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A little exhaust design indecision.

A little exhaust design indecision.

A little exhaust design indecision.

(OP)
I'm trying to design a new exhaust system for my Miata, and in this venture I’ve research a multitude of books but they all lead me down the road of build it and test it.  I find myself wondering if there are any exhaust companies out there that TRULY spend a large amount of money on R&D to make a muffler with both superior deadening/attenuating qualities coupled with superior flow at low pressure gradients.  

It seems illogical that any muffler i design could compare to a company that actually makes mufflers and has a large body of engineers working with them. I'm particularly interested in Flow master muffler systems because of their claim to using systems of chambers that create excellent scavenging effects yet flow the lowest of most main stream muffler companies. Its interesting however when I email asking for any technical specifications on they’re product they never respond, it adds in an opposite effect and leaves me asking myself if any aftermarket muffler company really puts a lot of R&D into the mufflers they just put together something that half way works and sell that.

Some of the ideas I had for increasing muffler flow performance if I do find out its not worth while to go with an aftermarket muffler are, creating trumpets on the wave length tuned pipes in the muffler chamber systems to increase discharge coefficient, creating ideal taper angles into and out of the muffler for better transition back into laminar flow, and insulating the muffler from ambient temperatures so that heat loss is minimized in order to keep heat and velocity high.  Can anyone tell me if my ideas are good leads into a better muffler design?

I guess responding to either question or both,  I’m struggling with what I should do, go with someone else’s design or find that maybe my thoughts are original or not cost effective for the major manufacturer and I could see increased attenuating qualities and increased flow.

RE: A little exhaust design indecision.

The whole thing is terribly complex, you only need to look at a factory system to realise how much trouble they go to to make a compact, light, robust, free flowing, quiet system free of resonances and with sufficient ground and body clearance. It sure ain't easy.

So what do you wish to gain, and what are you prepared to sacrifice ?

For a road car, perhaps try to copy the original layout, using all the original factory mounting points and pipe lengths, but with slightly increased pipe diameter and use mandrel bends. If the original had resonators or expansion chambers fitted in unusual locations do the same. I would just scale everything up in flow area, at least as a starting point.

With a bit of luck it may flow much more freely with only a slight increase in overall noise level.



RE: A little exhaust design indecision.

I think you are right to be cynical.

Your ideas for reducing back pressure seem to be on the right track, although I don't understand why you want to keep temperatures high, unless you have a tailpipe turbo.

Bear in mind that back pressure does not affect most of the driving range at all, and, typically, doesn't even hurt full power a great deal.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: A little exhaust design indecision.

I've just found a page that says that for a standard WRX turbo the exhaust +cat gives a back pressure of 12 psi at 225 hp. removal of the exhaust+cat gives another 20 hp.

Now, I'm a bit astonished by the 12 psi number, but guessing that it is a bit higher than your standard system, and your engine probably is no more than 200 hp, it seems likely that the best you could do with the factory cat is a 10 hp improvement. I think there are easier ways to get that sort of performance boost.

However, there is an easy way. Build a straight through pipe, put a butterfly valve at the tailpipe, and dyno the thing. Mess about with the valve setting and produce a graph of back pressure vs power.

Then post the results here.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: A little exhaust design indecision.

I can well believe that significant exhaust back pressure will hardly effect a modern production "emission" engine with zero valve overlap.

But surely it would not help a traditional "sports" engine with generous valve timing, and where some attempt at exhaust assisted scavenging is being attempted.

While it may all be horribly illegal and antisocial these days, some of us find the new ways difficult to adapt to after a miss spent youth.

I have had some success with a rear mounted exhaust butterfly. This is lightly spring loaded with a hole drilled in one side only to unbalance the pressure. Under normal road load it tends to maintain a fairly small constant back pressure, and keeps the noise in.

At above around 90% throttle a cable mechanically forces the butterfly to the fully open position. It may not go much faster, but the sound is glorious.

RE: A little exhaust design indecision.

Since most OEM exhaust systems are optimum for only a specific hp output, read that 'stock', I have always found ways to improve performance, mileage, hp, etc. by modifying those stock systems to suit ones individual requirements.

