I was just curious what the different components of the Air Box do, and why there's two different sections. I also know that within the throttle body air box there are individual trumpets. Does this mean putting open trumpets on would be simple, or would that involve extensive modification? Basically if someone could just explain the Air box in depth and the effects of modifying or editing parts of it.

For example what is the point of the open trumpets here, besides looking cool and how is it interacting with the air box system.


Thanks
DBRMN

Looking cool is all they're doing. I was instantly suspicious when I saw the plenum chamber, so I dumped the picture into PhotoShop and blew it up - sure enough, the airflow meter has been moved round and plugged into the back of the plenum chamber. No attempt to disguise it  

Because of the interplay between the airbox, the AFM, the FICU and the plenum chamber, it is just about impossible to modify the air intake system on a 2-valve K. It's a whole different ballgame on the 4-valve engine where there is no AFM. Motorbike Mike on here tried fitting a 4-valve FICU to a 2-valve engine and disposing of the AFM (yes, it needed a new wiring loom too) but found the the engine was wildly overfueling at times.

In short, the only modification you can do is to dispose of the airbox and to plug the AFM directly into the plenum chamber. You'll then need an air filter on the intake side of the AFM. What that does to the engine response I don't know, but I doubt it's good because by removing the airbox, you've removed a huge reservoir of air ready for the engine for the times when the intake airflow is disturbed.

@Dai wrote:Looking cool is all they're doing. I was instantly suspicious when I saw the plenum chamber, so I dumped the picture into PhotoShop and blew it up - sure enough, the airflow meter has been moved round and plugged into the back of the plenum chamber. No attempt to disguise it  

Because of the interplay between the airbox, the AFM, the FICU and the plenum chamber, it is just about impossible to modify the air intake system on a 2-valve K. It's a whole different ballgame on the 4-valve engine where there is no AFM. Motorbike Mike on here tried fitting a 4-valve FICU to a 2-valve engine and disposing of the AFM (yes, it needed a new wiring loom too) but found the the engine was wildly overfueling at times.

In short, the only modification you can do is to dispose of the airbox and to plug the AFM directly into the plenum chamber. You'll then need an air filter on the intake side of the AFM. What that does to the engine response I don't know, but I doubt it's good because by removing the airbox, you've removed a huge reservoir of air ready for the engine for the times when the intake airflow is disturbed.

So in short any modification is a bad idea. Can you explain the function of each section, for example what is the plenum chamber?

Brief explanation, because I'm damned if I'm going into the thermodynamics of it!  

The air box takes air from the air intake and both filters it and acts as a very short term reservoir. From there, it goes through the AFM to the plenum chamber. The AFM does exactly that; it measures the air flow demanded by the engine and passes the data on to the FICU to determine how much fuel is needed. The plenum chamber acts as an equalising chamber for the four intake trumpets (venturi) because the cylinders pull air at different times. Each of those venturi acts as a restriction in the air flow which accelerates the air into the cylinder and jams more air in than would be possible without the venturi. Google 'venturi effect'.

Anyone got anything to add to that very, very brief description?

@DBRMN wrote:For example what is the point of the open trumpets here, besides looking cool and how is it interacting with the air box system.

I think the only point of those open trumpets is decoration, similar to a 1953 Buick's Cruiserline Ventiports but with even less purpose.    The whole image looks like a concept illustration rather than a real moto. Do you have a link to it?

Good explanation, Dai. 

I would only add that requirements for a 100,000 mile service life on the air filter by the EPA dictated it's size and the size of the airbox below it.  The design criteria of the box is to allow uniform airflow across the entire surface area of the filter.

I just wanna know how they get them to keep from falling down when they're parked without a side or center stand.

@Point-Seven-five wrote:I just wanna know how they get them to keep from falling down when they're parked without a side or center stand.

It has a gyroscope linked to pump-activated helium-filled stabilizer bags linked to storage tanks. The whole system is hell on battery life.

