Author Topic: Some thoughts on velocity stacks  (Read 2666 times)

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Offline Beergineer

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Some thoughts on velocity stacks
« on: July 20, 2011, 10:01:20 AM »
Given that my '74 CB750K came to me with velocity stacks rather than an airbox I've been having a hell of a time getting it to run without the choke on. It has been pointed out to me that the issue is probably that the velocity stacks just supply too much air to the carbs (with stock jetting). So the question is, how can a velocity stack provide a metered air flow?

I'm an engineering student and am working on a project to test inline flame arrestors. The variety I'm dealing with are basically a steel pipe with crimped metal foil rolled up inside. The metal foil reduces the area of flow, which results in a pressure drop on the output side of the arrestor. Since the metal foil inside can be crimped to different heights, the pressure drop can be varied to suit your carburetors. The crimped foil also serves as a flow straightener, which will cause the flow to the carb to be more laminar in nature (which will supposedly make the carbs work better in general).

If I can get my hands on 4 arrestors of the right size, I'll do some simple tests to see what happens. Until then I'll be going to a stock airbox.

KingCustomCycles.com

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Re: Some thoughts on velocity stacks
« Reply #1 on: July 20, 2011, 10:14:47 AM »
Twotired will have a field day with this one...........until then, drill your main jets to .051", drop your needle clip 1 notch down, and have at it. "do not try this at home, results may vary and not typical".

Offline Skunk Stripe

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Re: Some thoughts on velocity stacks
« Reply #2 on: July 20, 2011, 12:16:27 PM »
That's funny stuff right there!

Offline TwoTired

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Re: Some thoughts on velocity stacks
« Reply #3 on: July 20, 2011, 12:42:37 PM »
... velocity stacks just supply too much air to the carbs (with stock jetting).
I wish people would stop saying that.  At air velocities below red line RPMs, the air volume is the same.
The filter membrane provides a pressure drop across the membrane which reaches toward the pressure drop source,into the carb throats where the fuel jet orifices are located.  Raising the pressure at those jet exit points reduces fuel flow through a given orifice size and thus removeing the filter and duct assembly reduces the fuel flow fro essentially the SAME amount of air inlet to the engine.

I'm an engineering student and am working on a project to test inline flame arrestors. The variety I'm dealing with are basically a steel pipe with crimped metal foil rolled up inside. The metal foil reduces the area of flow, which results in a pressure drop on the output side of the arrestor. Since the metal foil inside can be crimped to different heights, the pressure drop can be varied to suit your carburetors. The crimped foil also serves as a flow straightener, which will cause the flow to the carb to be more laminar in nature (which will supposedly make the carbs work better in general).
Laminar flow is air speed related, usually.  I'd be interested to know how an obstruction into a laminar air flow can allow it to remain laminar at all speeds.  Or, how it can change turbulent flow into laminar flow without it having an aerodynamic shape.
Are you accounting for the decreased cross sectional area when the "device" is placed inside an air duct?

Cheers,

P.S. I really don't have time for a field day right now.  House repair projects are dominating my life, along with the use of pain killers.  ::)

Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline Beergineer

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Re: Some thoughts on velocity stacks
« Reply #4 on: July 20, 2011, 12:46:45 PM »
Definitely some funny thinking, when you spend 40 hrs a week as an engineer, and another 4 hrs in a Thermo Fluids class, and another 8+ hrs on the labs, and whatever's left working on a motorcycle all the lines begin to blur...I'd like to think Edison felt the same way(when he wasn't busy killing elephants that is).

Offline Elan

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Re: Some thoughts on velocity stacks
« Reply #5 on: July 20, 2011, 12:47:51 PM »
subscribed just to se where this goes
Oil Pump Kits are Available on eBay!See my eBay store!
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k6 build   http://forums.sohc4.net/index.php?topic=79833.0

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KingCustomCycles.com

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Re: Some thoughts on velocity stacks
« Reply #6 on: July 20, 2011, 12:55:00 PM »
I totally understand twotired, my wife and I shoveled 5 tons of driveway gravel before 7:00 AM to beat the heat (100 in the shade here in IN) I took two Tylenol to ease the back pain and my doctor suggested I only lift 20 pounds for 6 weeks after my sinus surgery last Monday.  Take it easy out in Ca. we'll have plenty of time for this when the snow flies.

