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GT4
Nordschleife
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Joined: 08 Nov 2008
Posts: 30174
Location: Hertfordshire and Hampshire


PostPosted: Mon Jan 24, 2011 5:07 pm    Post subject: The Helmholtz Resonator the CAI tube and the Filter panel Reply with quote

NOTE: If your car has the additional Tiptronic gearbox "smog pump" vapour extraction breather line THIS WILL NOT WORK without modification. SEE PAGE 4 FOR REQUIRED TIP MOD


-------------------------------------------------------------------------------------

Thought I'd finally get round to summarising the process and all the replies I had made on this.

AIM: More noise and hopefully some more air flow.

Helmholtz Resonator chamber:


CAI:




Filter Panel:


So in reverse order ...

FILTER PANEL
The filter panel is pretty straight forward, just swap your old paper filter for a BMC, K&N et al high flow filter panel.

Improves the airflow and therefore, hopefully, the power. Irrespective, as long as you look after it (hoover every year and re-oil as appropriate), it will save you money on the paper filters over its 1,000,000 mile lifetime!

Just remember to wrap it in kitchen roll or newspaper overnight when it is new or re-oiled to wick out any excess oil and save your MAF from oil droplet contamination.

Also, it is alot easier to insert the panel in the airbox lid first (so with the airbox removed, fit upside down). This isn't an issue if you have already removed the intake tube (two/three jubilee clips) as it is held in with just one brass bolt at the front of the airbox. Just unclip the oil filler tube and remember to be careful with the MAF cable that runs though clips on the airbox rear. If you are thinking of washing your MAF too, do this first then it is off the airbox.

See here:
http://911uk.com/viewtopic.php?t=57605

CAI
Cold Air Induction (as opposed, pressumably, warm or hot), in this sense it is simply a more slippery tube from the airbox to the throttle body with the Helmholtz spur removed (more later...).

Standard 996 airbox



As the 997 is just a heavily face-lifted 996 lots of bits are interchangeable, like this 3.6 airbox

Standard 997 Airbox (in a 996)


There is no functional advantage, but it looks nicer.

The airbox is quite a nice design, the "snorkel" receives slightly pressurised air at speed from the airstream over the top of the engine bay. The GT models even have scoops for an even greater "Ram air" effect.

Ram Scoops



However, the standard "Y-pipe" that joins the airbox to the throttle body leaves a few things to be desired.

Y-pipe


For a start it is quite corrugated and hence turbulent, which seems a shame after the plastic grill and metal mesh air straighteners in the airbox exit. So fitting a nice smooth tube seems sensible. For a second there is that odd second pipe?

So what does the Y-spur do? - it allow the addition of a Helmholtz Resonator (anti-noise chamber).

Helmholtz Resonator

The second smaller leg of the intake tube, or the Y-spur of the Y-pipe connects the airbox-intake system to the Helmholtz Resonator chamber (also called the HHR).

The noise is increased (improved?) by the removal of the standard airbox Helmholtz resonator (this is a noise cancellation method using harmonic anti-sound resonance of the natural air pulses into the throttle body during cylinder induction - by creating an opposite signed standing wave in the cavity).

Its removal results in a louder (and more pleasing) noise. The original inclusion of the HHR is just for drive-by noise regulations, rather than some clever intake tuning, like the VarioRam.

HHR in the lid of the airbox:



Removal of the HHR from inside the top of airbox allows clean airflow across the full filter surface (upto 1/3rd of the filter surface is restricted or turbulent with HHR in place) and liberates between 0.5L and 0.75L of post-filter air.

As the HHR removal leaves a hole in the post-filter side (top) of the airbox, it is necessary to cap this hole otherwise dirty and warm air will enter from the engine bay.

HHR removed


The FabSpeed kits came with a ready made bung for this purpose.

They supplied a blanking piece in the kit, but this seems to have been dropped now (after 997.1 glued the HHR in?)




However, the Schnell one and others don't, so I made my own for £2.52 from plumbers merchant bits:

Quote:
1 1/2" STRAIGHT tank CONNECTOR + securing nut
1 1/2" socket plug x1
1 1/2" poly washer x2


Plumbing bits








The panel filter needs the post filter air-straightener to streamline the flow. Some people discuss removing the straightener THIS MUST NOT BE REMOVED - firstly the air will be more turbulent, secondly any large particles somehow getting past filter panel (either ill fitting or panel missing etc) will not get stopped prior to throttle body and engine ingestion!

