Topic: PVC velocity stacks

[davis96]

So up until now I was running the stock rubber velocity stacks which I pulled out of my airbox (if you want to argue about the dangers vs the benefits of unfiltered velocity stacks in general, please save your breath). I re-jetted and everything was working quite well. But my rubber stacks began to deteriorate and I decided to try my hand at making a set of stacks from PVC pipe using a plumbing fixture as a form and boiling water as a heat source with which to melt the pvc so it was pliable enough to fit the form. I have not found any aftermarket velocity stacks which have the correct inner diameter and allow smooth turbulence-free airflow.

the inner diameter of this particular pvc pipe is the same as the inner diameter of the carburetor inlet. It took some trial-and-error to get the process down right, but I am happy with the end result. The photos show the materials used, the resulting piece, and how the stacks look with tape securing them to the carbs.




The next step is to attach a piece whose inner diameter is the same as the outer diameter of the stack/carb inlet which will be used to secure the stacks to the carbs, like so:

[Duke McDukiedook]

So if you can deform this pipe with boiling water temperatures how will they hold up to a hot engine?
 

[davis96]

So if you can deform this pipe with boiling water temperatures how will they hold up to a hot engine?
 

We will see, but it seems to me that the intake side of the carburetor isn't exposed to very much heat at all, certainly not the 200 degree temps required to deform the pvc.

[70CB750]

Way to go, curious about some real life testing, but this is good.

[goofy3392]

I agree you should be just fine with the pvc tubing.  The carb area doesn't get that hot, otherwise fuel would boil right?

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[brooze72]

Excellent Davis, I like your thinking.  Goofy is right, heat shouldn't be an issue, PVC melts around 400F, if it gets that hot, there will be flames everywhere.  If you think you need more heat resistance, use ABS.

[Speedngheadphone]

This is so cool.  Nice work.  Keep the updates/progress coming.

[FunJimmy]

Seriously?
That is mighty cool.
Nice work.

Oh ya.....do you have any idea how bad unfiltered air is......bla, bla bla. 

[750resurrection]

I agree you should be just fine with the pvc tubing.  The carb area doesn't get that hot, otherwise fuel would boil right?

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Lots of air cooling going on there - so if it gets too hot he can crack the throttle wide open and really cool her down. 

[lone*X]

What is the plumbing fixture that you used to form the bell? 

[Retro Rocket]

What is the plumbing fixture that you used to form the bell?

Probably a flaring tool....

[HondaMan]

I agree you should be just fine with the pvc tubing.  The carb area doesn't get that hot, otherwise fuel would boil right?

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It does, if you have an enclosed fairing lowers in summer city traffic. Makes for interesting idle issues! It never did until ethanol came along, tho...

[davis96]

What is the plumbing fixture that you used to form the bell? 

It's the chrome thing in the first picture, its a shower flange of some sort:
http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=202562825&R=202562825#.UGp9KU2CkhQ

[davis96]

I agree you should be just fine with the pvc tubing.  The carb area doesn't get that hot, otherwise fuel would boil right?

It does, if you have an enclosed fairing lowers in summer city traffic. Makes for interesting idle issues! It never did until ethanol came along, tho...

I should be good to go, no fairing, no side covers even.

[750resurrection]

Seriously?
That is mighty cool.

Cool there is seriously cheap. I like.

[goofy3392]

I agree you should be just fine with the pvc tubing.  The carb area doesn't get that hot, otherwise fuel would boil right?

Sent from my LG-VS700 using Tapatalk 2

It does, if you have an enclosed fairing lowers in summer city traffic. Makes for interesting idle issues! It never did until ethanol came along, tho...

ah I see.  I did a search and found the boiling point for gasoline with 10% ethanol to be 425* F. Not sure of octane level that test was for..but also from same source, at 230*F 50% of fuel is evaporated? Not sure if I worded that right, but what I think is happening the liquid fuel is vaporizing and not flowing as well? Am I close hondaman?

