer cover, and hold the stubstacks and back cover in place. Hope this helps you, Carl!

Bob Muenchausen

Folks - thanks for all the advice and leads - very useful. Looks like the stock stacks would be my best bet, or a set of good billet ones (rather pricey though). I don't have the stock filters to go back to (previous owner had already fitted the K@Ns with no stacks at all), and so must get some new stub stacks.
Resh

Just ordered the billet stub stack from APT. I'll post whatever I find out.
Carl Holm

Thanks Carl - any comments would be good.

Does anyone know where to get billet stub stacks in the UK? I can only seem to find cast items...
Resh

Resh,

I've got a bag full of used stub stacks for K & Ns from various cars I have broken. Mail me and I'll see if I can help

Antony
Antony

This thread was discussed between 23/04/2002 and 27/04/2002

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