[Otto_Ing]

The inlet in the airbox should not have that huge hole as in the first pic, otherwise the pulsing (tuning) is lost, right?

[ace.cafe]

The inlet in the airbox should not have that huge hole as in the first pic, otherwise the pulsing (tuning) is lost, right?

Generally speaking, yes, unless  that hole just happened to be the correct size for the tuned vent frquency, which is unlikely.

[Otto_Ing]

Ok, without spoiling the external looks, I could imaging a velocity stack integrated in that cover of a certain dia and length, max. making a breathing hole in the side panel. Would that make sense?

I need to crunch some theory i think first, but the volume of the airbox should be ideally 10-12x the engine volume if I am not mistaken...

[ace.cafe]

It is permissable to make the inlet duct shaped like a velocity stack if you wish, but it should also have a similar bell shape coming into the airbox, because we would like to control the pressure recovery behavior in the airbox by the shape of the inlet diffusor.
So, without getting very technical and wordy, there should be as much flare as possible on the inside end of the inlet duct to keep the inlet air as laminar as possible, and to assist pressure recovery in the airbox(which is just a term for controlling the transition of energy from velocity to static pressure). This reduces turbulence problems.

General principles for airbox design would say it should be as large as the physical restraints can permit in the application, but never any smaller than 1.25 x engine displacement.

The actual inlet tract stack/bellmouth which resides inside the airbox, and is part of the extension of the throttle body may extend into the airbox for a distance, as long as the end of the bellmouth is at least 2 x diameter of the bellmouth from the opposite wall, and there is at least 1 x diameter of the bellmouth all around the edges of the bellmouth to the nearest walls, for flow clearance.

All these dimensions can be adjusted to less than ideal if necessary, but there may/will be a negative impact by doing so. So, try to get as close as you can.

[Otto_Ing]

The airbox is as big as it gets, my best guess is about 5 liters. Now the issue I see is the length of the snorkel/stack going in the airbox from as there is not much space behind the side panel. Also for what rpm should this box be tuned to? 1/3 - 1/2 max rpm?

[ace.cafe]

The airbox is as big as it gets, my best guess is about 5 liters. Now the issue I see is the length of the snorkel/stack going in the airbox from as there is not much space behind the side panel. Also for what rpm should this box be tuned to? 1/3 - 1/2 max rpm?

There are different goals for airbox tuning.
Some tune it at peak hp rpm, some tune it at peak torque rpm, and some tune it half-way between.

[Otto_Ing]

I did some theory crunching. Now i wonder if this can be true. Tuning for top hp at max. rpm would require a hole in the Airbox of dia. 10 cm (3,94") and a length of "only" 1,5 cm (0,6") under following assumptions. (The Airbox has to be measured, 5 liters is only a good guess.)

V, engine =   535     ccm    (Volume Engine)
V, airbox  =   5000   ccm    (Volume Airbox)
di =   10           cm            (Airbox Inlet diameter)
A =   78,54       cm^2        (Airbox Inlet area)
L =   1,5           cm            (Airbox Inlet length)
      
res.   5274,127781   rpm   (Resonance frequency)

Now looking at the dimensions of the hole in the first picture, it could indeed be tuned for max. hp at something like 5000-5500 rpm. Unless i calculated something wrong of course!

[ace.cafe]

It sounds to me like you used the actual rpm for the calculation.
The actual frequency is only half, because there is only one inlet stroke per two engine revolutions. So, you need to calculate for about 2750 rpm in this case of max rpm.

Generally, I would expect this to require significantly more air mass volume inside the resonator, so it would be expected to be longer.

[Otto_Ing]

Actually I didn't use engine rpm at all. I calculated at what frequency the air in the airbox should resonate having the snorkel dia 10 cm, and length 1,5 cm. And that should be half the rpm, so it has been taken into account....

[ace.cafe]

Okay, well give it a try, and see if you can feel a power increase at the top end. It should be enough to feel it in the seat of the pants, if it is working.
You may find some anti-resonance effects at harmonic multiples, so also watch for that. It would appear like a cough or sputter at a numeric multiple of the tuned frequency.

It should be fun to try out!

You can also cut some tube lengths that are longer than your first try, and install them on the airbox to see what kind of tuning effects you get.
Any length will affect some rpm. The trick is to get it at the rpms where you want it.
I have always found that the calculations will only get you close, and that some "cut and try" methods to adjust length for fine tuning is always needed.

Also, I have found that typically a duct that has a diameter that is not much  different than the length seems to work well. But 10cm diamater is a pretty decent diameter that shouldn't restrict the air flow, so that seems like a good diameter to begin with.
Do not use a diameter that is smaller than the throttle body diameter, obviously.

[Otto_Ing]

Well my bike is taken apart as you know ACE  . It is being prepared for a new big bang head. So I wait for the first dyno charts and than adjust this for peak hp at the corresponding rpm. Till than I can calculate a few 100 times and make sure this is correct.   

[ace.cafe]

Well my bike is taken apart as you know ACE  . It is being prepared for a new big bang head. So I wait for the first dyno charts and than adjust this for peak hp at the corresponding rpm. Till than I can calculate a few times and make sure this is correct.   

Oh yeah! 
I forgot that you have it apart!

[Otto_Ing]

...ok, i've redone everything in SI units as something must have gone wrong. The air in the box is the spring and the air in the snorkel is the mass. Let's see if this new result meets your experience better.

305 cubic inch Airbox
1,97 inch Inlet dia.
21,65 inch Inlet length
Eigenfrequency of this is = 2745 1/min, which
would kick in at top rpm of 5490 rpm.

if this is correct than this will be a bloody elefant trunk. I attached a pic with the formula I derived. I used it on the Suzuki TL box, and it seems to match now.

the issue of course is, it is only one slow running cylinder so the eigenfrequency has to be very low. Would it be usefull in this case to use a multiple of the frequency so that the lenght of the trunk comes down a bit?

[ace.cafe]

The duct is probably too narrow,  and that will cause very peakyresults that may have undesirable results at other rpms.
Try a larger duct with shorter length.

[Otto_Ing]

The calc. corresponds exactly with the Helmholtz resonator / Single cavity resonator.

http://hyperphysics.phy-astr.gsu.edu/hbase/waves/cavity.html

Increasing the diameter of the snorkel, it needs to be even longer. Here some examples.

Exampel 1:

Airbox volume         V =   0,005   m^3   305,12   cu. inch
Snorkel diameter         d =   0,1   m   3,94   inch
Snorkel area         A =   0,007853982   m^2   12,17   sq. inch
Snorkel length         L =   2,2   m   86,61   inch

Exapel 2:

Airbox volume         V =   0,005   m^3   305,12   cu. inch
Snorkel diameter         d =   0,08   m   3,15   inch
Snorkel area         A =   0,005026548   m^2   7,79   sq. inch
Snorkel length         L =   1,4   m   55,12   inch

This all would not seem such a problem if it was a 2 cylinder engine, running some 10000 rpm. In this case the result would be (realistic):

Airbox volume         V =   0,005   m^3   305,12   cu. inch
Snorkel diameter         d =   0,08   m   3,15   inch
Snorkel area         A =   0,005026548   m^2   7,79   sq. inch
Snorkel length         L =   0,1   m   3,94   inch
   
So after this, it doesn't seem to be too hopefull for our bike to get a realistic solution for a tuned airbox done. Unless a multiple of the frequency could be used?