resonant airbox theory

[RobT]

I have finally found an airbox that will fit my ibiza, this one,

http://www.pipercross.net/competition_carb_600_airbox.asp

took loads of measurements off the one on the pipercross stand at autosport and have today made an exact dimensional copy in cardboard, and it fits ! finally may get to go to donny again...

BUT

Is it going to be restrictive ?????? 100mm inlet to the box but is only approx 6.3L in volume.....

Have emailed pipercross to find out what type of power have people had through these boxes

in the meantime....

hugely interesting webpage about resonant theory in airbox design:

http://www.calsci.com/motorcycleinfo/Airboxes.html

"A modern engine with valve overlap will naturally have a dip in the torque at about a third to a half the red line rpm. If the air box is tuned to have minimum resistance to air flow at this rpm, the dip in the torque curve will be partially filled in by the ease of pulling air into the engine.

So, your air box is most likely designed to add horsepower in the mid-range. The air box will have little or no effect on peak hp"

(it may not be a good idea to bin those std airboxes boys....)

pipercross box is approx 6300cc
10cm ducting has intake cross sectional area = 78.55 cm2

resonant frequency of box will be following rpm for (inlet tube length in cm)

7279 (10)
5147 (20)
4202 (30)
3639 (40)
3255 (50)
2971 (60)
2751 (70)
2573 (80)
2426 (90)
2302 (100)
etc

So if you have a dip in your torque curve, perhaps by tuning the intake length it can be improved upon.....

If take 30cm long duct as a point of interest, resonant frequency for a 6300cc box is 4202rpm. If that was an 8000cc box, freq would be 3729 rpm. If it was a 4000cc box, it would be 5274rpm.

Air speed in a 10cm diameter pipe at 8000rpm is 0.49mach (169m/sec) so the pipe may be getting restrictive to flow

Now, if 75mm bore tube is used (cross sectional area 44.18cm2) and assuming our 6300cc box again:

resonant frequency of box will be following rpm for (inlet tube length in cm)

5459 (10)
3859 (20)
2729 (40)
2228 (60)
1929 (80)
1726 (100)

gas speed in a 75mm bore tube at 7000rpm is 300m/s of getting scarily close to speed of sound 340m/s

75mm bore tube is not a good idea

starting to understand why mini miglias have such huge air intake ducting:

http://www.andy1.packham.btinternet.co.uk/whatmigengine.jpg

so sounds like a short intake pipe is a good idea for beefing up midrange, approx 30cm with 100mm ducting, but this has to be balanced by the need for an effective cold air feed......and it could do with being abit longer to achieve this goal...hmm....

[RobT]

interesting links

http://e30m3performance.com/tech_articles/engine-tech/airbox_symposium/index.htm

How BMW ddesign pukka airboxes

Cheap and cheerful airbox upgrade idea

http://www.mgf.ultimatemg.com/group2/airbox/index.htm

[F2 Stu]

Serious food for thought there as I'll have the same issues.

I desire an F2 Ibiza style intake but no damn good if I'm kicked out of circuits over noise:(

Those Bimmer intakes are sex but having one made would be extremly expensive, guess you could fabricate up a 3d master with a lot of time & patience & have a composite place mould it up:shrug:

Will have to mke up my mind before I get Radtec to make me a shorter rad.

[RobT]

thought you wanted the F2 style duct to vent an oil cooler ? this style of duct will not work so well for an air intake.

All being well, I will be running the PX600 airbox when noise is an issue (or its raining hard) and a regular dome filter when its not or dry - both will fit on the same backplate from pipercross so very easy to swap over

Air will enter through the grill above the rad - this is a very high pressure point on the car and thus there will be some ram air effect hopefully - more so with the airbox setup which may prove to be better all round than an open filter.....tests coming up

[F2 Stu]

F2 bonnet vent for oilcooler exhuast yes, not for an intake.

Would love an enclosed airbox fed from the front grill ala F2, but noise & water ingress would be a concern - tank taping the front end in the pissing rain isnt a practical solution

like the fact the backplate is universal with a conventional filter too - kicking myself I bought an ITG instead.....

