Air filter/stub stack mod/upgrade

Started by 94touring, June 25, 2015, 08:35:06 AM

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During my extensive reading lately came across articles on air filters and stub stack.  See attached pic.  What these do is allow better air flow into the carb by eliminating the sharp edge on the intake port on aftermarket air filter setups.  Turns out pancake/cone filters actually DECREASE performance over a stock air box due to these airflow characteristics.  There are several styles of stub stacks you can buy, some are pretty fancy and prices vary.  They simply bolt inside your filter.  I bought a generic pair as pictured off the E of Bays for $20 shipped.  They are cast and rough and poor quality, however guys are simply sanding them and polishing and having a nice looking product with some minor elbow grease.  For the money seemed like a good deal to me.  Rolling road results are showing 2-6hp and torque gains throughout the whole powerband and much better throttle response.  Not bad for $20.  On my carb setups I have the thin inexpensive pancake style with foam filter.  Too thin for the stub stacks.  I thought about ditching those and buying the cone style or k&n pancake style filter but thought it would be ashamed not using my brand new polished pancake filters, plus extra money spent on new filter assemblies.  What I did find however were replacement filters that I can simply swap into the basic pancake filter, and for a lot less money than buying new filter assemblies.  They are 2 inches in height, twice the height of the basic pancake filter.  All that's needed now is to buy longer bolts to fit the polished top and bottoms of the filter assembly.  Here is the product info on the filters if anyone wants to upgrade what you currently have.  K&N E-3200

Also pictured is the basic pancake that the K&N will fit.     


Coolness - thanks for posting this!

I used pancake air filters for a few months about 10 years ago - other than that, the remaining 18 years nothing.  But I did just put my K&Ns back on in hopes of richening up my mixture...


Those stubbies are pretty cool for $20!

I'd like to get one of those on my car but there's no room between the carb and firewall unless I get a completely different air cleaner setup.
Complete failure at retirement

1989 Cooper Racing Green
2009 Clubman S
2014 Audi Allroad


I have similar pancake filters to the ones shown.  I've been wondering if they are the cause of some of my mixture problems.  I seem to have a real rich mixture when at speed but normal when running slower.  I thought maybe to pancake was to restrictive and was causing the engine to suck fuel out of a lack of ability to pull air.  Does that make sense?  Anyway, these inserts definitly sound like a good idea.  Should save me some bucks as well.



The basic pancake filter is pretty thin.  Vizard has apparently wrote a section in his book about the negative effects that the sharp edges have on air intake on these aftermarket bolt on filters.  The stub stacks effectively open the mouth up to allow more air in but also create a more fluid flow for the air, eliminating turbulence.  Otherwise you're better off running a stock airbox. 


Got a couple sets of stub stacks in today.  Actually not half bad quality.  Won't take much to polish up the ports.

John Gervais

I once made a 1 cm thick aluminium plate with an offset generously radiused oval-hole that was tapered thin on the perimeter edges and screwed via 3 screws inside the HS4- type plastic air filter box.  I guess you'd call it a 'flat' velocity stack for the plastic box to round off the edge of the oval orifice. 

Shortly afterwards, I installed twin HS2's and if I could find it today, I'd take a picture of it.  I'm back to the plastic box and the HS2's need rebuilding, so I'd like to try it again - I'm sure it helped, so if anyone has a block of aluminium sitting around, I'd like to try to carve another one...
- Pave the Bay -


Side by side comparison.  Filter fits perfectly.


Today I installed these guys.  Took some 1500 wet sandpaper and gave the ports on the stub stacks a nice polish.  5 minutes of elbow grease was all that was needed.  The bolts required are 1/4-20 x 2-1/2 and the acorn nuts to match. 

John Gervais

Cool - I bet they'll sound awesome.  I've got a pair of those same pancake filters in a box somewhere and it's good to know that the tall K&N filters will fit.

Now, if only I rebuild those twins ...
- Pave the Bay -


Yeah I have a pair of hs2's coming that will get this mod done as well.  These are a fair amount cheaper than the cone style too.   


I got a pair for HS2s as well. Quick shipping from the UK.
A little touch up and should be nice.


Have everything on the way to rebuild my hs2's I picked up,  including these filters and stub stacks.  Added this to the DIY as well. 


Quote from: 94touring on July 28, 2015, 09:15:30 AM
Today I installed these guys.  Took some 1500 wet sandpaper and gave the ports on the stub stacks a nice polish.  5 minutes of elbow grease was all that was needed.  The bolts required are 1/4-20 x 2-1/2 and the acorn nuts to match.

Do you happen to have any pictures of this setup finished in the car? Showing master cylinder clearance/ fire wall clearance? I have a local mini expert saying these don't have clearance and to go with the K&N cones. I would very much prefer to go with this look.





The problem with the K&N's is they let in a lot more dirt. I've know this for a long time, but have it on my Corvair because that is the only thing I've found. After my last oil analysis I have made a paper sleeve for my K&N that I hope will help. There was just way too much dirt and wear in my oil for my comfort.