Stub Stacks

In the MGOC 'Enjoying MG' August 2011 edition there was an advertisement for airflow stub stacks. Having never seen these before I discussed the subject on an MGA forum.  Stub stacks provide improved airflow into the carburettors from the Volkes filter boxes.  However, these canisters are narrow and the consensus was that a stub stack in an MGA should be 5mm to 8mm thick maximum. As a result, I decided to make my own.  Below is the story.  I am only a basic self taught lathe user so I am always ready to listen to good tips, but it goes to show that they are not difficult to make.


The MGOC Stub Stacks that started off my interest.


I decided to make my stub stacks from a 3.5" aluminium bar.  The diameter of the bore would be the same as the inlet bore of the carburettor = 1.39".  The sloping face would be 15 degrees; the radius of the throat would be 0.25"; the other holes would be taken directly from the gasket, although the main retaining stud hole would have to be increased to 12.5mm to allow the stub stack body to slide down the Volkes canisters internal attachment posts.  This was the first drawing.  In the eventuality I machined the edges thinner than depicted.


Basic set up of aluminium bar in lathe.  The tailstock was used when required to hold the piece steady.


Boring out the centre hole to 1.39".



Cutting the 15 degree sloping face.  This will result in a thickness of between 5mm to 8mm depending on how much the centre hole is blended at the edge.  I went for a 0.25" radius. Note the tailstock in place to provide stability while cutting.




Basic stub stack.


Drilled out using the gasket to place the centres of the holes.  The air vent holes to the carburettor were drilled to 6mm.  The attachment post holes were drilled to 12.5mm to provide a comfortable but tight fit.  They were then over-drilled part way through to give clearance over the post welds.  I also had to dress the welds significantly.

Trial fit in the Volkes filter canister.
The stub stack was finished with 400 wet and dry and then polished with BlancoPolish stainless steel cleaner.  Brilliant stuff.
Glued into place with 2-pack adhesive.  The fit was tight enough to not require gluing, but I applied the belt and braces principle.
Finished stub stack in place.  Now to repeat for the second filter box. Incidentally, I use K&N filters in the canisters.

I have been very impressed with the improved performance of these stub stacks.  I run the 5-bearing 1800 engine in my MGA.  I found throttle response much improved and the feeling of increased power.  A second set of stub stacks was tested on an MGA owned by Neil McGurk and fitted with a 3-bearing 1800 engine.  The car was tested before and after resulting in a peak power increase from 88.03HP at 4641rpm to 93.31HP at 4601rpm and peak torque increase from 104.4ft-lb at 4350rpm to 107.4ft-lb at 4350rpm.