StanSoph

I wanted to move up in the world, so I removed my Coke Bottle ( orange cap)( actually red cap). It has been replaced with 2 Stainless washers with a bolt through the center. The washers bolted together sandwich the airbox and seal off the opening. They are identical in size to the opening and the tube then fits over it perfectly. My mod, sans orange cap. LV Dell; I used an aircraft grade bolt and lock nut for safety. . I thought to myself, " self, why would you let a simple piece of ingested plastic, orange or otherwise ruin your engine?" That is why I went with stainless, it will total the engine. Just like Dad always said "Any mod worth doing is worth doing right." Now I can have a Coke and a smile. Now, what's this I hear about cards and making the engine louder? Is it a D.I.Y. ?

penguinking

LOL

well done, well done!

pics? size and specs of washer and bolts? i'm still under warranty, i want to grenade my motor too

Re-animator

Incorrect. Heimholtz (sp) resonators are on post combustion gases only; hence exhaust "pulses".

The pulsating flow from each cylinder's exhaust process of an automobile petrol or diesel engine sets up pressure waves in the exhaust system-the exhaust port and the manifold having average pressure levels higher than the atmospheric. This varies with the engine speed and load.

At higher speeds and loads the exhaust manifold is at pressures substantially above atmospheric pressure. These pressure waves propagate at speed of the sound relative to the moving exhaust gas, which escapes with a high velocity producing an objectionable exhaust boom or noise.

A suitably designed exhaust silencer or muffler accomplishes the muffling of this exhaust noise. The basics of silencing can be understood by recalling a few principles of physics. The velocity of sound in the gas at a given temperature is directly proportional to the square root of the product of the pressure and the ratio of the specific heats (at constant pressure to that at constant volume), and inversely to the square root of the density of the gas.

As the temperature varies, the velocity also varies directly as the temperature by another square root law involving the product of the coefficient of thermal expansion of the gas and the temperature. The exhaust noise can be reduced appreciably by providing resonance chambers to offset the noise wave effects.

This is accomplished by the principle of the Helmholtz resonator. In principle, it comprises the exhaust pipe, which goes through the large volume of a chamber.

The axial holes in the exhaust pipe enclosed by the chamber allow the gases to vibrate with the large mass of the gases in the chamber (forming a spring-mass vibrating system) and generate the sound of the same frequency but in opposite phase to that which has to be nullified (called anti-sound).

To achieve this the muffler volume should be proportioned to the engine piston displacement, and inversely proportioned to the engine speed and the square root of the number of engine cylinders. The usual length to diameter (l/d) ratio of the resonator is about 4:1 to 8:1.

A small l/d ratio muffler attenuates the sound well for a narrow frequency band, where as the large l/d muffler attenuates the sound to a lesser degree but over a wider frequency band. The effectiveness of the exhaust system in silencing the exhaust depends also on the relative lengths of the exhaust pipe (from the exhaust manifold to the muffler) and the tail pipe. A ratio of 1:2 is better than 4:1, and 1:1 is the poorest ratio.

Since the narrow frequency range limits the resonant chamber application, other features are incorporated in the resonant chamber to produce friction effects and filter off noise effects of other offending frequencies. Provision of baffles, resonator mufflers with end baffles, resonator with centre baffle chamber and four-chamber muffler are illustrative examples. In early stationary engines, muffling of the sound was accomplished by allowing the gases to expand by changing the direction of flow or by cooling them with injected water.

Also, the Fabspeed version of the orange cap mod boasts increased acceleration from 3,000-6,000 rpm which can only come from more power.

"Better throttle response and power gain you can feel. - 100% Guaranteed.

.42 seconds faster acceleration 3000-600 RPM with BMC F1 airfilter, major improvement in power and acceleration that you can feel!"

mdillian3690

StanSop,

Hey post some pics and specs! Great Job!

stiles_s

Reanimator: I appreciate the effort, and perhaps Helmholtz resonators are used to tune exhaust pulses as well, but they are absolutely used in intake systems. I spent a bunch of time in the Miata tuning community, and removing this resonator (on the intake) would lose about 5 lb-ft at lower RPMs, which mattered in this car You would get a placebo effect of a high-RPM rush that made it feel faster overall, but the hole in low end torque was obvious. My guess is the resonator in the intake box has something to do with tuning the intake frequencies. I'd also wager low end torque is being lost, but our cars make so much power that it may not matter to you guys.

Here's a random link I googled: http://www.mecc.unipd.it/~cos/DINAMO...suonatore.html

Here's one that talks about the "active helmholtz resonator" in the new 997 3.8. Since it's in the top of the airbox, like the one you guys are messing with, my guess is we can agree that we're dealing with tuning the intake tract using a helmholtz resonator...
http://www.germancarfans.com/news.cf...porsche/1.html

Here's another one that talks about the intake-path resonators. They say they are all about noise cancellation, but that hasn't been my experience...
http://autospeed.drive.com.au/cms/A_1969/article.html

penguinking

is this similar to exhaust tuning on 2-stroke motors?

Re-animator

Thanks for the debate on this. Can't speak for Miatas. It appears Fabspeed has specific data on NA 996 airbox increasing acceleration when the resonator is removed. Based on your link, it also appears that Porsche has a patent pending on a Helmholtz airbox resonator on the 3.8 997S motor. It also states that this new design is for noise. The other stuff talks of intake runners for increased performance. Varying intake runner length and diameter in multiple stages is not new technology. BMW uses similar venturi variability in their runners. This other stuff is related to the intake after the MAF. The 996 silencer is before the MAF. The patent pending intake noise suppressor on the 997S is between the MAF and the throttle butterfly. The links you provided actually support that the 996 airbox resonator is only for noise suppression, which is exactly why we are plugging it up so the intake air will flow more freely into the MAF unsuppressed. Thanks for the links. It is nice to know that the Weissach boys are still coming up with useful noise suppressive technology.

