[Etnier]

I have installed a slight variant of the now-famous 9Magazine airbox mod: in my car the holes are 2 3/8" diameter, and there's no trim since I have front trim piece in the car.



It does really 'bark' when the throttle is opened. Made me wonder about this:



What if the airbox bottom were all but removed? What if you cut it laterally so as to only leave enough of the box to secure the filter and retain the box mounting brackets? I'm wondering if the noise we're getting might be from either-

• harmonic vibration of what's left of the front panel or...
  
  
• turbulence around the hole edges or...
  
  
• a combination of the above.
  
  

Removing the last of the structure and creating sort of a 'rectangular cone filter' might be worth investigating in this regard. Thoughts?

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[Silver BLT]

Sounds like a brilliant Idea! It would ensure available complete flow across the filter area, having said that I believe there is an inherent design flaw with the location of the intake AFTER the filter (in the corner). Unless that portion of the filter directly nearest the air box output is somehow restricted, that area of the filter will be doing most of the air intake / filtering work. Just something I have noticed. I think your idea for a rectangular air box mod is a winner. I'd try it but I have already opened up and ducted mine. I'll post my pics soon.

I think your idea is sound. (pun intended)



IIRC, I read a post over a year ago about this mod that had your name on it.

Silver BLT

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[flash]

there is NO performance benefit to be had from anything larger than the 2" holes - 2.375 works, but exactly as well, no better - zero restriciton is as low as you can go, and as much air you can get in there with this MAF and throttle body, and without forced induction - removing more material will only weaken the box structure



the sound you hear is mostly the throttle body induction noise, and not the holes - only a small bit happens there, and the trim rings help "tune" that a bit, just like a subwoofer port



i was curious at one point, and removed the rings - the sounds got a bit "fllappier" without them - i didn't like it, and reinstalled the rings - not sure if the difference was the lack of the bell mouth that the rings give a bit of, or if it was the side of the box vibrating without the rings there to hold it still



this is why i did not recommend running without them - it's not just an aesthetic thing - you're a bit committed now though with the larger holes

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[Greimann]

[quote name='Etnier' date='Aug 14 2005, 05:41 AM']What if the airbox bottom were all but removed?

[right][post="8740"]<{POST_SNAPBACK}>[/post][/right][/quote]

Look at what is directly under the airbox - the radiator. There is little clearance for air and what would be coming in would be hot.



But like Bob said, you can't get better than zero, which is the current solution.



You will get used to the extra bark. Give it a few weeks of driving.

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[Silver BLT]

Flash,

I think Etnier was simply pontificating (love that word) an alternative, not suggesting any improvements to the air flow were needed or possible.



Silver BLT

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[flash]

no worries - i didn't mean to sound critical - i just didn't want to see somebody hack up their box and then not get anything but more expense out of it



i know the natural thought is "bigger is better" - heck, that's what started dave and i on this course in the first place, but there is a limiting factor that exceeds the holes



now, if you want to get a bigger throttle body, and MAF, that may allow for larger holes, but then you may get into retuning the resonant induction manfold, which is a potential nightmare



film at 11 on that one (come on - you knew i couldn't leave it alone)

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[Etnier]

• i know the natural thought is "bigger is better"
  
  
• you can't get better than zero
  
  
• there is NO performance benefit to be had from anything larger than the 2" holes
  
  

I need to clarify here: issues of maximizing CFM weren't raised in my post, and did not drive my thinking. I left the house after posting and realized I had failed to buttress premptively against that misunderstanding.



I'm simply thinking: what good is the box, particularly if it's been all but destroyed? It's a resonant chamber with four holes of identical diameter sucking in air at substantial velocity. Flash feels that issues about the specifics of the hole size and edge design affect noise one way or the other. So why not just lose the thing...?

Look at what is directly under the airbox - the radiator. There is little clearance for air and what would be coming in would be hot.

Dave: the hot air argument might hold true, but I need to make it clear I'm not talking about moving anything downwards. The air filter would stay where it is, maintaining the same clearance to the radiator. Unfortunately someone would have to drag out the temp gauges to find out if removing the directionality (and insulation?) of the box affects temps.

IIRC the conical filter mentioned (here?) recently would be subject to the same issue, only more so.

IIRC, I read a post over a year ago about this mod that had your name on it.

Not true, AFAIK.

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[flash]

well, again, if you remove the front of the box, there is nothing there to give it any stability, so the pressure of the air going in could deform the filter upward, creating a whole new set of problems with interference with the top of the box as it raises up, thereby decreasing flow to the MAF - we stopped because since we had achieved maximum performance, we didn't see the point in risking any other issues



also, the tone would change again, and likely not for the better, but this is pretty subjective



the box was designed primarly to attentuate sound - germany has extremely high standards when it comes to noise - that's why all the little flutes and everything are inside - they all play a role in lowering intake noise - removal of more material would lower the note, and raise the volume, but it would also add more of the "farty" noise - this is where it becomes a subjective thing



quick explanation of ports, hole sizes, and what they do to sound (but i think you already know this one): bigger hole = lower note - smaller hole = higher note - this is an air velocity thing - the bigger the hole, the slower the air that goes through it



that being said, from a performance standpoint, there is probably a point at which the holes can actually be too big, though this would likely only affect very low rmp modes



i hope that helps - i'm not sure what you are after, but i think it's going in the wrong direction

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[Etnier]

Thanks, Flash. Good points.

ON EDIT: Just to clarify:

...if you remove the front of the box, there is nothing there to give it any stability, so the pressure of the air going in could deform the filter upward...

I was talking about removing the bottom of the box, leaving the top edge where the 8 spring clamps and the two airbox mount flanges are. I was betting that the clamped assembly would be rigid enough. Perhaps not. At any rate- "the sound you hear is mostly the throttle body induction noise, and not the holes" kind of settles things in my mind.