Offline Beergineer

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Re: Some thoughts on velocity stacks
« Reply #7 on: July 20, 2011, 01:07:30 PM »
TwoTired, you're right about my statement:it's not really true. However, a carburetor/intake assembly is a control volume system and the volume of air is always the same, regardless of RPM. What I've been able to come up with is that an unobstructed velocity stack enables air at atmospheric pressure to be more effectively (not necessarily efficiently) sucked into the carb. An air box/filter/sock/obstruction will cause a pressure drop between the atmosphere and the carb throat, which is what Honda designed for. By removing the pressure drop device the amount (mass flow) of air to the carb is increased. Am I right, or is there something I missed?

As far as Laminar/Turbulent flow is concerned they're affected by a lot things. In a pipe the primary factors are surface roughness and velocity. However turbulent flow can be made laminar by using a flow straightener. google 'laminar fountain' for some examples of laminar flow at 'turbulent' velocities. A flow straightener is essentially a bunch of straws packed into a pipe, which simply encourages the fluid to flow in straight lines.

As to the original topic; A lot of people buy velocity stacks because they look cool, the drawback is that you need to re-tune your carbs. A Velocity Stack with a built in pressure drop device would allow you to have the looks without the re-tune.

Offline TwoTired

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Re: Some thoughts on velocity stacks
« Reply #8 on: July 20, 2011, 02:17:45 PM »
TwoTired, you're right about my statement:it's not really true. However, a carburetor/intake assembly is a control volume system and the volume of air is always the same, regardless of RPM.
Wait?  What?  How can that be?  RPM is related to air speed, right?  And, for any given orifice size, volume is a function of pressure differentials present at the inlet and outlet of that orifice.  Since the inlet (or vacuum source) for the SOHC4 engine is pulsed, the average pressure depression is related to the spacing between those pulses.  Surely, sustaining an engine operating at higher RPM requires more volume of air than at a lower RPM, won't you agree?

The throat pressures at the jet exit points are also affected by the venturi effect. (Bernouli's principle), which is certainly affected by air speed and volume.

What I've been able to come up with is that an unobstructed velocity stack enables air at atmospheric pressure to be more effectively (not necessarily efficiently) sucked into the carb. An air box/filter/sock/obstruction will cause a pressure drop between the atmosphere and the carb throat, which is what Honda designed for. By removing the pressure drop device the amount (mass flow) of air to the carb is increased. Am I right, or is there something I missed?
I think that's a fair grasp of the concept.
Also, a true velocity stack, also slightly compresses the air at very high velocities.  Putting more oxygen into the carb throat/engine cylinder can make the engine produce more power.

As far as Laminar/Turbulent flow is concerned they're affected by a lot things. In a pipe the primary factors are surface roughness and velocity. However turbulent flow can be made laminar by using a flow straightener. google 'laminar fountain' for some examples of laminar flow at 'turbulent' velocities. A flow straightener is essentially a bunch of straws packed into a pipe, which simply encourages the fluid to flow in straight lines.
Agreed, I've seen these.
But laminar flow does not mean the air molecules are all traveling the same speed.  The air molecules touching or right next to the tube wall are either not moving or moving much slower than the air near the center of the tube.  It is a layered effect within the tube when viewed at a 90 degree axis to the air flow.  At some increased speed, the friction between layers induces turbulence within the tube.  The flow straighteners, therefore, have a speed limitation. To keep the speeds down within the "straightener tubes" the major diameter of the tube bundles must be increased, as anything you put in the air flow channel effectively reduces total cross sectional area AND adds more area/volume of stationary molecules in the air path.

As to the original topic; A lot of people buy velocity stacks because they look cool, the drawback is that you need to re-tune your carbs.
Very true, also because they saw a track racer with them on it.  And it is a simple and cheap part to install.

A Velocity Stack with a built in pressure drop device would allow you to have the looks without the re-tune.
I'm not trying burst your bubble.  But, I do have trouble seeing where putting anything but moving air within a velocity stack, will maintain velocity stack function.  Yes, you can put a restrictor inside it, hidden from view, that may mimic the stock air box.  But, won't it also displace air volume (and the performance boost oxygen it contains) above and beyond what the stock air induction arrangement offers?

So my question is:
Is the fundamental idea to purposely sacrifice engine capability for the benefit of stack looks without carb retuning?