Linear (non-turbulent) air-flow is required for accurate MAF (air-mass flow) readings, non-linear (turbulent) air-flow will result in incorrect fuelling.

STRAIGHTENER (and finer metal mesh behind this grid)




Conclusion:
The noise is a lot more interesting and louder, perhaps not quite an induction roar (well it might be, but my ears are at the front and the engine is in the back). The noise through the rear bulkhead is a pleasing engine sound and the throttle response is definitely a lot crisper.


EDIT: FOUND MOST OF MY PHOTOS THAT HAD BEEN LOST TO NTL/VIRGIN HOSTING, SO INITIAL POST EDITED TO ADD BACK:

Last edited by GT4 on Tue Sep 20, 2016 8:31 am; edited 10 times in total
 
  
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MARKY T
Trainee


Joined: 17 Jul 2010
Posts: 50
Location: durham


PostPosted: Mon Jan 24, 2011 5:40 pm    Post subject: Reply with quote

hi gt4,
nice too see a pic of my car on there [997 airbox with the blue induction pipe],realy love the sound that mine makes almost like a mild exhaust mod.
im sure it is a little faster.
go out and get a kit ,very easy to do just make sure that you remove the resonator in the airbox lid.
the 997 airbox is a direct fit if you have the complete airbox but you can fit just the upper as i did but it needs a few mods with the fastening screws.
cheers mark
 
  
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GT4
Nordschleife
Nordschleife


Joined: 08 Nov 2008
Posts: 30174
Location: Hertfordshire and Hampshire


PostPosted: Mon Jan 24, 2011 11:01 pm    Post subject: Test Reply with quote

Your picture was to show the various makes (colours!) of identical CAI (ie silicone tube!)

DesignTek = Blue
FabSpeed = Red
Schnell = Orange

Although, as a coloured tube can't be copyrighted, there are various manufacturers with the same or similar colours, some may even allow you to choose (why no one seems to do a less conspicuous black is beyond me!).

Interesting, I didn't realise you could (sort of) mix and match the lid and base.

Why did you need to do this? Was the lid only available?

The yellow car (996) has a complete 997 'box.
 
  
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Jet Jock
Montreal


Joined: 18 Jul 2005
Posts: 563
Location: Dublin


PostPosted: Tue Jan 25, 2011 12:13 am    Post subject: Reply with quote

I did this with my 996. Kept the standard airbox, used a BMC panel filter and Design Tek tube and bung but left the HHR in the airbox. Does this make a big difference to sound if I remove the HHR now? My Cargraphic Sports exhaust is loud enough at the mo. Thumb

Thanks for the write up GT4.

Baz
 
  
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GT4
Nordschleife
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PostPosted: Tue Jan 25, 2011 12:41 am    Post subject: Reply with quote

Well, I did the mod two years ago this month, so I thought it was probably about time I wrote it up properly (rather than drip feeding bits and pieces here and there in replies).

The HHR removal (assuming CAI in place and hence resonance chamber effect negated) will just free up post filter free air volume and de-restrict (clean) the airflow across the filter panel. So in that sense it probably won't make any more noise.

The free volume is between 0.5L and 0.75L and allows the induction system to quickly suck a gulp of air before additional filter flow begins by the lowered pressure in the airbox top. So it is a freebie to the induction system prior to really sucking air through the filter panel, therefore instantaneous acceleration is slightly improved.

Of course, in an ongoing fashion, the unhindered air flow from the LHS filter surface is a good thing too. The HHR box almost smothers 1/3rd of the filter.

Next time you change (or look at) your filter panel, check out the underside of the panel. The section directly below the HHR box will be a lot cleaner than the rest of the panel (possibly more visible on an oiled filter panel than the paper ones, as the dust sticks cumulatively and you don't usually bother to look at the paper ones as you chuck them away rather than cleaning them).

Why the clean patch? Because a lot less air is drawn through the panel here than everywhere else.

Not very fair, and not very effective.

What is the point of having a large, approximately 725 cm^2, surface area and not using approximately 240cm^2 of it?
 
  
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Jet Jock
Montreal


Joined: 18 Jul 2005
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Location: Dublin


PostPosted: Tue Jan 25, 2011 12:48 am    Post subject: Reply with quote

Cool Thanks again for the swift reply.
 