@Dai wrote:Brief explanation, because I'm damned if I'm going into the thermodynamics of it!  

The air box takes air from the air intake and both filters it and acts as a very short term reservoir. From there, it goes through the AFM to the plenum chamber. The AFM does exactly that; it measures the air flow demanded by the engine and passes the data on to the FICU to determine how much fuel is needed. The plenum chamber acts as an equalising chamber for the four intake trumpets (venturi) because the cylinders pull air at different times. Each of those venturi acts as a restriction in the air flow which accelerates the air into the cylinder and jams more air in than would be possible without the venturi. Google 'venturi effect'.

Anyone got anything to add to that very, very brief description?

Wow I love this kind of stuff its amazing how complex just the air system can be, ye I'm definitely not going anywhere near that to modify it. Thank you so much this explained alot.

@Laitch wrote:

@DBRMN wrote:For example what is the point of the open trumpets here, besides looking cool and how is it interacting with the air box system.

I think the only point of those open trumpets is decoration, similar to a 1953 Buick's Cruiserline Ventiports but with even less purpose.    The whole image looks like a concept illustration rather than a real moto. Do you have a link to it?

This is a video about the bike but the original creators are from Brazil, its one of the bikes hat inspired me to chose the K100.

@DBRMN wrote:This is a video about the bike but the original creators are from Brazil, its one of the bikes hat inspired me to chose the K100.

Thanks for the video link.

As soon as I heard the guy with the robo-voice that sounds like he's about to swoon in his excitement about this moto, I recalled having viewed it previously. I'd sure like to understand the function of those velocity stacks because, as Dai has observed, the engine still seems to have an air flow meter attached to it elsewhere.

Found something really interesting, look at the video below at about 6:30, this guys completely gotten rid of the right hand side air box. What does everyone think of that, I think it's brilliant makes so much space for other stuff.

BTW this guy is doing a brilliant series on his K100 build finding it so useful.

Seems to be a fairly common mod in the cafe racer crowd. Don't forget you'll now need some form of filter on the end of the AFM.

@Dai wrote:Seems to be a fairly common mod in the cafe racer crowd. Don't forget you'll now need some form of filter on the end of the AFM.

This is defo a mod I'm going to consider because it would provide space for ECU and ABS unit.
He he used this beautiful K&N Filter:

@Dai wrote:Don't forget you'll now need some form of filter on the end of the AFM.

Just curious.   Why does a cafe racer need an air filter???

When you race a cafe it often wins and they do kick up a lot of dust

I’m thinking of hiding my battery, ECU and ABA unit there will it be too hot? What about with heat sheilding and a gap?

Rick G makes good points you wouldnt know the problem in your stock world 😂 lol

And the coffee grounds...Oh, the coffee grounds.  Cafes spew them everywhere.  It's the reason I avoid cafes.

I've been thinking about those velocity stacks and I believe they are to allow you to still reach the big brass screws. I'm not big on Café racers but then again I have a preference to just sip coffee even when it's Loffee. Although I guess I can gulp down the last of it if they're all waiting on me.

@Dai wrote:Brief explanation, because I'm damned if I'm going into the thermodynamics of it!  

The air box takes air from the air intake and both filters it and acts as a very short term reservoir. From there, it goes through the AFM to the plenum chamber. The AFM does exactly that; it measures the air flow demanded by the engine and passes the data on to the FICU to determine how much fuel is needed. The plenum chamber acts as an equalising chamber for the four intake trumpets (venturi) because the cylinders pull air at different times. Each of those venturi acts as a restriction in the air flow which accelerates the air into the cylinder and jams more air in than would be possible without the venturi. Google 'venturi effect'.

Anyone got anything to add to that very, very brief description?

@ Dai : Awesome explanation !

@Dai wrote:Brief explanation, because I'm damned if I'm going into the thermodynamics of it!  

