True Function Of A Velocity Stack?

[Boost_Retard]

If you take a look at these 2 pics you will get an idea of what i am up against..

The carb is a 40 IDF 15, those brass tubes you see sticking up out of the throats are part of the high speed enrichment system..they can be removed if need be....


This is the FAT performance IDF filter setup i bought off of e bay..


im not so sure how this high speed enrichment system works, but the carb came with the stock 40 IDF cast top plate/velocity stack combo..the stacks are about 40mm tall..and the large end opening (top) is 41mm, the small end (against carb) is 37mm,

the top of the carb has a 37mm opening and the butterfly end is 40mm..

What i need to figure out is what these velocity stacks actually do..and what would happen if i ditched them infavor of the off road air filter system, wich needs to bolt directly to the carb..no room for velocity stacks..

the air filter runners have a ID of 47mm, so if i drop it ontop of my carb, i can see about 5mm of carb top plate per side of the 37mm throat opening..

will this larger 47mm tube still do what a velocity stack should do? or should i have something lathed up, and pressed..or welded..or glued..into the large 47mm tube..that will taper down to 37mm..to make a nice smooth transition of air from the filter into the carb..

im not sure if having that 5mm flat lip will screw up air flow enough that some sort of tapered insert would be needed...and im not sure if removing the stock velocity stacks and installing this tall filter setup will mess with the high speed enrichment function...

its just a 40 IDF 36mm venturie, but its gotta feed a 2.0L motor that will be screaming most of the time..

SO im looking to make the best of what i got...

i dont have any problems makin a trip down to a machine shop and having something made up..

but i also dont know if a velocity stack needs to be taperd..or if this off road filter system really is just a large pair of stacks..

LOL ya followin me?

let me know what you think...
thanks
Cale
 

[Bruce]

The top edge of your carb is a sharp corner.  The air being drawn through the carb gets interupted by this sharp corner.  Thus there is an inlet restriction due to the sharp corner.  When you add a velocity stack you can reduce this inlet restriction.  Reduced inlet restriction means more air flow.  More air flow means.......

The ideal shape of a velocity stack is a hyperbolic curve of rotation.  Think of the cooling towers usually associtated with a nuclear power plant.  Those are hyperbolic.  Chop one of those at it's minimum diameter and the shape from there up is the theoretical perfect shape for a velocity stack.  Now look at that goofy thing Weber bolts onto their carbs.  Not even close.  But better than nothing.  If you take the stock one off and put that FAT thing on, you will be effectively running without velocity stacks.  If it were me, I'd try to machine something to go in there.  If you do it right, you can exceed the performance of the stock velocity stack.