  
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MARKY T
Trainee


Joined: 17 Jul 2010
Posts: 50
Location: durham


PostPosted: Tue Jan 25, 2011 5:39 pm    Post subject: Reply with quote

Hi gt4,
i got the top brand new from ebay,you can buy the base from the dealers or the full airbox but not just the top got a great deal.
The screws that hold the 996 airbox together are on the top of the lid but on the screws on the 997 airbox are on the base [opposite side] so a few mods were made with the screw holes too fit them both together.
i also put in a bmc filter for good measure.cheers mark
 
  
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GT4
Nordschleife
Nordschleife


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Posts: 30174
Location: Hertfordshire and Hampshire


PostPosted: Tue Jan 25, 2011 7:10 pm    Post subject: Reply with quote

I did think about putting a 997 airbox in mine, but as it is really just for show, and you don't really show that bit off (unless want to show off to your technician, or more likely confuse him) it seemed like a bit of a way down the list of priorities.

They do look nice though.
 
  
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rajkumar911
Monza


Joined: 23 Nov 2009
Posts: 169



PostPosted: Wed Jan 26, 2011 10:42 am    Post subject: Reply with quote

I have done this mod - and removed the HHR, blocking off with plumbing bits. The noise was louder but it just sounded like a drone.

I personally perfer the noise with the HHR kept there and blocked off with the alloy cap (from DesignTek).

Each to their own!
 
  
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GT4
Nordschleife
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Joined: 08 Nov 2008
Posts: 30174
Location: Hertfordshire and Hampshire


PostPosted: Wed Jan 26, 2011 2:14 pm    Post subject: Reply with quote

rajkumar911 wrote:
I have done this mod - and removed the HHR, blocking off with plumbing bits. The noise was louder but it just sounded like a drone.

I personally perfer the noise with the HHR kept there and blocked off with the alloy cap (from DesignTek).

Each to their own!


That is odd, as once blocked, the HHR is just an obstruction in the airbox lid. The blocking off negates the anti-noise.

Admittedly, if you still retained the spur you would get some form of Venturi type turbulence (noise effect).
 
  
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tbrown
Paul Ricard


Joined: 05 May 2007
Posts: 3430
Location: Fra Mauro Highlands

2007 Porsche Cayman 987

PostPosted: Thu Jan 27, 2011 12:58 am    Post subject: Reply with quote

GT4 wrote:
rajkumar911 wrote:
I have done this mod - and removed the HHR, blocking off with plumbing bits. The noise was louder but it just sounded like a drone.

I personally perfer the noise with the HHR kept there and blocked off with the alloy cap (from DesignTek).

Each to their own!


That is odd, as once blocked, the HHR is just an obstruction in the airbox lid. The blocking off negates the anti-noise.


I was thinking the same.
I had to remove the HHR in order to fit the Deign tek alloy cap in the airbox,
D tek assured me they only made one size of cap Confused
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Cayman S
Ex 996C4 Polar Silver 125k
Ex 996C2 Ocean Blue 143k
 
  
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GT4
Nordschleife
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PostPosted: Thu Jan 27, 2011 10:06 am    Post subject: Reply with quote

Perhaps the cap is tapered?

Certainly the HHR neck and airbox aperture (post removal) are at least 2-4mm different diameters.

Looking at some photos I found on the web, it appears they fit in just the neck (ie implied retention of the HHR, which is just bizarre as it is now only good for a door stop!)





It does also state in the advertising blurb:

Quote:
We include a DesignTek Vortex filter to complement this air flow and a CNC aluminium bung to block off the factory muffler.


As in the HHR, rather than "block off the airbox".

If you can remove the HHR, you are certainly better off for all the reasons previously given.
 
  
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baptistsan
Monza


Joined: 19 Dec 2006
Posts: 164
Location: suffolk


PostPosted: Thu Jan 27, 2011 10:39 am    Post subject: Reply with quote

Thank you for a very useful post.

Am I just really sad that I love the look of the 997 airbox?

Assume the internal design is identical to the 996 version?
 
  
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GT4
Nordschleife
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PostPosted: Thu Jan 27, 2011 11:24 am    Post subject: Reply with quote

Yes, and if you like it buy one (but realise you are just buying the looks, it has no functional differences or advantages).

They are plug-and-play into a 996 engine bay.

AFAIK OPC prices are mental though, so seek out breakers or eBay etc.