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[flash]

no worries - always here to help if i can



now go out and drive the car and enjoy it! i need to live vicariously through you guys this weekend - i'm jammed getting everything ready for the denver run

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[Greimann]

[quote name='flash' date='Aug 14 2005, 09:42 AM']the box was designed primarly to attentuate sound - germany has extremely high standards when it comes to noise - that's why all the little flutes and everything are inside ....



that being said, from a performance standpoint, there is probably a point at which the holes can actually be too big, though this would likely only affect very low rmp modes

[right][post="8761"]<{POST_SNAPBACK}>[/post][/right][/quote]

I disagree on a couple of points.



The ribs are most likely stiffening elements for the thin plastic rather than noise baffles. The small curvy snorkel is the anti-noise trumpet.



The whole (hole) idea is to reduce the pressure differential across the filter. The bigger the better. Gives a margin of safety if you go with a larger hole. The tests we did with the high tech flow test rig did not corrleate to a specific engine RPM. It just measured relative effectiveness among the modifications.

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[flash]

nope - the ribs are defintely a sound thing - they wouldnot have needed to go to the expense they did in that molding to stiffen it up



it's called a helmholtz absorber - also referred to as a "bisquit tin" by the brits - a very old and very effective principle for baffling sound, found in almost every recording studio



the bend in the snorkel also attentuates the sound



the larger hole thing i referred to would be a function of velocity - lower velocity air happens when going through a larger orifice - higher velocity air results in more power - the low rpm thing was a theory besed on lower velocity air happening with a larger hole and that at higher rpm you would likely see high enough velocity air that it would overcome the larger hole failings

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[Greimann]

[quote name='flash' date='Aug 14 2005, 08:40 PM']nope - the ribs are defintely a sound thing - they wouldnot have needed to go to the expense they did in that molding to stiffen it up[/quote]

Run down to the garage and look at it again. The entire preiphery is webbed with intricate stiffening ribs on the outside. The ones on the inside are there for cosmetic / space consideration.

the larger hole thing i referred to would be a function of velocity - lower velocity air happens when going through a larger orifice - higher velocity air results in more power -

[right][post="8787"]<{POST_SNAPBACK}>[/post][/right]

Then it follows that the small snorkel flows the fastest air of all. There is no resonant tuning benefit in front of the filter. You want the most open structure possible. Behind the filter, resonance is the name of the game.

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[flash]

i'm talking about the ones on the inside - it's one hell of a coincidence that they follow the exact priciples of one of the most effective sound deading devices known - happy accident, or by design?



i think there is definitely a tuning effect in front of the filter - all the work we have done is in front of the filter - we've done nothing behind the filter - you can even feel the difference with the stock grille versus your grille - i know you doubt the effect of ram air, and we disagree on that one



certainly the big thing is behind the filter, but if you don't have velocity going in, you don't have the maximum possible effect - i'll agree that the effect may be minimal, and that this may be a moot point, but i was only addressing the issue of maximum size and possible effects - you may never feel it, but i'll be it can be measured

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[Greimann]

i'm talking about the ones on the inside - it's one hell of a coincidence that they follow the exact priciples of one of the most effective sound deading devices known - happy accident, or by design?

Could be either. Unless a Porsche engineer involved in the process gives up the design process, we can only speculate.

i think there is definitely a tuning effect in front of the filter - .

For sound yes, flow no. The acoustic and air flow principles are different. Air filtration engineering principles say that a reduction in the flow restriction is the best solution.

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[flash]

wel, i know what it would sound like if you removed the ribs inside - i've done that before - louder and more hollow



sure a reduction in flow restriction is the best solution - i'm not disagreeing with that at all - never did



this is a pretty academic discussion, albeit fun



the only thing i am saying is that changing the velocity of the air will affect the effeciency of the intake - to what degree on this setup would have to be measured - i do know that increasing the velocity will lower the temp, and result in more power - that's why velocity stacks work - there's a formula for that one - whether or not opening up the front of the box would make for any amount of detrimental change is up for testing (somebody else is going to have to do this one), but i can see how it possibly could, and if it did, it would likely be at low rpm levels where the amount of air being drawn through is at its lowest, and the size of the openings would show the most difference in flow characteristics



it is very likely that the biggest difference would be too much heat getting in the box without the smaller holes and ports to keep it fast and cold

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[Greimann]

I don't think velocity and temperature play any role here. Velocity stacks work because they tune resonant pulses to coincide with the intake valve opening, Temperature is not a factor. The air box is too far away from the intake valve to be considered into the resonant pulse scene. Whether the ribs inside act as a shroeder diffusor may or may not be the case.



Anyway, here are some guys who have done mods on other Porsche models (scroll down a bit). Damn, I totally missd the red silicone strap market for this one!

http://www.fabspeed.com/911sc.html

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[flash]

i know you don't think velocity and temperature play a role - we have had that disagreement before



there is a forumla though (something like 15 degree drop in intake temp = 2 hp) - the intake temp drop with the modded box on this car was so significant to totally screw up the dyno and make me have to retest everything



the change is in the front, so there has to be something going on there



not so sure about the distance thing either - there is another formula for loss per foot



anyway, none of it really matters much - we are really way out in "theory land" here

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[flash]

by the way - you get serious internet points for that research

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[ds968]

oh crap. I was supposed to drill the those holes in the FRONT of the air box ??!!

now it makes sense why I get a noticeably better pick up in reverse gear ..

<img src="/forum/images/smilies/968/tongue.gif" class="smilie" alt="" /> <img src="/forum/images/smilies/968/laugh.gif" class="smilie" alt="" />

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