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[RAFster122s]

You need to study velocity stack design more. There are lots of articles and engineering papers on the proper design. The bell mouth needs to be a certain percentage of the throat diameter. There is an optimal curve for the design as well.
the opening lip cant be sharp like you have it or it causes turbulence at the lip opening which restricts air flow and velocity into the carbs. They need a thick radius opening that curves back on itself to minimize this turbulence. This is because the air is drawn not only directly in front of the carb stack but it is also pulled from around to behind the opening. Take a vac hose from your shop vac and a lighter or match and approach the hose opening from the "backside" of the opening, you will see the flame being pulled towards the opening.  Or use a smoking piece of paper or other smoldering combustible that is only smoking a lot. The smoke  will show you the rough approximation of the air molecule movement. Adapt your stacks with a long tube to the proper opening to your shop vac  so you dont introduce a choke point and you can run crude tests using your shop or home vac. You are Going to need a significant amount of smoke for this testing that you can direct a good sized stream of smoke.
 (Think: Charcoal grill with fatty meat being cooked, plumbing 4" pipe over the lid outlet with a small computer fan in the tube to provide a pressurized source, block that to a clear 1" diameter hose to provide your pressurized smoke source. )

What you have looks interesting and could become the mold or basis for a molded velocity stack with more work. Casting resins and even injected molded plastics are within reach of the home hobbist.

Your lack of filters is your decision, if you want to rebuild your motor sooner rather than later, then you are contributing to the continued demand for parts. Good on you.

Running filters with velocity stacks then get into the minimum recommended distances from the velocity stack openings. But then you run into issues such as the frame tubing intruding into that recommended space. so you have design constraints for retrofitting the stacks and potentially the/a filter.
Have you looked at the velocity stack antipod design with the ITG filters that another forum member has designed and  produced for other SOHC/4 forum members to buy?

[Old Scrambler]

Rafster has some good points about the radius return-edge, but the effect is deminished with longer stacks.  The rubber originals are stacks with flanges......you could use thin-wall ABS and recreate them OR you could use aluminum, copper, or brass tubes and shape with heat and a similar mold for an effective shape.  Mounting them to the carbs is critical for fine tuning. 

I use a short PVC bell with a 180-degree flange on a Mikuni 29mm carb....then attach a 3-inch diameter air-craft intake hose leading to an air-cleaner about 8-inches distant from the bell. Because the source of my air is all from one direction, the shape of the bell is not as critical. The distance from the valve seat to the main-jet is about 9-inches which is just beyond the second harmonic pulse. The shorter CB750 intake is between the 1st and 2nd harmonic and works best for mid-range torque.

[phil71]

they look awesome, great job!
  I hope you're doing this outside. Melting PVC gives off a very deadly toxin... and you should be ventilating well.

[davis96]

You need to study velocity stack design more. There are lots of articles and engineering papers on the proper design. The bell mouth needs to be a certain percentage of the throat diameter. There is an optimal curve for the design as well. The opening lip cant be sharp like you have it or it causes turbulence at the lip opening which restricts air flow and velocity into the carbs. They need a thick radius opening that curves back on itself to minimize this turbulence. This is because the air is drawn not only directly in front of the carb stack but it is also pulled from around to behind the opening.

I knew there would be at least one naysayer to come out of the woodwork.  I am not an engineer, but I have done enough research to understand the elements of velocity stack design and their functions (i.e. increase laminar airflow, minimize turbulence, etc). I do agree with your assessment of my design to a certain extent. The stacks are not a perfect solution, and they are not finished yet. I plan on a few more modifications before calling them complete, namely some more sanding to take any remaining hard edges down. However I don't agree with such an unequivocal assessment of v-stack design.

My stacks are based on the design and dimensions of the Honda factory plenum stacks. I am aware that a fully radiused lip reduces turbulence and improves laminar airflow. But to say that a velocity stack "needs" to have such a lip in order to function is simply incorrect. A simple bell mouth design may not render the absolute maximum airflow/minimum turbulence, but it is not fundamentally flawed as you seem to have alluded here, and it is certainly a cheaper/more functional alternative to aftermarket velocity stacks or pod filters. To quote Tintop (designer and manufacturer of the Antipod device mentioned below): "Honda (500/550/750), Keihin CR & FCR, Weber, and Dellorto, all use a bell mouth shape.  As an ultimate example of why it is not nessescary, look no further than the no holds bared CanAm injected big blocks.  If a fully rolled lip would have made any extra power those cars would have had them."

Running filters with velocity stacks then get into the minimum recommended distances from the velocity stack openings. But then you run into issues such as the frame tubing intruding into that recommended space. so you have design constraints for retrofitting the stacks and potentially the/a filter.
Have you looked at the velocity stack antipod design with the ITG filters that another forum member has designed and  produced for other SOHC/4 forum members to buy?