In racing, my passion, the higher the ultimate hp the engine makes, the greater the exhaust system mods effect overall performance.
To be PC these days, most tracks mandate some form of silencing device to hold noise levels to some arbitrary limit.  None of the builders/owners/drivers that I know, care much for the idea save the rare time when a silencer actually improves output.  Rare, but it still happens on occasion.
In my case, I am currently using the Flowmaster Hushpower II on both a 1380cc Mini and a 1594cc DOHC Lotus...both have several dyno sheets and generally all results tend to support Flowmaster's claims of very little hp loss (~2%).  However, I have seen a drop in midrange torque output of aroung 5%.  My tests were done on a Clayton chassis dyno back to back with an open exhaust. I have dyno sheets available for the last three or four 1380 tests from June and Sept. of this year, if your interested for comparative value.
If your  going to do your own R&D, Flowmaster is a great place to start. It beats the heck out of starting from scratch as I did many years ago.


Rod

RE: A little exhaust design indecision.

(OP)
My intent is to have a vehicle that is quiet enough for long distance drives and has little resonance from 3200-4200 rpms. Everything else is really fair game as those are cruising speeds with the gearing in the car and while driving around town i don't mind the noise. Even if the muffler didnt have enough dampening qualities on its own I could still build in helmholtz systems to knock the resonance down at those rpms... but heres a further question, what flow losses do helmholtz systems cause?

RE: A little exhaust design indecision.

(OP)
Thanks for your inputs, and i was interested in your dynographs, I was also interested in the attenuating qualities of the hushpower muffler, mayby if you had a sound clip or video of the dyno before and after?? That would be great.  Thanks !

RE: A little exhaust design indecision.

On the dyno at full chat, I measured the sound at some 92 dba at 50 feet distance to the rear of the exhaust pipe outlet.  The instrument I used belonged to the dyno operator and I have no idea how accurate it was as all I was shooting for was <103db...SCCA legal for my class at the Laguna Seca track the end of this month.  I have no idea how it would function on a street car but, IF it was really 92db at 7600 rpm that I saw, I would think it ok to try.

Rod

RE: A little exhaust design indecision.

Qoute from GregLocock:
"I've just found a page that says that for a standard WRX turbo the exhaust +cat gives a back pressure of 12 psi at 225 hp. removal of the exhaust+cat gives another 20 hp.

Now, I'm a bit astonished by the 12 psi number, but guessing that it is a bit higher than your standard system,"

There is no way that is a correct backpressure.  12" of water maybe but even that would be high.  Turbo engines do not like backpressure plus the turbo does a good job of converting impulse energy to boost so they are inherently quiet.  Typical turbo backpressure would be about 5" water.

RE: A little exhaust design indecision.

Although I learned long ago not to doubt our 'chief engineer', 12 psi does seem a bit excessive...However...We had my crew chief's car, a 2004 WRX, on track at Las Vegas last year to do a little testing (playing?)after he had replaced the OEM exhaust system with a proprietary ($$$) system...BIG 'seat of the pants' improvement in performance.  Perhaps the stock exhaust has high backpressure for some reason, maybe emission related?
I have only had a couple of turbo anythings and the only one that was dynoed in the 80's did not have backpressure anywhere near that number.

Rod

RE: A little exhaust design indecision.

Most medium and high speed marine Diesels with turbos specify a maximum backpressure between 27 and 40 inches of water.  Diesels, especially turbo Diesels, don't like backpressure.  Recent 'high performance' versions allow 20 inches, which is barely achievable with a noisy exhaust system.  Locomotive engines specify 10 inches, which is basically not achievable with a muffler.

Exception:  Some recent Volvo engines specify 10 min, 25 max kPa ( 1.4 min, 3.6 max psi).  I have asked; nobody outside of Sweden knows why a minimum is specified, or why it's so high.

I'm also astonished by the 12 psi number.  That would be more credible, upstream of a turbo.

Mike Halloran
Pembroke Pines, FL, USA

RE: A little exhaust design indecision.

I have never measured the back pressure in a WRX, but I have certainly recorded similar pressures with other high output turbo engines fitted with original factory exhaust systems. Most production cars end up being several psi flat out. It is the only way to meet EPA decibel limits these days.

