Topic: Velocity Stacks & filtering?

[dummkauf]

Is this possible?

I am in the process of rebuilding my carbs and I love the look of velocity stacks on a bike, however I am concerned with the air filtering ability of velocity stacks.  Every velocity stack I've seen for sale looks like someone just glued a screen onto the end of which while great for keeping sticks and small animals out of my engine, doesn't look like they will do much for keeping dirt out.

I have been toying with the idea of taking a K&N filter and trying to cut it down and retro fit the pieces into a velocity stack, however since most K&N pods are huge compared to a velocity stack I'm not sure if the little pieces of filter would allow enough air flow, as well as I am still unsure on exactly how I would accomplish this either way.  Ideally I would want the filters recessed into the stacks if possible so they are not visible.......so basically I want form & function if at all possible 

I did some searching on this site and the interwebs and couldn't find anything on this.  It appears that unless I make something myself I am left with 2 options.
     1) Velocity Stacks which = poor filtering
     2) Pods or Airbox = best filtering

So my questions to all of you knowledgable folks are:
    - Are there any velocity stacks available for my carbs that have a quality filter builtin?
    - Would cutting down a K&N filter and somehow fitting it into a velocity stack allow enough airflow?
    - Since I am in the middle of my carb rebuild....what sized jet should I be going to for velocity stacks(or filtered stacks )?

Oh, and I have stock keihin carbs on a 76' CB750 K6

[Sam Green Racing]

I think you know the answer to that one

You can't have it both ways, one idea cancels the other out.

Sam.

[Joel]

It is possible to buy foam material for custom applications.  You could fit a chunk into the stack and use a screen over the end to hold it in.

http://www.unifilter.com/online%20catalog/accessories.html

[scunny]

if you want to put foam in the stack, have a gauze filter between the foam and your motor(stops sucking it in)
what Sam said, foam filters and stacks don't match. choose one or the other.

[ieism]

A filter in a stack will seriously upset airflow, and defeats the purpose of a stack. In fact, those grills to stop the rabbits getting sucked into your carbs are already a very bad idea on velocity stacks.

There was a discussion about making a short velocitystack INSIDE a k/n filter here for a while. That seamed like a good idea. But I have K/N filters, and don't see how you could get a stack in there, you'd have to saw it in half.

[dummkauf]

There was a discussion about making a short velocitystack INSIDE a k/n filter here for a while. That seamed like a good idea. But I have K/N filters, and don't see how you could get a stack in there, you'd have to saw it in half.

I don't understand the point of the stack inside the filter?  What possible benefit would that provide?  Atleast to me it seems that would reduce airflow since the stack would be effectively reducing the amount of air flow to the engine that the filter would provide without the stack inside of it?

I'm not going to try this, but I am curious what the reasoning behind this is.

[dummkauf]

Another question to all of those out there that are running velocity stacks:  How long do you run before needing to clean your motor out?

Or would I be correct in thinking that velocity stacks are generally saved for show or race bikes that are not driven very often?  When I am done with this project I have no intention of using the bike for my daily commuter(I have a 70mpg thumper for that ), however I do plan on driving it on a regular basis.  My weekend rides can easily take up an entire day.

[scunny]

ran stacks on my 500 for about 2 years, some roundies,drags and to work every day. never had a problem. obviously wear will be increased(don't do dirt roads)

[Sam Green Racing]

Another question to all of those out there that are running velocity stacks:  How long do you run before needing to clean your motor out?

What do you think gets in there that needs to be cleaned out ?

Sam.

[Joel]

I can see how a stack with a filter won't flow as well as one without.  However, is a stack with a filter any worse than a pod filter?

A wire or mesh basket could hold a foam filter element for normal operation and be removable to allow open operation when desired.

[Sam Green Racing]

You would need to keep changing your jetting if you swapped between the two.

Sam.

[dummkauf]

What do you think gets in there that needs to be cleaned out ?
Sam.

I'm picturing dirt, rocks, bugs, and anything else flying around that could get sucked in getting into my motor and causing problems

[Joel]

You would need to keep changing your jetting if you swapped between the two.

Sam.

True, I hadn't thought of that.

[dummkauf]

I can see how a stack with a filter won't flow as well as one without.  However, is a stack with a filter any worse than a pod filter?

This is an excellent question?  Does anyone know?

[scunny]

Sam, he wants to keep rodents out.
during normal use there is not much to worry about. my concern was always people putting things in there so I had bungs to solve that problem
regards filter media, if you look at say a pod filter, the entire(well almost) of the pod is used due to the fact that it reduces air flow, so to counter this you increase the surface area of the filter media. TT would?might be proud of me  

[ieism]

It's not all about increased airflow. Why do velocity stacks always have the "bellmouth" shape?
Why are the intake rubbers on the stock airbox shaped like small stacks?
Why does a bike run better with stacks than without them ?

I don't know the exact science behind it (TwoTired is an expert on this) but it's got something to do with the velocity of the airstream and vacuum.

