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HAhaha!


Hitler said:
"What we ended up with are non-functional art projects"

I felt somewhat similar at the end of my Winter Build-off last year. :(
 

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lb/hp is what it's about!
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I listened to this at Lunch today. The shouting in German made the whole lunch room go quit and everyone was staring at me as I laughed at the video. :LolLolLolLol:
 

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Just watched this, hysterical. I'm now scared to try and start my bike. I should just trailer it to a couple of shows when I'm done and save myself the headache.
 

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lɐʇuǝɯᴉɹǝdx&#4
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I'm either modding my airbox (won't fit with the new rear shock in place) or building a custom one that has the same volume and 'choke' tube. Filter will be one of those big cold air cones.... so basically one giant pod.
 

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Is my bike ok?
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Discussion Starter · #10 ·
I have a very hard ime believing stock air boxes create a "vacuum" need for the bikes to run properly.

Um..carbs work off pressure differential. If both sides were under vacuum what would draw the fuel out?

Edit: the above was not directed at you, Seb. I just keep hearing " the factory airbox creates the needed vacuum for the carbs to work properly. " I dont see it.

All bikes have trumpets, velocity stacks, etc. of some sort in their factory air box configuration. I think it's shitty air entry that causes the issues, not the fact that they no longer share a common plenum.

I've got some ideas on this, but so far, no time to build/test any of them.

Owning a twin cam CB with stock CV carburetors and pods makes me the poster child for a non pod - friendly bike. I really wanna do some experimenting on this.
 

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lɐʇuǝɯᴉɹǝdx&#4
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I have a very hard ime believing stock air boxes create a "vacuum" need for the bikes to run properly.

Um..carbs work off pressure differential. If both sides were under vacuum what would draw the fuel out?

Edit: the above was not directed at you, Seb. I just keep hearing " the factory airbox creates the needed vacuum for the carbs to work properly. " I dont see it.
I don't see it either... but that's because I hear a very different and more logical explanation.

The explanation I hear is some of the early dirt bikes put the intake inside a shroud to keep muck from getting thrown into the intake (or the filter, if they used em back then). It was noticed that the engine ran better, and somebody figured out that the shroud created a zone of non-turbulent air that was stationary relative to the intake. Stationary air has higher pressure, so the engine was getting more air more easily.

Then some clever person who knew about
decided to make the shroud a rigid, air tight box with a tuned port, sized such that the resonance of the box corresponded to the frequency at which the engine opened its intake valves (or equivalent for a two stroke), making it so that (at a certain rpm) resonance actually increased the pressure at the intake even more, above what is possible with ambient pressure.

In short, a properly designed airbox is essentially like the box you mount a sub woofer in; at a specific frequencies it gives a kick to how much air your engine moves into the intake tract, just like a sub box give a kick to how much air the cone moves at certain frequncies. Its worth noting that at certain other frequencies this effect is REVERSED because the resonance is 'out of phase', but with a good design that's usually down where you don't really care much what the power band is doing, and / or is above the redline. Some modern sport bikes even go so far as to have servos that change the volume of the airbox or length of the 'snorkle' so that they can gain this benefit at multiple RPM, giving a broader boost to the power band and / or eliminating the 'out of phase' frequency issue.

Doesn't matter if the system uses cv carbs, slide carbs, any sort of fuel injection, or even is a two / four stroke or diesel; higher pressure at the intake when the valve (or port or whatever) opens is a Good Thing, and a proper airbox can increase that (at certain RPM). In effect, its passive super charging (in the sense that any increase in intake pressure above ambient is super charging).

All bikes have trumpets, velocity stacks, etc. of some sort in their factory air box configuration. I think it's shitty air entry that causes the issues, not the fact that they no longer share a common plenum.

I've got some ideas on this, but so far, no time to build/test any of them.

Owning a twin cam CB with stock CV carburetors and pods makes me the poster child for a non pod - friendly bike. I really wanna do some experimenting on this.
I agree on that. With pods on CV carbs in particular, there seems to be a common problem that some poorly designed pods have weird airflow patterns or even actually block off a bit of the rim of the carb intake. This means air pressure often goes to shit right where the channel that sends air to the slide lift is. Putting the pods on the ends of correctly fitted velocity stacks should avoid that, but most people don't bother... while most stock airboxes effectively have (flexible) velocity stacks as part of the design, namely the carb boots. Good airflow patterns are probably the main purpose of an airbox, making engine tuning easier mostly due to the boots and the creation of a source of non-turbulent 'dead air'. Getting rid of those boots also effectively changes the intake track length, which may further alter tuning. The resonance effect is just a further tuning trick that an airbox happens to allow, and probably only adds a few percent to HP at the specific RPM it is tuned for. Still, a few percent HP to put on the spec sheet, with effectively no weight penalty, and a manufacturing cost equivalent to a decent tupperware container; you can see why almost every bike comes from the factory with one. A notable exception would be all the HD's out there...

