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Velocity Stack Formula

This is a discussion on Velocity Stack Formula within the Technical forums, part of the Caferacer.net Forums category; Hi guys. Searched and looked around but couldn't find a direct answer to this question. I just put together a '72 CL175 for an 660 ...

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  1. #1
    Senior Member nixoid's Avatar
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    Velocity Stack Formula

    Hi guys.

    Searched and looked around but couldn't find a direct answer to this question.

    I just put together a '72 CL175 for an 660 mile endurance race. Will be running WFO for 11 hrs min and wanted to put a set of Velocity Stacks on it. I am running stock carbs on a stock motor which currently has the stock exhaust that I ran a 3/8ths bit up the center of about 12-15". My plan is to add a set of CB200 head pipes with turnouts if I can get them to fit around my motobit rearsets.

    Really what I am looking for is a list of the variables which I will need to collect and the formula to use to choose the proper Velocity stack. You may have already figured out that I am a mechanical novice so any help written for dummies would be appreciated.

    In my search I noticed that Johnny B seems to know this stuff fairly well... If you're out there listening please chime in or hit me directly with an email

    I found a reference to the Helmholtz Resonance and looked it up - mind boggeling to a simpleton such as me: http://en.wikipedia.org/wiki/Helmholtz_resonance nice reading though.

  2. #2
    Moderator jbranson's Avatar
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    My first question would be why do you WANT to put a set of velocity stack on the carbs? Yes they look cool, but in your application I don't know if you'll net any extra power from them, maybe even lose some power. And running at WOT for that long you'll want some filters. You should consider running some K&N pods if you absolutely don't want to run the stock setup.

    For stacks, the first thing you'll want to check is availability. Can you get them to fit your carbs? If so, about 3" long is going to be very close to what you want.

    Maybe some of the west coast 160/175 racers will chime in, I know some of them have made a study of velocity stacks on these bikes.
    If it's any help, my little helmholtz program specifies an intake tract length from valve head to end of carb/stack of 8" tuned for 9,500 rpm, on a 175 with stock carbs and intake.
    JohnnyB

  3. #3
    Senior Member nixoid's Avatar
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    My intention is to streamline the bike as much as possible so I planned on doing this to help break up the air turbulance as much as possible while saving weightand reducing the amount of gas washing down the cylenders. My brothers own a screening business and they gave me a really nice screen with stretch that has a fine weave to reduce the junk going into the carbs

    The 9500 rpms is where this thing will live. Does this program also tell you how wide the final dimension should be? BTW thanks for running that for me and the explanation.

    If you think this is a useless mod then is there any other suggestions you might make to get the same results? Coils? wires? plugs? Points? Anything a shadetree mechanic can do with 4 weeks and decent mechanical knowledge?

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    Senior Member hahnda's Avatar
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    I'm guessing this is for the Lake Erie Loop coming up?
    http://www.scramblercycle.com

  6. #5
    Senior Member nixoid's Avatar
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    quote:Originally posted by hahnda

    I'm guessing this is for the Lake Erie Loop coming up?
    You got it. Watch the video: http://www.lakeerieloop.com/video.html a real gas.

  7. #6
    Senior Member Bateman147's Avatar
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    If you're keeping the stock carbs (which are tiny little 18mm units) I wouldn't bother using anything but the stock airbox/filter.

    Using any other carbs though will mean that you will be custom fabricating intake manifolds for it. You could try 160 carbs - they might fit in place of the 175 carbs on the stock manifolds and are 20mm. I haven't tried that.

    If you're trying to do it on the cheap I'd stop there. If you're not I'd go straight to Keihin PE22 or PE24 carbs and custom made intake manifolds. Use the biggest foam sock or K&N air filter you can fit. I can supply the carbs if you want and might be able to be talked into making manifolds, but they wouldn't be cheap. If it'll be raining when you're running you'll have to fabricate rain shields as well or some sort of airbox.

    If you're dead set on using velocity stacks I can make you some adaptors that will fit the PE carb inlet bell and allow you to use the velocity stacks that are sold for Keihin FCR carbs and allow you to use big foam sock air filters over the whole thing. Stacks are available in either plastic or aluminum in various lengths - but not particularly cheap. You'll also have a harder time fitting the carbs on that bike with that setup and your intake manifolds will be harder to make. JB's intake length is a great target for starting to experiment. You'll spend hours on a dyno final tuning the length of the whole getup.