[RobT]

dont think this setup will fit your motor without chopping the bonnet - nearly touching the underside of mine and its only a single fibreglass skin

tried so many airfilter setups I've lost count - ITG panel, cossie DFV ITG dome (£££££), home made plywood affair with (1) twister then (2) bmc, the list goes on

[F2 Stu]

Ive got a 90mm high ITG Megaflow spaced 40mm from the bodies which will clear one a bit of reinforcing webbing is removed.

Said airbox is 75mm high.

[RobT]

you gone for a domed or square filter ?

[DPJ]

in the meantime....

hugely interesting webpage about resonant theory in airbox design:

http://www.calsci.com/motorcycleinfo/Airboxes.html

...................
(it may not be a good idea to bin those std airboxes boys....)



Wow, that's fascinating. :cartman:

I know this is a NA forum - I've thought hard and I can't see any relevance to turbocharged applications where the path from airbox to inlet valve is massively longer.... (but someone please tell me different...:blink: )

[F2 Stu]

you gone for a domed or square filter ?

Domed - can see why you ask as the airbox is square and bigger in certian places

[Fl@pper]

gotta start looking at my trumpet base and collector as it were soon before i make the box so cheers rob - i know have another 101 jobs to think of :)

mines basically an aluminuim version of this in reverse (71mm t/b)

http://www.mez.co.uk/rovermanifolds/act_westfield.jpg

reversed to allow IC connection easier later on ;) but thats a different section DPJ ;)

[RobT]

Wow, that's fascinating. :cartman:

I know this is a NA forum - I've thought hard and I can't see any relevance to turbocharged applications where the path from airbox to inlet valve is massively longer.... (but someone please tell me different...:blink: )

if you dont like it **** off elsewhere to another section

this is the NA section, no interest in turbo's here (and what I am talking about is the path length from the outside world to the airbox not from box to valve...an entirely different subject)

[RobT]

Domed - can see why you ask as the airbox is square and bigger in certian places

slight difference then, forgot to mention that eh ?......:cry: ....it will not fit your car without chopping the bonnet

[Fl@pper]

im only here to show the shape and to double up your findings :) but thats the box alone and a whole new ball game - sorry

on a more serious note - although a smaller bore would be restrictive to speak of but surely air would theoretically speed up if the same flow was being inputted

there is a fine lined window to catch though between the 2

[F2 Stu]

slight difference then, forgot to mention that eh ?......:cry: ....it will not fit your car without chopping the bonnet

Or smack the underside with hammer til theres room - I do have another bonnet to play with:)

[m0rk]

I'd play with the 'spare' if i were you stu.... it's already dinted...

[F2 Stu]

Yeah, wont need to do anything just yet tho, albeit notching the webb - or flattening the top of the filter

[Fl@pper]

years ago i messed with cones and the mathematics on a multi stage 2 stroke exhaust that enabled the pipe to effectively draw the fumes out

i wonder if the same could be done on an intake in reverse ?

http://upload.wikimedia.org/wikipedia/commons/5/52/Arbeitsweise_Zweitakt.gif

[RobT]

im only here to show the shape and to double up your findings :) but thats the box alone and a whole new ball game - sorry

on a more serious note - although a smaller bore would be restrictive to speak of but surely air would theoretically speed up if the same flow was being inputted

there is a fine lined window to catch though between the 2


not having a go at you m8, the pic was useful - I've got a flyer from the guys who make that box and they do some lovely work, lots with tvr's

the issue is the mass of air moving through the pipe - the air at the edges moves slower than the air in the middle and this gets to a point where the air dont move much at all...even though its just air

I think the speed of air comes in further downstream, in the region box to engine (but again I am not sure)

(any fluid dynamics experts on here to explain better ?)

[RobT]

years ago i messed with cones and the mathematics on a multi stage 2 stroke exhaust that enabled the pipe to effectively draw the fumes out

i wonder if the same could be done on an intake in reverse ?

http://upload.wikimedia.org/wikipedia/commons/5/52/Arbeitsweise_Zweitakt.gif

This kind of thing is used in the airbox to valve area - the length of the intake runner / rampipe being tuned for certain points in the power curve - I have some good links to this kind of stuff, will stick em up tonight

(but this should not be confused really with what this thread was about - airbox resonance)