Airbox vs Pods vs Velocity Stacks

thwrightstuff said:
I hate to admit it, but I think I may have to go back to the stock air box then. In the case of a custom box, I'd imagine that as long as the inlet of the box can allow the same flow rate as the stock box, the amount of dead air inside the box is negligible. So couldn't I theoretically get a custom box that would fit and look like the pods I have on and still get the same results?

It is hard to imagine that any company has spent as much time and money researching and developing air flow solutions for that motor as Honda.
 
In theory maybe, but the interior design of the air box has as much to do with intake velocity as the intake runners do. Spare yourself the headache and go back to the stock box and jetting. I agree that pods look cool on certain bikes, but the CB650 really isn't even one of them because of the way the frame is put together.
 
julian.allard66 said:
I've wondered on my own CB400 whether you could move the airbox further back to hide it, and then use tubes disguised as pod filters to get the look of k&ns without the hassle of setting them up.

Quoted my own post because my fat fingers had trouble with the phone.
 
I think we all want a stock airbox that look exactly like some trashy pods! :D if people just could get over the "airboxes are hideous" thing and move on with their lives..
Guess im a lucky dog.. all my bikes run right. Clever as well, since i dont ride inline 4's with mangy pods. They are an insult to any carb.. and will only work half decent if you lengthen them at least 2" and make sure they have bellmouths inside.
 
Lots of voodoo around this, stock airboxes use acoustic resonance to increase volumetric efficiency.
Nothing us lot would understand without the proper degree in engineering unfortunately, and we dont need to. Its all been done a million times already.
 
Run a stock plenum and make an adapter for a round clamp on k&n or just get a uni drop in. It's not worth playing with pods and they'll never be as efficient.


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www.lostboysatomicgarage.com
 
I have loads of build plans but never do anything about them because I'm too busy riding the bike - in the UK a lot of show bikes don't run very nicely and are horrible to ride.
 
datadavid said:
Lots of voodoo around this, stock airboxes use acoustic resonance to increase volumetric efficiency.
Nothing us lot would understand without the proper degree in engineering unfortunately, and we dont need to. Its all been done a million times already.


I doubt that is true when talking about 1970s and 1980s bikes. There has been a lot of design with modern bikes, but that takes computing power. The 1980 Honda 650 was still designed with slide rules. Plus if you look at it, it is probably just a box designed to be a large as possible to fit the space in the frame and accommodate all the other items they wanted.
 
Rusnak_322 said:
I doubt that is true when talking about 1970s and 1980s bikes. There has been a lot of design with modern bikes, but that takes computing power. The 1980 Honda 650 was still designed with slide rules. Plus if you look at it, it is probably just a box designed to be a large as possible to fit the space in the frame and accommodate all the other items they wanted.

Well the simple answer is no.
Yamaha did that in the 70's, both intake and exhaust.
The xs1100 stock exhaust is still to this day a highly accomplished engineering piece. Google it!
 
Uh Huh. tell that to all the engineers before the computer age. My grandfather's both were engineers at Fisher Body in Lansing MI and they would likely Gibbs you in the back of the head for that comment ;)

GW300H169
 
Rusnak_322 said:
I doubt that is true when talking about 1970s and 1980s bikes. There has been a lot of design with modern bikes, but that takes computing power. The 1980 Honda 650 was still designed with slide rules.

This is a silly statement. Why don't you now explain why the velocity stacks on the 650 are the shape they are if Honda were just throwing things together.
 
datadavid said:
Lots of voodoo around this, stock airboxes use acoustic resonance to increase volumetric efficiency.
Nothing us lot would understand without the proper degree in engineering unfortunately, and we dont need to. Its all been done a million times already.

This is true, but the V.E. Is only for certain rpm's.
Its like a Helmholtz resonator, or when you open your car window going down the highway.
The airbox is designed for the rpm range the engine is most likely to run at.
When the valve opens, the vacuum is there and the air rushes towards the intake, as the valve slams shut, the air is carried by its momentum and pressure builds up at the intake.
This is when you want the valve to open for max V.E.
At some point, the momentum dies and the air shoots back into the air box, creating a low pressure zone at the intake.
If the valve opens then, the engine will not get as much air/fuel as it could have
The pressure wave will at some point hit the entry of the airbox and push air out, before bouncing back to the engine intake. The wave losses amplitude over each oscillation

These waves travel at the speed of sound, so it's some fun calculations to determine how long it takes for each pressure oscillation.
The higher the velocity of the air, the more momentum it will have, the higher pressure that will build up at the intake valve, the more time the pressure will be there before bouncing back.

Edit: thats why stacks are much better when used in a high revving engine
 
farmer92 said:
This is true, but the V.E. Is only for certain rpm's.
Its like a Helmholtz resonator, or when you open your car window going down the highway.
The airbox is designed for the rpm range the engine is most likely to run at.
When the valve opens, the vacuum is there and the air rushes towards the intake, as the valve slams shut, the air is carried by its momentum and pressure builds up at the intake.
This is when you want the valve to open for max V.E.
At some point, the momentum dies and the air shoots back into the air box, creating a low pressure zone at the intake.
If the valve opens then, the engine will not get as much air/fuel as it could have
The pressure wave will at some point hit the entry of the airbox and push air out, before bouncing back to the engine intake. The wave losses amplitude over each oscillation

These waves travel at the speed of sound, so it's some fun calculations to determine how long it takes for each pressure oscillation.
The higher the velocity of the air, the more momentum it will have, the higher pressure that will build up at the intake valve, the more time the pressure will be there before bouncing back.

Edit: thats why stacks are much better when used in a high revving engine
Yes! Thats exactly what my automotive engineer buddy tried to explain, sending my welder brain spinning, but i got the message somehow..
Also, if you dont get these pulses tuned correctly, it will drag a/f mix back and forth along the intake tract, creating seriously rich conditions in worst case.
This can be clearly seen when running open stacks at certain rpm's, carbs shooting vaporized mix out the mouths, then sucking it back in.
 
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?
 
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
 
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
Thats why i fabricate extensions of at least 2" length when running pods. Though i prefer oval k&n's
 
Tune-A-Fish said:
Uh Huh. tell that to all the engineers before the computer age. My grandfather's both were engineers at Fisher Body in Lansing MI and they would likely Gibbs you in the back of the head for that comment ;)

GW300H169
Haha! Yes, and since yamaha is a musical instrument maker since day one, they are bound to have an army of engineers doing nothing but calculating acoustics..
 
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