Airbox vs Pods vs Velocity Stacks

On a bike like a TZ250, you can actually see a fuel standoff behind the carbs at certain revs. It's the resonance effect.

MOTA is a piece of simulation software for 2 strokes and one of the things it does is to create a graphic of waves in each of the ducts (intake, transfers and exhaust). It is amazing to see that the pressure waves oscillate 3 or more times even at high revs. That means that the pressure wave is basically going in and out. Think of that as pulses back and forward. Each reversion is smaller than the one before it, but it's easy to see how it would screw up fueling.

I hear about helmholtz resonating air boxes but have no reason to believe that any manufacturer was doing that sort of analysis back in the day. What they did want was smooth flow and a still air box is the way to get that. That's also why many vehicles have a plenum chamber in the intake system design. Not so much to make it work really well at one rpm, but to make it work at all revs.

The other advantage on a still air box is that it is essentially not affected by cross winds or airflow past a rider's leg which often causes issues with pods and stacks. And that's why they typically have the intake in a relatively still area like under the seat.
 
farmer92 said:
Close, physicist
I think it would work better is if the carb boot was longer... Way longer.
The pods/stacks might not be so bad then.
You don't want the pressure to build up in the carb, it will force the fuel back down the jets...

The momentum of the air is an amazing thing, look up pulse jets if you haven't heard of them

When you say way longer, how long do you mean? 2 inches? 3 inches? 4 inches? I have two, dual oval K&N filters for all 4 carbs. I'd rather modify those to make it work than screw around with the air box.

Teazer, a plenum chamber would be an awesome solution, but I don't think I have the knowledge or skills or money to create one for my bike. It almost seems like over engineering for a simple bike.
Can't stacks help with issues of crosswinds?
 
thwrightstuff said:
See, this is the problem I seem to run into, lots of conflicting arguments. Sounds like pods are pointless and only run stacks i'm highway riding a lot of racing a lot. I'm not against using my air box, but the problem is it's a massive pain in the ass to get the carbs in and out with that thing in there.

Farmer, are you by any chance a mechanical engineer?

What you have is people with opinions vs. people with facts. The fact of the matter is, you can run pods and get them to work well, but there is a way to do it and a way not to. It can run "well enough" for some people if you just slap some pods on and tune. However, if you pull the intake boots off of your stock airbox, you will see that they are, in fact, velocity stacks which are well tuned for your application. If you can find a pod or pancake filter that fits on the end of that stack you might alleviate a lot of the tuning issues you're having. But then again, maybe not.

Carbs are a fickle beast.
 
coyote13 said:
What you have is people with opinions vs. people with facts. The fact of the matter is, you can run pods and get them to work well, but there is a way to do it and a way not to. It can run "well enough" for some people if you just slap some pods on and tune. However, if you pull the intake boots off of your stock airbox, you will see that they are, in fact, velocity stacks which are well tuned for your application. If you can find a pod or pancake filter that fits on the end of that stack you might alleviate a lot of the tuning issues you're having. But then again, maybe not.

Carbs are a fickle beast.


So then by this logic, Couldn't I just get stacks with uni filters and attain the same results?
 
There is pretty much no escaping the airbox if you are keeping drivability as a priority. Anything else will simply have drawbacks that you will have to weigh against the benefits. Perhaps it is easier to wrap your head around if you start with the fundamental fact that the intake tract will always be a tube of some sort with an infinitely variable frequency wave traveling inside it. The waves occur because of the stop/start nature of the airflow due to valve opening and closing. The variable is due to constantly changing engine rpm. While it is a fairly simple in concept to want to capitalize on this phenomenon, the actual calculation gets complex as wave speed is impacted by temperature and pressure which is continuously variable along the length and ultimately you make something close and then experiment to fine tune. So on a racing application, you may be mostly concerned with max power at some rpm that will (hopefully) occur at some calculated rpm (i.e you have designed your cam(s) for max power just before the crankshaft grenades). You design the intake length to agree with that, and you use velocity stacks for the volume flow characteristics and sharp entry for the wave activity. With a little fussing with the actual length you hopefully get optimal power. Then you ride the bike and continue adjusting for the problems created by the inconsistent air speed around the stack inlets. Hopefully you arrive at a compromise that gets good lap times. This commonly results in either a dreadfully running or slower than necessary street bike setup because if you get it right like the race bike, the intake tract tuning will be poor everywhere except max power rpm so driveability will suffer and the wind problem will still be there for all rpm.