The air box takes air from the air intake and both filters it and acts as a very short term reservoir. From there, it goes through the AFM to the plenum chamber. The AFM does exactly that; it measures the air flow demanded by the engine and passes the data on to the FICU to determine how much fuel is needed. The plenum chamber acts as an equalising chamber for the four intake trumpets (venturi) because the cylinders pull air at different times. Each of those venturi acts as a restriction in the air flow which accelerates the air into the cylinder and jams more air in than would be possible without the venturi. Google 'venturi effect'.

Anyone got anything to add to that very, very brief description?

yes ,
just to mention that the size and location of the air box (in the system ) reduces induction noise very markedly

a requirement to attempt to satisfy the noise regulations across all manufacturers ,  in some cars you will find a small box attached to the inlet path after the air cleaner ,. which is designed just for that reason , but other wise seems of little value , until you remove it and hear the change in induction noise, it is quite apparent .

the intake trumpets , commonly called velocity stacks are designed to affect the peak induction velocity at certain revs (as dai says , to do with cylinder filling ), and can often be designed to improve performance of the motor at the designed rev range expected , so enhances the whole system if designed correctly , like exhaust but in reverse

the length of the air induction system , from valve seat to air inlet snorkel opening can also enhance low end engine efficiency and torque,

@charlie99 wrote:

@Dai wrote:Brief explanation, because I'm damned if I'm going into the thermodynamics of it!  

The air box takes air from the air intake and both filters it and acts as a very short term reservoir. From there, it goes through the AFM to the plenum chamber. The AFM does exactly that; it measures the air flow demanded by the engine and passes the data on to the FICU to determine how much fuel is needed. The plenum chamber acts as an equalising chamber for the four intake trumpets (venturi) because the cylinders pull air at different times. Each of those venturi acts as a restriction in the air flow which accelerates the air into the cylinder and jams more air in than would be possible without the venturi. Google 'venturi effect'.

Anyone got anything to add to that very, very brief description?

yes ,
just to mention that the size and location of the air box (in the system ) reduces induction noise very markedly

a requirement to attempt to satisfy the noise regulations across all manufacturers ,  in some cars you will find a small box attached to the inlet path after the air cleaner ,. which is designed just for that reason , but other wise seems of little value , until you remove it and hear the change in induction noise, it is quite apparent .

the intake trumpets , commonly called velocity stacks are designed to affect the peak induction velocity at certain revs (as dai says , to do with cylinder filling ), and can often be designed to improve performance of the motor at the designed rev range expected , so enhances the whole system if designed correctly , like exhaust but in reverse

the length of the air induction system , from valve seat to air inlet snorkel opening can also enhance low end engine efficiency and torque,

Wow so it could potentially be a very worthwhile upgrade, I'm assuming you'd  have to get it done by a specialist though. What kind of kit would you need? I don't even know what type of shop could do something like that

a couple of pockets of money , a dyno , and some good engineering and maths skills

as usual, I have none of the above .

these days an additional complexity , actually an advantage is the new modern programable ecu  to get the most out of it all

BSK has some cool looking throttle bodies with stacks

http://www.bskspeedworks.co.uk/k-throttle-bodies.html

Stacks will require a different ECU and programming to work with the throttle position sensor that replaces the MAF for determining air input for mixture calculations.  Mapping the correct mixture requires a fair bit of time on a dyno at around $100+ per hour.

Since most cafe projects seem to be just for photos and don't really need to run, I guess you could use the stacks and trailer the bike back and forth to photo locations.

@jahearne wrote:BSK has some cool looking throttle bodies with stacks

No use. They're for the 4-valve engine only where the Moronic unit does all the fuel/air calculations.

@Dai wrote:

@jahearne wrote:BSK has some cool looking throttle bodies with stacks

No use. They're for the 4-valve engine only where the Moronic unit does all the fuel/air calculations.

ben runs an after market  ecu

so suitable for his builds …
just a heads up

but yes  in normal practice  motronic ecu would be required