Topic: Velocity Stack Filters

[Rocketman]

Thinking about adding air filtration to my CB550.  Currently using Keihin CR29s with open velocity stacks.  First thought was a still air box or something like the ITG filters discussed in the forum post.  http://forums.sohc4.net/index.php/topic,79491.0.html

While search around the internet I cam across a firm offering both their own stacks for CR carbs with metal filters or filters for stock Keihin velocity stacks.

http://www.pjmotorsports.com/velocity-stack-filters.html

Anyone have any experience with velocity stack filters they can share?  What ever I do will be dyno tested when the motor goes back together.

[KeithB]

Any kind of filter or screen in the throat of a velocity stack will negatively impact the whole point of the stack.
The "still air box" is a better idea, like the stock box on a CB750.
But I suggest you first get your bike on the dyno and see what really works to make a good HP and torque curve.
 Two Tired has a great post on velocity stacks on this site. Have a look for it...good reading!

[bwaller]

Martin, I use Mic smoothbores and had stacks made then use dual sock foam filters over the stacks. Mine will have a different spacing but you can surely find a spacing to fit. From bellmouth inlet to back of the filter is 45mm. Just an option.

No filter is ideal obviously but anything directly over the end of the bellmouth will create negative issues.

http://www.ramair-filters.co.uk/shop/motocross/sock-filters-motocross/2-x-twin-inlet-carb-socks-2/

[slikwilli420]

I'm using the same setup as Brent on mine. I cut down Weber DCOE stacks to fit my carbs and added a filter from ram-air.

[turboguzzi]

in modern superbikes, it's common knowledge that airboxes actually help power if designed right, so not a bad idea. im considering building one for my 500/4.

mesh over the stack feels wrong, but air never does what you think, so now that i have my flow bench up and running, will try some mesh over an open stack and tell you exactly if and how much flow you loose.

[simon#42]

you wont loose any flow at all , the drop in performance comes from messing up the harmonics of the inlet . anything close to the end of the inlet of the carb will cause a pulse wave reflection
which almost always is bad . 
why do you need air filters anyway , you dont go racing to make engines last a long time !

[bwaller]

Well actually Simon....I don't mind a bit if they last!  This is middleweight vintage, but each to his own.

I do understand what you say however, we piss money into the wind on the best of days. What's an extra top end work now & then?

At Barber in October when my bike slid into the gravel trap I was pretty happy to have those filters. We were back out in no time. 

[slikwilli420]

I would never run without filters. My head cost $2 grand and replacing items more often than necessary for a SLIGHT power advantage is foolish. Hit a gravel trap and suck in an engine full of crap and not only is your weekend over but it may cost enough to keep you from other events.

[simon#42]

be thankful you race a honda 4 brent  back when i raced rg500s and crashed most weeks i spent far to much of my life looking for where the carbs had landed
and digging mud out of the crankcases . during one particularly graceless crash at snetterton it actualy ate a turnip !

[bwaller]

Hahaha, I love turnip! Things do get airborne. One of my more spectacular get-offs popped the crabs off, plus launched my brand new Krober 200+ feet. Tyler found it all and the Krober even worked although we needed a new frame.

[NickO]

My mate raced his TZ750 this year for the first time in many many years.

It was set-up very "safe" (i.e. rich) and had the gauzes fitted to the stacks.

Afterwards on the dyno it was only showing about 100hp, but removing the gauzes saw an immediate improvement to 110hp+.

[bwaller]

Well I'd be a tad careful with a big TZ too. Not so easy finding parts anymore, really have to make those things last.

Good on him for getting it out.

[simon#42]

parts are not so bad these days , the main problem was getting a good set of crankcases . the design is not very good and they damage easily .
they never make the same power after they have been repaired . there where some very good french [ jpx ] made cases made in the 80s but these have long gone
there are now some being made in australia which look to be very well made and should solve that problem . other than that all other engine parts can be found .
if you wanted an original master cylinder or something you would struggle though . not sure why more people dont race them these days they are easy to set up
and nice to ride , its only when you have got one right on the limit ...... then they have a habit of showing you who the boss actually is .

[MessnerMoto]

I designed and made for my project... Velocity stack plus UNI filter. 3d printed.... I dont have any numbers but vacuum pull from carbs sounds crazy

[turboguzzi]

flow bench results are in...

didnt have metal mesh, so used some mosquito net that has 0.13mm strands

for the sport, also tried a layer of 1/2" gas tank foam.

so then:

bellmouth free - 80 CFM @ 28" H2O

one layer of mosquito net - 78 CFM

two layers mosquito net- 77 CFM

tank foam - 76 CFM

Simon is almost right, very little impact, just -2.5% for the mesh, honestly was expecting lot worse for the net and better for the foam, turned out the other way around. air flow surprises you indeed.   

the metal mesh could of course make things worse if strand diameter/mesh density is higher

Regarding foam, with more area for filtering like in the nice stacks of mirko and brent, loss would be way lower for the foam.

FYI

[Rocketman]

Good information. Definitely going to add filters. Multiple ways to go. Thanks to all who responded.

[bwaller]

Good sleuthing TG.

Very nice solution Mirko

[MessnerMoto]

Yoss you can experiment, if you have time, and create rounded mesh/wire.... something simple. Probably losses would be way lower like u commented... without my fancy stuff

would be great to see some actual numbers....

[simon#42]

i normally see a loss of 6 to 8% on the dyno ,  the engines dont seem to care how pretty they are sometimes the ugly ones are better sometimes not .

[gschuld]

Mirko,

Very impressive work..... . 

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

George

[scottly]

flow bench results are in...

bellmouth free - 80 CFM @ 28" H2O

The open stack flowed less than your port??

[MessnerMoto]

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....

[turboguzzi]

flow bench results are in...

bellmouth free - 80 CFM @ 28" H2O

The open stack flowed less than your port??
[/quote

flow bench results are in...

bellmouth free - 80 CFM @ 28" H2O

The open stack flowed less than your port??

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

[gschuld]

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....

🙂

[KeithB]

I designed and made for my project... Velocity stack plus UNI filter. 3d printed.... I dont have any numbers but vacuum pull from carbs sounds crazy

How did you arrive at the shape and size of the stack?

[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.