You can run stacks on a streetbike, but it will slowly sandblast your engine.

[Sam Green Racing]

What do you think gets in there that needs to be cleaned out ?
Sam.

I'm picturing dirt, rocks, bugs, and anything else flying around that could get sucked in getting into my motor and causing problems

Dirt dust and bugs will go right through. Rocks on the other hand
That's why we have fillter or pods for every day use.
Ram stacks are for performance where there is little chance of anything getting in there.
Although a moto crosser is a performance bike, you never see them running in all the sh1t without a filter.

Sam.

[Joel]

The bell shape allows a smoother transition for the incoming air.  Less turbulence allows increased velocity.

[dummkauf]

I think I'm going to end up getting a set of pods.

Stacks would be nice but ultimatley I want my engine to last and be functional, which from this discussion rules out my stacks

[Sam Green Racing]

I think you know the answer to that one

You can't have it both ways, one idea cancels the other out.

Sam.

 

Sam.

[scunny]

Although a moto crosser is a performance bike, you never see them running in all the sh1t without a filter.
bugger there goes another brilliant idea
yep bellmouth is to lower turbulance, check out some of the carb rubbers from filter box to carb, some of these are bell mouthed.

regards as to run stacks or not... if you like the look go for it
                                              if you want to prolong the time between top end rebuilds, don't
 stacks perform best when on the throttle, couple this with a 4 into 1 and you can(sometimes) increase top end performance but sometimes compromise general road use

[dummkauf]

I think you know the answer to that one

You can't have it both ways, one idea cancels the other out.

Sam.

 

Sam.

Yeah yeah yeah.....I'm stubborn and don't give up easily

Thanks for all the input though.  Sometimes bouncing ideas off of others helps to clear up the situation.

[scunny]

they do look good tho 
if it's any consolation I like stacks but am probably going to go pods now because my track days are probably over

[tsflstb]

Stacks are dangerous - there was a small child in that stroller before I fired it up.

[scunny]

ha ha ha..............nice stack

[ieism]

Ha ha!
Did I mention the cool noise they make? It will make your bike sound like Satan smoking a bong. It doesn't get much cooler than that.

[TwoTired]

A true velocity stack requires laminar air flow to operate properly.  A filter of any type near enough to the bell mouth to cause turbulence, pretty much relegates a velocity stack to the function of an ordinary tube.

If the air is laminar, the reduced cross section from entrance to minimum diameter of the stack compresses the air entering, when the velocity is high.  This packs more oxygen into the charge, enabling more fuel to be burned and more power output.  A stack by itself will not create a gain.  It must be accompanied by a fuel metering adjustment, and some high intake velocities (Mach number).

The stock set up has rubber "stacks" just ahead of the carbs.  The "stacks" are fed air from a plenum of filtered air.  Filters turbulate air due to their obstructive properties.  The plenum allows the air the settle some into a common pool before entering the "stacks".  The bell mouthed "stacks" do more to keep the air flow laminar (for the purpose of keeping high and low pressure variations from reaching the fuel jet outlet ports in the carb throat) than to behave as true velocity stack.  At least, at the velocities normally encountered here.

The drag racers of old used Velocity stacks to advantage, poking the inlet bells high above the vehicle where scooped air was unfettered body work.  They were long, too.  14 inches on some screaming V8 motors.

The short V stacks on MCs probably do more to keep the fuel metering within the carbs consistent than they do to actually compress the inlet air.  Of course, if your RPM is high enough to give a significant inlet air Mach numbers...

But, then if you are operating at very high RPMs, that alone is going to shorten your engine life between overhauls.  Most likely more than dust inhalation.  Speaking of dust inhalation, what kind of dust will your open stack suck in? The soft stuff, or the gritty stuff that sand blasts cylinder walls and erodes valve seats and faces?  And, if you are doing frequent overhauls, as racers do, will you care?

If you want the look of stacks without the hassles, you could do what Honda did on the CB700.  Faux stacks.  See Pic.

Cheers,

[KingCustomCycles.com]

I like the sucking noise they make. 

[scunny]

+1 on the sucking noise.
 in an ideal world the movement of air thru an obstruction is best done by sucking rather than blowing, that's why your radiator fan is behind the radiator.

[KeithB]

It will make your bike sound like Satan smoking a bong.

THAT is very funny!

I was reading the thread and working out a suitable response when Terry nailed it.
This whole velocity stack thing is a real tuning issue.
If you dont do it right, why bother?
I see on my ST1100 that Honda went to a great deal of trouble to "stack" the carbs and provide a proper plenum to correct air flow.
A lot of this type of tuning is to even out the torque curve for street riding.
In a race application, you can tune the engine to be fierce in a narrow RPM range and then select the correct tranny and sprocket ratio for the track style.
Any contamination of the engine is not an issue due to the usual re- builds that racing requires.
I put pods on my '78 K Cafe and a header and then tuned the carbs to that setup.
Works well, looks good and keeps infants out of the engine...