Follow along as I build a high performance 914 engine
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!
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).
However, the bore in these was actually a little bit too small. I fixed that on the 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:
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:
Using HTS-2000, an aluminum brazing product, I was able to "add" material up to my plug:
And then pop the plug out on the shop press:
Looking good so far!
On the back side, I smoothed any excess material down with a belt sander:
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).
Test fitting everything together:
And, here's the result of all that work - a smooth bore for the carburetor!
|All pictures and text copyright 2008|