Carb Velocity Stacks

Now my carburetors are back together, it's time to figure out what I want to do with velocity stacks. As I understand it, they serve two functions: a) the curved bell allows air (a fluid) to make a smooth transition into the throat of the carb, thus minimizing turbulent flow in the "air column", which delivers more air and at a more steady rate; b) the length of the velocity stack has a dynamic effect on "pulses" caused by the intake valve opening and closing (think of the inch-worm effect in a line of stop-and-go traffic on the freeway - the air molecules bunch up, then spread out, bunch up again, etc.) Here's a Wikipedia article on velocity stacks.

This sounded reasonable enough for me to want velocity stacks on my engine, so I bought a set from CB Performance. Imagine my surprise when I sat one on top of the carb and saw a 3/16" step do disrupt my smooth air flow!

Oversized velocity stack on a Porsche 914 engine


That's going to increase turbulence - not decrease it! However, according to CB Performance, "That's what everyone uses - you won't notice a change in performance." Well, that's not good enough for me... So I picked up a set of velocity stacks for 40mm Webers (pictured on left) which are a little smaller than what I had (pictured on right).

2 different velocity stacks on a Weber 44 IDF for a Porsche 914


However, the bore in these was actually a little bit too small. I fixed that on the lathe:

Boring velocity stack on lathe


I'm happy with the velocity stacks, but I have the same problem with the air-cleaner baseplates - they are oversized, which causes a big step. However, I couldn't find these with a smaller diameter. Adding material is harder than removing material, but it can be done... First, I sanded the inside edge to expose bare aluminum:

Sanded edge on aluminum air cleaner baseplate for Weber 44 IDF carb


Then, I machined a stainless steel blank to the diameter I wanted (I have a 1 degree taper machined on this), and by making a little fixture, was able to position it right where I wanted it:

Metal plug fixture used to modify air cleaner baseplate for Weber 44 IDF carb


Using HTS-2000, an aluminum brazing product, I was able to "add" material up to my plug:

Adding material (HTS-2000) to air cleaner baseplate for Weber 44 IDF carb


And then pop the plug out on the shop press:

Pushing plug out of air cleaner baseplate for Weber 44 IDF carb


Looking good so far!

Metal added to air cleaner base for Weber 44 IDF carb


On the back side, I smoothed any excess material down with a belt sander:

Sanding back side of air cleaner baseplate for Weber 44 IDF carb


Now, with this flat surface, I was ready to fixture it to the mill and clean up the top side. I traced around the bases of the velocity stacks so I'd know exactly where I needed to machine it flat (this also gives the velocity stacks a better place to sit, rather on the cast surface).

Machining air cleaner baseplates for Weber 44 IDF carbs


Machining done:

Machining complete on air cleaner baseplates for Weber 44 IDF carbs


Test fitting everything together:

Weber 44 IDF carbs on high-performance Porsche 914 engine


And, here's the result of all that work - a smooth bore for the carburetor!

Inside throat of Weber 44 IDF carburetor

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All pictures and text copyright 2008