"Helmholtz resonator for unmarred sound
The new, individual-control Helmholtz resonator accommodated in the air filter housing is a typical feature of the acoustic refinement that has gone into the new power unit of the Carrera S. This acoustic damper offering approximately 0.3 litres of additional resonance volume is integrated in the intake system between the hot-film air mass meter and the throttle butterfly. The Helmholtz resonator is activated when accelerating between 5000 and 6000 rpm, reducing oscillations of the intake noise under such conditions within the 260 Hertz frequency range. Porsche has already registered this new system for a patent, the new 3.8-litre power unit benefitting as a result of this technology from a deep, throaty sound under all engine loads and at all speeds, without any aggressive peaks."

stiles_s

Yep, I realized that my links argued that the resonators were primarily for sound.

As far as Fabspeed's claims, I've got to admit I'm biased. I've seen way too many false or "aggressive" claims from tuners selling intake systems to believe that beating Porsche at their own game is an easy task. I take any claim by a tuner with a huge grain of salt. Did Fabspeed plug the tube because it made power, or because it sounded like it did?

Based on some other digging, I've found that the Helmholtz resonators can also be "tuned" to increase power at a certain point in the rev range. That must be what was happening w/the old Miatas -- there was a big torque hole if you removed the intake resonator.

I agree that this is an interesting debate. Neither of us really knows whether the unit in the 996 is there for only noise suppression, or if it also serves to smooth the power band. It would be interesting to hear it from the guy who designed the intake for Porsche. Intakes these days are pretty carefully tuned parts of the entire system -- a far cry from the old days when you just threw a K&N on and went faster

I would still really love to see a before and after dyno. Maybe I'll do it just for grins.

leedav

Interesting. If you dyno, I'd like to see a lot of granularity in the 3300-3600 RPM range. It may be my imagination, but I believe there is a slight surge in that range (my only powertrain mod is GHL mufflers)

Re-animator

Yup, I certainly wouldn't argue that the infamous cap does anything power wise. It just sounds louder and subjectively cooler in the higher revs. Thanks again for the research.

Re-animator

Also, I can understand why they went ROW on the intake design for Swiss noise laws. Hard to drive around Western Europe without going Swiss. It sure takes a lot to get American cars in there.

"Importing American Vehicles into Switzerland

Successful discussions between officials from US Embassy, Bern and the Swiss Federal Office for Police Matters have led to new procedures which will greatly facilitate the importation of American - made cars, sport-utility vehicles and mini-vans.

After the car arrives in Switzerland, the sole testing requirement is a noise test, which can be performed at one of four testing stations in the cantons of Zurich, Vaud, Geneva, and Ticino, for Sfr 240. For model year 1995 and thereafter, the EPA label on all US-manufactured passenger cars will satisfy Swiss exhaust emission standards, with the exception of some vehicles that bear the EPA light-duty truck label. These are classified under the most stringent Swiss and EU exhaust emission standards. Parking lights and turning signals, should conform to Swiss standards, the speedometer must indicate Km/Hr, tires should conform to the top speed of the vehicle, and windshields must be laminated. The owner is also responsible for providing certain data on the engine and dimensions of the car. Apart from these requirements, no additional testing is required."

Importing a car to Swiss land

leedav

That does it. I demand an inspection on all imported Swiss Gruyere and dark chocolate. The inspection station will be in Portland, OR. Details available here.

StanSoph

I am in Portland again this week, car is in Seattle. I will post ASAP when I get back home. I drove down to PDX last week in the P-car. I-5 beat me to death. I am in the truck this week, much more compfortable not to mention more intimidating to the slow drivers in front. I feel the extra noise from the mod definitely sweetens up the sound especially after 4k rpm's. Blends nicely. If I remember correctly the washers are a shade over 1.75" and the bolt was 2.5" long. I opened up the air box and put one washer ouside an one on the insie and bolted together. The bolt head is on the top. The nut and bolt are aircraft hardware, the washers are everyday stainless ; some extra hardware in my shop/garage, I work on planes for a living so I had it laying around. For some reason I always end up wiith extra hardware after working on a plane....... You could cut a circle of aluminum to fit inside the resonator tube against the lip. A stiff piece of plastic or metal cut to fit in the resonator tube is all you really need. The size of the piece is not going to allow it into the intake. If it falls into the airbox it still won't be able to go through the other side. Maybe you have a 5 liter bottle of Cabernet , open it , enjoy a glass or two aand presto chango you have a large cork that will work. If you do have a 5 liter Cab. call me and several people and we can come over and drink and I'll supervise the mod , glass in hand.. If you are really desperate you can duct tape it or a 1 liter Coke bottle......... Sorry about the poor DIY written, I cheated in Technical Writing in college.

Re-animator

Were able to eliminate the silencer from the inside of the airbox this way? I would love to give the box more volume to load the intake charge. TIA

mdillian3690

StanSoph, I look forward to your pics. Thanks! By the way, my orange caps came in. They are not a solid has i thought they would be. I will try the mod this weekend. But wouln't mind a more solid mod.