You will need a 997 3.6 model though, 997 3.8 airboxes are slightly different.
 
  
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tbrown
Paul Ricard


Joined: 05 May 2007
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Location: Fra Mauro Highlands

2007 Porsche Cayman 987

PostPosted: Thu Jan 27, 2011 11:41 am    Post subject: Reply with quote

GT4 wrote:
Perhaps the cap is tapered?

Certainly the HHR neck and airbox aperture (post removal) are at least 2-4mm different diameters.

Looking at some photos I found on the web, it appears they fit in just the neck.


Cap is not tapered, but it may be the airbox on the newer model has a neck
[rather than a hole] that the HHR neck fits into. That would give the same internal dia for the alloy cap to fit into?
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GT4
Nordschleife
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PostPosted: Thu Jan 27, 2011 12:50 pm    Post subject: Reply with quote

Good point, the two pics I found were of 997 boxes, but how does the HHR fit if the later airbox lid has a neck Dont know
 
  
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rajkumar911
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Joined: 23 Nov 2009
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PostPosted: Fri Jan 28, 2011 3:35 pm    Post subject: Reply with quote

Just looking at the HHR and the positioning in the box and where the air passes into the tube housing the MAF - I really don't see that there would be an increased airflow into this tube by removing the HHR as the air is coming from below the tube (if that makes sense). if anything, the HHR removal means that air now occupying this space is likely to cause disturbance to the air that direct goes upwards and into the tube. Even if that is not true, for me at least, the noise was too overwhelming and wasn't crisp enough.

As for noise, there is quite some discussion on rennlist re the orange cap mod. That mod blocked the HHR (keeping the spur) and resulted in a change of sound. Therefore it is possible that air passing through the spur and HHR does cause some harmonics that influence sound.

This feel all too nerdy!
 
  
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GT4
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PostPosted: Fri Jan 28, 2011 3:54 pm    Post subject: Reply with quote

I don't understand what you are describing, but you have either contradicted or reiterated what has previously been written.

So I am not sure what to add, without contradicting your contradiction or reiterating your reiteration.
 
  
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rajkumar911
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PostPosted: Fri Jan 28, 2011 7:52 pm    Post subject: Reply with quote

it'll be much easier to explain face-to-face over a beer Wink
 
  
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GT4
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PostPosted: Fri Jan 28, 2011 10:40 pm    Post subject: Reply with quote



I'm not being funny, but that seam/flange is where the filter panel sealing/seating ridge sits.

That means the HHR box suffocates the portion of the filter panel sitting under it (given air is drawn through from underneath when installed). That's almost 1/3rd panel surface.

The HHR was almost certainly an after thought added to enable passing drive by noise regs. Why else would it take up some of the airbox and cover up the panel. It would be quite easy to have an external chamber or if it was incorporated into the airbox, use a smaller square filter panel rather than a panel 30% bigger than useable. As for the fact its presence or absence changes the noise characteristics and loudness, that is no accident, its anti-noise capabilities is the ONLY reason it is added.

Now I am not debating the attractiveness of the noise (which is personal) and I even added a question mark to the statement "improvement?" on page one as I realise everything from PSE to sports suspension to colour is objective. But the HHR will change the noise charcteristics and the noise level (loudness).

Additionally, leaving the old Y-pipe and capping (Orange Cap Mod) will result in a different effect as the spur will produce some sort or Venturi turbulence and hence airflow effect and noise effect.

Starting from the premise a clean, smooth CAI tube will lead to less turbulence and a faster, less restricted airflow, leaving the HHR in place after Y-pipe removal is pointless.

After HHR removal the full filter area is available and the airflow enters the airbox lower left from the snorkel, travels up through the entire panel, then makes its way out of the airbox to the top right, via the air-straightening grid and mesh and past the MAF, up the CAI and enters the throttle body.

Admitedly the extra airflow has to make its way up through the filter panel, then right, then left to exit the airbox, whereas the original airflow makes its way up through the filter panel, then left and out without changing direction, but water doesn't seem to have too much problem leaving a sink from all directions (left, right, front and back), and that has alot more momentum and no compressibility.

Turbulence is either time varying density, direction or speed (strictly, usually chaotic). Whereas the HHR and spur add resonance (ie frequency modulated turbulence), the airflow after HHR removal should be stable (for a fixed engine speed/induction rate) and most importantly laminar after the air-straightener.
 
  
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