TLDR ZOMG WALL OF TEXT - a good air box actually increases intake pressurr via acoustic resonance. Air boxes and boots also create smooth even airflow, which pods may or may not have, and which most cv carbs really need.
 

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hate us cuz they ainus
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I thought the whole problem with CV and pods was the fact that the rubber boots on the pods block the little air passages around the mouth of the carb? Like the idle circuit..... I think that's what its called then you have the humongous one that I think operates the slide right? After the pods they now have a huge wall beside them and they don't work as well. Its pretty much the same as having the valves in your head to close to the cylinder wall. Anyhow thats how it was explained to me once by someone here.
 

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lɐʇuǝɯᴉɹǝdx&#4
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I thought the whole problem with CV and pods was the fact that the rubber boots on the pods block the little air passages around the mouth of the carb? Like the idle circuit..... I think that's what its called then you have the humongous one that I think operates the slide right?
Yeah, some people claim that's the problem, and that they have fixed it by either altering the pod not to block that port, or altering the carb by drilling a hole from the carb throat into that passage. I dunno, but I suspect results are highly variable depending on the carb and pod in question. In theory there's no reason that it shouldn't work if you can ensure a good even airflow to the entire carb oriface... in practice, airflows can turn into a turbulent mess for no obvious reason. If I wanted pods on my carbs, I'd put my money on having pods at the ends of velocity stacks, or something similar.
 

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lɐʇuǝɯᴉɹǝdx&#4
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As do I, mostly for looks (using clear / colored plexi, for example). But designing one from scratch seems to require figuring out what the actual resonant frequency would be with it hooked up via the boots to the carbs is beyond me. That's why I'm just gonna modify the stock box, or build one with the same volume, and then run the stock snorkle, or something with the same cross section area / length. Its tempting to think about some method of dynamically altering snorkle length to continually keep resonance matching engine RPM, but I expect its more hassle than its worth.

I suppose if you don't have a box for a bike, finding one for a bike that has similar intake setup (carb size, aprox intake length) and number of cylinders and basing your build off that could work. Probably also best if the bikes have similar redline, as well.

One thing to keep in mind is that you may want to avoid flat, parallel walls in your 'box'. You'd still get the desired Helmholtz resonance, but you'd additionally have resonance at the frequency determined by the distance between the 2 parallel walls. From what I've seen, stock air boxes seem built with sloped / curved walls, maybe just to fit in the space the get put in, but maybe also for this reason. The same idea applies building a music studio room; if none of the walls are parallel, you don't need to worry as about the acoustics coloring the frequency balance.
 

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hate us cuz they ainus
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I'm bored so.... Here's what I found.

Helmholtz theory can be applicable to silencer design (intake and exhaust), but it is not the right intake duct performance design criteria for a high-speed 4-stroke engine. Helmholtz has been used in intake tuning for low and medium-speed engines (below approximately 2000 rpm), but open-closed end organ pipe theory provides the predominant tuning effect at higher speeds.
The only application of what you're describing that I'm aware of, with one cylinder charging another with a positive reflected wave at port closing when the other is opening, are naturally aspirated Mazda 2-rotor Wankels. Look up the RX-8 intake design and you'll see that it's a complex network of several Y-branches with valving to enable different Y-junctions with speed and variable port area/timing changes. This works because it operates with the firing frequency of a 2-cylinder 2-stroke and the second rotor needs the first rotor's ramming wave less than 180 degrees later. On a 4-cylinder with 1-2-4 manifolding to accomplish what you're describing, the tuned lengths would be much longer. I'm curious how it would work though and it wouldn't hurt to parameterize a few things in WAVE and give it a try.

More common is to treat the plenum as an atmosphere from which the cylinders draw and assume that there is no acoustic cross-talk from cylinder to cylinder. Using organ pipe theory or someone's constants based on testing and organ pipe theory (my testing and simulations always matched Blair's), the ramming depends on the speed of sound and the length of the duct from the bellmouth to the valves, following the centerline of any bends. This is the length over which the wave will travel and reflect when the intake valves are closed. You will have your choice of several lengths that create several ramming peaks at different speeds that you can use to shape the torque curve as you desire.

http://www.fsae.com/forums/showthread.php?11903-Another-Helmholtz-Resonance-Post
 

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lɐʇuǝɯᴉɹǝdx&#4
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I don't think one rules out the other- the airbox can have pressure waves caused by port mass / volume relationship and the runners can resonate like pipes. For somthing that's not intended to be a Helmholtz resonator, it sure seems to have all the parts needed to tune one.

At 6000 rpm, the esonant frequency needed for an I4 airbox would be 200hz, which seems quite possible to tune for.

It would be interesting to set up a small speaker and a sound pressure meter inside an (installed) airbox, see if you can actually produce resonance at a useful freqency.

Relevant video - https://www.youtube.com/watch?v=rBXe2FV9WnE&t=4m20s
 
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