Yes, I am familiar Tintop's anti-pod design. As beautifully engineered as Tintop's product is, this is rather a moot point as I have no qualms about running filter-less stacks.

I appreciate the input, and I will update this thread when the design progresses

-Davis

[Duke McDukiedook]

So if you can deform this pipe with boiling water temperatures how will they hold up to a hot engine?
 

We will see, but it seems to me that the intake side of the carburetor isn't exposed to very much heat at all, certainly not the 200 degree temps required to deform the pvc.

Like Mark said on a hot day with ethanol blended fuel in stop and go traffic deformation of your PVC could be a problem. You may want to go with a material with a higher melting/glass transition points than PVC.

[davis96]

So if you can deform this pipe with boiling water temperatures how will they hold up to a hot engine?
 

We will see, but it seems to me that the intake side of the carburetor isn't exposed to very much heat at all, certainly not the 200 degree temps required to deform the pvc.

Like Mark said on a hot day with ethanol blended fuel in stop and go traffic deformation of your PVC could be a problem. You may want to go with a material with a higher melting/plastic yielding points than PVC.

I really feel like this will be a non-issue. The only reason is that even with the temperature topping 200-250 degrees the PVC was by no means easy to form, I really had to wrestle with it. I'm all for 'better safe than sorry', but rather than toss all this stellar craftsmanship in the trash (at least as far as the temperature issue is concerned), I'm going to go ahead and take a the 'wait-and-see' approach on this one.  I am, however apprehensive with regard to the possibility that if gas were to find its way out of the carb throat, it might have an effect on the pvc material. We shall see

[Mo]

[\quote]
I really feel like this will be a non-issue. The only reason is that even with the temperature topping 200-250 degrees the PVC was by no means easy to form, I really had to wrestle with it. I'm all for 'better safe than sorry', but rather than toss all this stellar craftsmanship in the trash (at least as far as the temperature issue is concerned), I'm going to go ahead and take a the 'wait-and-see' approach on this one.  I am, however apprehensive with regard to the possibility that if gas were to find its way out of the carb throat, it might have an effect on the pvc material. We shall see
[/quote]

PVC in contact with gasoline can become brittle. This, mixed with engine vibrations can become a problem. Also, if gas does leak back from the carbs (granted it's hard to imagine that this would happen), comes in contacts with the pvc, making it brittle and cracking, then gas could leak onto the hot engine case...asking for trouble if you ask me.

They do look very good though.

[750resurrection]

So if you can deform this pipe with boiling water temperatures how will they hold up to a hot engine?
 

We will see, but it seems to me that the intake side of the carburetor isn't exposed to very much heat at all, certainly not the 200 degree temps required to deform the pvc.

Like Mark said on a hot day with ethanol blended fuel in stop and go traffic deformation of your PVC could be a problem. You may want to go with a material with a higher melting/plastic yielding points than PVC.

I really feel like this will be a non-issue. The only reason is that even with the temperature topping 200-250 degrees the PVC was by no means easy to form, I really had to wrestle with it. I'm all for 'better safe than sorry', but rather than toss all this stellar craftsmanship in the trash (at least as far as the temperature issue is concerned), I'm going to go ahead and take a the 'wait-and-see' approach on this one.  I am, however apprehensive with regard to the possibility that if gas were to find its way out of the carb throat, it might have an effect on the pvc material. We shall see

I'm sorry, after all the dire warnings I've decided to phone this in to OSHA.

  I feel so good being a responsible citizen. Think of all our children, who could...well, I can't even say it.

Is Will Robinson around ? We need some sound effects.

 (of course I'm kidding)

Can hardly wait for the running report after 5k miles.

[davis96]

I'm sorry, after all the dire warnings I've decided to phone this in to OSHA.

  I feel so good being a responsible citizen. Think of all our children, who could...well, I can't even say it.

Is Will Robinson around ? We need some sound effects.

 (of course I'm kidding)

Can hardly wait for the running report after 5k miles.

HAH the bike only has 9000 miles as it is, but I'll keep ya posted... From some of these responses you'd think I was riding around with live ammunition in my gas tank. For a bunch of grisly bikers we sure do have some nervous nellies on the forum.