Cheers,

P.S. Thanks for giving me an excuse not to go back out and work in the heat for a time.  ;D
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline excerpt

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Re: Some thoughts on velocity stacks
« Reply #9 on: July 20, 2011, 04:15:50 PM »
How about you two brainiacs tell everyone how to get pod filters to work.

From hearing you two go on, I would imagine you could tell me the perfect jet sizes and needle clip position.

Don't worry about the airscrew. I'll figure that adjustment out on my own ;) lol

Offline Retro Rocket

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Re: Some thoughts on velocity stacks
« Reply #10 on: July 20, 2011, 04:23:02 PM »
Quote
How about you two brainiacs tell everyone how to get pod filters to work.

Throw them in the bin...... ;)
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Offline TwoTired

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Re: Some thoughts on velocity stacks
« Reply #11 on: July 20, 2011, 05:46:39 PM »
How about you two brainiacs tell everyone how to get pod filters to work.

From hearing you two go on, I would imagine you could tell me the perfect jet sizes and needle clip position.
Ah, you've missed a primary concept!
Jet sizes and clip positions are a specification.
The generic term "pod filter" has no specification.  (Certainly none published.)

A #100 jet from two different manufacturers will have 1mm opening in each one.

A "pod filter" from one source can have a wildly different pressure drop than one from another source, depending on filter media selection and how the media is constructed in/on the pod. 
Shortening the inlet duct brings atmospheric pressure closer to the where the low pressure is created, (even though that is a bit more predictable), while the jet exit location remain the same distance from the low pressure source.

Style changes are far more complex when physics are involved, and give a whole new meaning to "cheap".  ;D

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline excerpt

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Re: Some thoughts on velocity stacks
« Reply #12 on: July 20, 2011, 06:25:48 PM »
How about you two brainiacs tell everyone how to get pod filters to work.

From hearing you two go on, I would imagine you could tell me the perfect jet sizes and needle clip position.
Ah, you've missed a primary concept!
Jet sizes and clip positions are a specification.
The generic term "pod filter" has no specification.  (Certainly none published.)

A #100 jet from two different manufacturers will have 1mm opening in each one.

A "pod filter" from one source can have a wildly different pressure drop than one from another source, depending on filter media selection and how the media is constructed in/on the pod. 
Shortening the inlet duct brings atmospheric pressure closer to the where the low pressure is created, (even though that is a bit more predictable), while the jet exit location remain the same distance from the low pressure source.

Style changes are far more complex when physics are involved, and give a whole new meaning to "cheap".  ;D

Cheers,

That's the fanciest way of saying "I have no idea" I've ever heard! I award you 2 internets!

Offline Beergineer

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Re: Some thoughts on velocity stacks
« Reply #13 on: July 20, 2011, 07:21:17 PM »
TwoTired, you're confusing volume with mass. Since air is a compressible fluid any given mass of air can fill any number of volumes, the pressure just changes. This is why turbo/superchargers work; they cram more air into the same volume. The higher the RPM, the quicker the pulses, the more air (by mass) is sucked into the carb.

"So my question is:
Is the fundamental idea to purposely sacrifice engine capability for the benefit of stack looks without carb retuning?"

Yep. I wouldn't say sacrifice capability, just not taking advantage of full potential. These Pressure Dropping V-Stacks would not be for the performance enthusiast. Another reason is because (to me atleast) tuning a carb is a PITA. Also I had a hell of a time getting the stock air box in/out of the bike. Although that could be a personal problem.

Excerpt, I only bought my first bike 2 months ago; All I know about carbs I've learned right here. Everything else is just applying the fluff my head has been filled with over the past 4 years. If I was an expert I'd be riding by now instead of trying to figure out another solution. One of my professors worked designing carbs for 20 years, his answer to my recent questions was "fuel injection."

I don't really see the point of pods, outside of aesthetics. Instead of having one filter to provide standard flow to 4 carbs you now have 4 filters providing different flow to each carb. Just seems like a step in the wrong direction to me. Although the Sling Shot cars I've seen (which run bike engines) used pods-so there must be some advantage.

PS-TwoTired, glad I could help. Tomorrow the heat index is supposed to be 105-115 here in Rochester, guess where I'll be.

Offline Free Booter

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Re: Some thoughts on velocity stacks
« Reply #14 on: March 29, 2012, 12:22:33 PM »
what about variable  stacks?


Or a stack that could vary in the diameter of the intake orifice?