If you abandon the stacks and go with pods, your tuned intake length will simply be the carb entry more or less. The individual filters will have a softening effect on the air speed problem but it will still be there. This is why you can have a situation where you get the thing to run pretty well in 2nd and 3rd gear but very poorly in 4th and 5th. Still, most bikes (in my experience) can be made to run pretty nicely with pod filters if they are of good quality and their physical construction does not create another problem like covering air jet supply inlets. They are often the most practical means of choosing the lesser of evils or more accurately just a preferred compromise. Lots of old bikes have pretty restrictive stock air filters so possibly going to pods makes an improvement after proper tuning that make them worth the sacrifice of a still air box. And if you like the look maybe it is a good plan as open stacks will greatly shorten engine life expectancy - catastrophically on a two stroke.

The still airbox is the obvious solution, which is why your bike was manufactured with one. Solves the wind problem (mostly at least - pretty hard to contrive the inlet to it that is actually not afflicted somehow by the wind, but certainly massively better than pods or stacks stuck directly out in the breeze). Solves the air filtering problem potentially as you at least in theory could have a very large super nonrestrictive element and now rain is no longer an issue as well. And now you can design the intake length with intake trumpets like the race bike except that now you have filtered air and uniform pressure supply air at all speeds. And if you are exceedingly clever you design the box volume (this would be the Heimholtz resonator part) and the tuned length of the inlet to the box itself to help out bad spots in the rpm range created by any focus on the max power tuned length of the primary intakes. Of course all that is also compromised by the need to actually fit everything on the bike along with other incidental parts(like an electrical system!) and you can see the results on most any new performance vehicle.

So what do you do? Obviously the straight forward path is to keep the stock airbox and alter it to incorporate the biggest least restrictive filter possible and tune to that. Otherwise design your own to incorporate intake bells, resonant frequency and inlet (quite a job even for the manufacturers), or go with pods or stacks and resign yourself to the fact that likely there will be compromises and sometimes very frustrating and time consuming carb tuning. LOTS of people do this and are very happy with their choices. Good idea to save all the oem parts though - just in case you want to go back.
 
Subscribed :)
I've always recommended to my customers that they use a high flow filter inside their stock airbox.Couldn't we find a way to do some special custom painting to the stock airbox exterior and then say it's a 'custom' airbox :D ;D
 
There must be a custom option that allows for easier install and uninstall of the carbs with an air box. I just tried, unsuccessfully, to install my oem air box again and it was so enraging I wanted to take a sledge hammer to my whole bike and give up on motorcycles. There either must be an easier way to install these damn things or a custom box to make this easier. There is no way in hell I can see myself doing this several times trying to tune the carbs with this setup. I might literally go insane if that's the case.
 
Autodesk's Simulation CFD
Wonderful software for fluid/heat flow, requires some good computing power to work well though
 
thwrightstuff said:
There must be a custom option that allows for easier install and uninstall of the carbs with an air box. I just tried, unsuccessfully, to install my oem air box again and it was so enraging I wanted to take a sledge hammer to my whole bike and give up on motorcycles. There either must be an easier way to install these damn things or a custom box to make this easier. There is no way in hell I can see myself doing this several times trying to tune the carbs with this setup. I might literally go insane if that's the case.
Sounds like you should just leave the thing alone and ride! Wrenching is supposed to be fun..
Cb650's also dont have the room to play around with intake lengths and pods, frame gets in the way. Which you have already noticed! ;)
Plus that motor just runs better with the box and gains are hardly noticeable without it. Rode a few and they are hopelessly civilized and tired old lumps..
 
In the 80s a UK company called Ledar made extension tubes to mount K&Ns further from the carbs, about 2" if I remember correctly.
I had a tuned RD350LC that ran TZ750 reed cages, it wouldn't run properly with the K&Ns directly on the carbs but ran fine with the extension tubes.
 
