Topic: Velocity Stack Filters

[gschuld]

I was wondering the same.

Below are examples of stacks made for the round top factory cb750 carbs.  Two vintage aftermarket stacks, and the factory airbox stack.

The graph at the bottom is the same as a previous post above, from David Vizard’s book on performance Carbs/intakes, the text below that is the text relating to it.  That man practically ate and slept in a room with a dyno and flow bench.  Given the general length we commonly use for velocity stack on our 750s, the #7 and the #10 shapes seem nose appropriate to our needs.  Notice the factory stack is very similar to #3, which isn’t too bad really.

But in general terms for those wondering, shorter stacks tend to favor more high rpm power, longer stacks favor more torque in the low to mid range. 

George

[simon#42]

they are not designed to work they are designed to look nice .   

[MessnerMoto]

I used intuition.... The main idea is to increase as much as possible area of UNI filter and still keep surface continuity of velocity stack.

Does it work regarding performance? Probably
Does it work better then just putting UNI filter on v-stack? I bet
Does it look nice? Yes

[gschuld]

Well, I think the basic idea of the shape from the filter to the generously rounded entrance of the stack throat follows good practice, it’s the area from the entrance to the carb entry that could be taylored in shape to work very well as a proper velocity stack/filter combo.

60mm is about the average overall length for velocity stacks that I have seen on cb750 racers using FACTORY round top carbs(those using CR or Smoothbore Mikuni carbs tend to have shorter stacks).  The 60mm is including the 10mm of the overlap on the carb entrance, so an effective 50mm from the front edge of the carb entrance.

Since modification to your design to fit a factory round top carbs would require creating a new stack shape from scratch anyway, that would be a logical place to start.

The shape I used in the drawing with the gradual bell shape to a short constant section before reaching the carb would also be about idea IMHO.

George

[Tintop]

George, the 50mm length relates to the carb throat dia.  Also modern stacks use a rounded bell mouth, not the sharp edge type.  This creates smoother air flow at the edges, and improves flow with-in the throat / carb.  You also need to allow for a minimum distance inside from the throat mouth to the back of the filter.  This is generally accepted to be the same as the stack length.   I covered this in my filter thread, which has pics of the stacks.

edit: my .02 the best in your chart is #9 for our bikes.

[bwaller]

I'm no expert but when I took a stab at a design what I read was best was a 7 degree angle from entrance to carb.

[scottly]

Look at the pic. No head just a stack over the 80cfm hole in the calibration plate

Then the baseline test of the open stack is faulty; flow is limited by the calibration plate. Re-run the tests without the restriction. (Or not, we already know that placing filter media directly on the ends of stacks doesn't work very well. )

[scottly]

edit: my .02 the best in your chart is #9 for our bikes.

#11 looks more like a short Weber stack inner profile than any of the others.

[wannabridin]

Mirko,

Very impressive work..... . 

Have you considered adjusting the design to cover factory cb750 round top carbs?

George

Send me ID, OD and distance between ODs of two outer carbs....

I'm digging this!  Well done sir.  Care to help share the stock carb designs?  I've a FDM printer and can do some test prints for folks if they'd like.  I would likely print with nylon or HD nylon to prevent any heat or fuel related issues.


Also, I've used a velocity stack inside a Uni pod, worked a treat!  Simply cut down some or all of the anti-collapse spring and carefully insert the stack.  Don't have any before/after results, but it was a great compromise project.