    I wouldn't bother with screen. It'll be <way> more restrictive than filters and won't filter out anything but the big rocks. I highly recommend running filters. You'd be amazed how much grit is in the air and how fast your motor will wear out from it. Your intake valves will become mirror image studies of the shape of the intake valve seat, and your rings and bore will wear rapidly. I won't even run a race bike without filters unless it's an RS125 that gets a new piston/rings every 250 miles.

    Other suggestions would be to ditch the points and replace them with a Pertronix ignitor setup. I have the kit in stock. It's a set it and forget it type of system. Also not particularly cheap though.

    Michael
    www.batemanracing.com



  8. #7
    Moderator jbranson's Avatar
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    Nix, Bateman is the man when it comes to these kinds of mods. There's not much of a halfway to do what you want. If it's power you want...it's better all the way or leave it stock.

    If you are concerned about durability, and sustained long range speed. My first labors would be directed towards making the bike lighter, and more aerodynamic.

    Michael...what do you think...a fairing? Wouldn't that be of considerable value at sustained 60+ mph speeds? Maybe more-so than trying to squeeze out another couple hp?

    Technically I'd think that the stock setup with side covers installed is slipperier than stacks or pods.

    You could probably stick 160 carbs right on there if you used the head side of some 175 intakes, and the carb side of some 160 intakes and welded them in the middle. Heck...maybe even just cut them, smooth the cuts, splice them together with some rubber hose and clamps?
    JohnnyB

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    Senior Member nixoid's Avatar
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    Well now I've heard from two guys I feel comfortable trusting their opinions on that say I'm nuts to continue down this road unless I'm willing to go full on no BS. Not the road I'm looking to travel. I have a set of UNI filters that have an ID which is too big for the OD of the Carbs [smallest set available].If I can figure out a way to seal that up do you think these will work out alright guys?

  10. #9
    Senior Member Bateman147's Avatar
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    I agree with JB - a fairing would be good, especially for the distances you're planning on covering. Would really help with comfort and will help top speed and mileage as well.

    www.batemanracing.com/temp2.html shows some pics of one of the Glass From the Past fairing options mounted on Tim's vert 175 racer. There are similar fairings made with a headlight hole cut out. You could also use a handlebar mounted screen though it'd be easy to get it too big. If you were comfortable in a racer crouch using rearsets and clipons the GFTP fairing would do you very well. A little pad on top of the tank for you to lay on so you could rest your arms would make it pretty comfortable, depending on how tall you are.

    Craig Vetter made a cool fairing for 160s at one time but I doubt you'd be able to find one. See http://www.craigvetter.com/pages/Vet...Series800.html

    You could try the pod filters under the stock sidecovers but I doubt they'd help you much. Those little carbs are going to run out of breath pretty early anyway. Is there a chance of rain? If so I strongly recommend sticking with the stock airbox - cut holes in the top somewhere if you're worried about breathing - but I doubt it'd make any difference.

    You could probably make 160 carbs work by cutting stock 160 manifolds and putting them back together with hard rubber hose and hose clamps - shorten them a bit and clock the flanges around to match the 175 head. Might just fit in and allow you to keep the airbox. Would make a pretty big difference to top end power compared to the 175 carbs. I haven't tried to fit that all up though so you'd have to look at how the manifolds matched the head and heat isolator - I think the 160 manifold will bolt onto the 175 head as the mounting bolt spacing is the same. Assuming you could find 160 carbs and manifolds cheap that were worth saving.

    Michael


  11. #10
    Senior Member nixoid's Avatar
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    Thanks for the thoughts Michael. I'm kind of travelling down that road already.

    I put a set of rearsets on it, a 4.5 gallon tank and I have a tankbag that will act as a pad/ rain gear holder with a generic Emgo fender. I'm going to try and fashion a clear screen to place over the headelight to help decrease the drag created by the headlight cavity. Maybe Next year I'll get more adventureous but at this point my budget is pretty much reached it's end and I'm more worried about having the bike tested and reliable at this point. If I can't do it on the cheap and quickly I'll have to table it till next years build.

    Got any more suggestions? for this year or next.

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