Finnigan said:
Farmer, do you have working simulations in autodesk?

Used it a few years ago, have not used it in a while, I like Inventor also, good for strength analysis
 
I'm determined to have a smooth riding bike so I'm just going to power through it and get the air box to work. I care more about function, not form, even if it means busted knuckles and cursing up a storm.
 
thwrightstuff said:
There must be a custom option that allows for easier install and uninstall of the carbs with an air box. I just tried, unsuccessfully, to install my oem air box again and it was so enraging I wanted to take a sledge hammer to my whole bike and give up on motorcycles. There either must be an easier way to install these damn things or a custom box to make this easier. There is no way in hell I can see myself doing this several times trying to tune the carbs with this setup. I might literally go insane if that's the case.

Usually there are 2 reasons why this is hard. First, old bikes have shrunk and stiff rubber manifolds both on the inlet and outlet sides of the carbs. They can be awful - really awful. Replace them if they are. Second is the unwillingness to do enough disassembly (and re-assembly!) to make the job easier. It can be time consuming. After 20 (or 30 or 100!) times you will find it a lot easier! If several times sounds like a lot, I suggest a new mind set or seek a pro to do the work. One of my favorite and most invariably true sayings is "If it's hard, you're doing it wrong". This is one of the most notable examples of a sometimes exception.
 
jpmobius said:
Usually there are 2 reasons why this is hard. First, old bikes have shrunk and stiff rubber manifolds both on the inlet and outlet sides of the carbs. They can be awful - really awful. Replace them if they are. Second is the unwillingness to do enough disassembly (and re-assembly!) to make the job easier. It can be time consuming. After 20 (or 30 or 100!) times you will find it a lot easier! If several times sounds like a lot, I suggest a new mind set or seek a pro to do the work. One of my favorite and most invariably true sayings is "If it's hard, you're doing it wrong". This is one of the most notable examples of a sometimes exception.
Or send them off to ichiban. He knows all about that shit
 
Going to be interesting for me, running pods in a cb750 that's now an 811 and a 4into1 motogpwerks pipe. I know cognito has a really nice filter that goes over all 4 carbs. I don't know much about airflow buy maybe it will give a more even air flow because of its design? Pricey at $230 but it's re-buildable


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That Cognito box looks nice, very similar to the Steel Dragon breadboxes. I had the SD box on for a couple rides on my CB750 before I pulled the motor, seemed to be a little flat through the middle of the powerband (though I didn't take the time to really tune it before pulling the motor out)
 
julian.allard66 said:
In the 80s a UK company called Ledar made extension tubes to mount K&Ns further from the carbs, about 2" if I remember correctly.
I had a tuned RD350LC that ran TZ750 reed cages, it wouldn't run properly with the K&Ns directly on the carbs but ran fine with the extension tubes.

Absolutely correct but the biggest change was the use of LEDAR (Lincs Engine Development And Racing, IIRC) emulsion with taller spray bars to tilt the fuel slope. Check out a really late RD400 and you will see that the #284 series emulsion tubes look like a copy of the LEDAR parts with tall spray bars. That was an innovative solution combining two stroke spray bar technology onto a bleed, 4 stroke, style emulsion tube. But Leon Moss was a clever dude. RIP Leon.

Here's one site to start thinking..http://www.calsci.com/motorcycleinfo/Airboxes.html

http://www.tl1000s.info/Randys_TL1000S_Site/Airbox_design_v1_resonance.html

http://www.autospeed.com/cms/article.html?&title=Building-and-Testing-an-Airbox&A=113248
 
teazer said:
Here's one site to start thinking..http://www.calsci.com/motorcycleinfo/Airboxes.html

http://www.tl1000s.info/Randys_TL1000S_Site/Airbox_design_v1_resonance.html

http://www.autospeed.com/cms/article.html?&title=Building-and-Testing-an-Airbox&A=113248

Now you've done it!

(great info though - gonna archive it and pretend I didn't read it - mobius project #962: design good looking Heimholtz resonator air filter housing to correct flat spot on latest engine project!)
 
This is a conversation that needs to be read. Absolutely wonderful input from everyone. God I love this forum.
 
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