There is no magic muffler that can even out violent explosive pressure pulsations and have zero back pressure, think about it.  

Some high frequency absorption is the best that can be hoped for in a straight through muffler. That will take off some of the "nasty edge" of the sound, and mellow it's character, but in reality the measured sound pressure level will not always fall as much as subjective hearing tests may suggest.  Meeting a decibel limit is much more difficult.


  

RE: A little exhaust design indecision.

After re reading all this, I just thought I would add the best part about many/most of the aftermarket exhaust systems for the WRX...they make it sound just like a VW bug!   After driving Matt's at Vegas, I am not a big fan of the WRX...maybe I am just not talanted enough to make that stock 5 speed work...or maybe the linkage is junk?

Rod

RE: A little exhaust design indecision.

Donaldson in Australia manufacture truck exhaust systems, and the usual design figure for a road diesel is 2psi (about 56 inches). Mike's figures sound about right too.

Now Donaldson give a range of recommended pipe sizes for
various rated engine horsepower to give approximately 2psi.

75kW 2"
120kW 2.5"
165Kw 3"
230Kw 3.5"
375kW 4"
550kW 5"
635kW 6"

Truck and car systems might be roughly the same overall length, so the pressure drops may be comparable. These pipe sizes are well above what production cars use, and that should tell you that production cars are going to have much more than 2psi pressure drop.

Noise attenuation is an entirely different matter.

RE: A little exhaust design indecision.

(OP)
So how does pipe diameter and sound relate to each other? It would seem atleast conceptually that the larger the pipe the lower the frequency emission...  so as you increase pipe size you decrease the need to muffle high frequency sound, and increase the need for a muffler that dissapates low frequency sound....  which causes you to switch from straight through mufflers to some kind of baffling muffler to cut the same amount of sound out...

 Seems there might be a trading point between acceptable frequency /magnitude of the waves and pipe size/different mufflers needed to attenuate those frequencies... anyone concur?

RE: A little exhaust design indecision.

Bear in mind that a legal cat has several psi of back pressure all by itself (there is a very expensive tradeoff involved).

If I had to pick a number, I'd guess most production exhausts, with cats, have 4-6 psi back pressure at max power.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: A little exhaust design indecision.

The biggest noise problem is the explosive wavefront coming out from each port during the initial exhaust valve opening. If you have ever started an engine up without any exhaust manifold fitted, even at idle, that crackle and harshness is really unpleasant. The other issue is low frequency pulsing of bulk gas flow from each cylinder which is quite different.

One is a steep acoustic shock wave, the other a low frequency high amplitude pressure wave.

Anything like tuned pipes or extractors that are going to maintain gas velocity, is going to be more noisy.

A plain pipe is a wonderful transmitter of sound. The old traditional ships bridge/engine room speaking tube is a good example.

Absorption mufflers can only attenuate the higher frequency components which changes the character of the sound, but does not reduce the overall measured sound power by very much.

To reduce a violently pulsing high pressure gas into smooth constant steady flow, there must be some restriction somewhere, there is no other way.

Any sort of pipe resonance will most likely increase the amplitude significantly at certain Rpm. Preventing drone at particular speeds is yet another problem to overcome.

None of this is simple or easy because it is not just one problem.

RE: A little exhaust design indecision.

(OP)
So basically what your trying to do is dissolve the pressure waves...  what if you converted them to a frequency that was above or below human hearing...   Know of any frequency converting devices like that?

RE: A little exhaust design indecision.

Yes, a turbocharger does exactly that. It converts higher frequency pressure waves into static pressure.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: A little exhaust design indecision.

(OP)
Well yea...    we need a complete list of frequency conversion devices though to attack this problem. restriction like a turbocharger is definitely one.

RE: A little exhaust design indecision.

The main reason a turbo reduces noise is the conversion of a lot of waste heat into intake boost.  

RE: A little exhaust design indecision.

No it isn't.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: A little exhaust design indecision.

(OP)
Basically it takes the big puts and cuts them into lots of little puts.  What your trying to do is desychronize the wave over t.  That is reduce the pressure gradients on each side of the air pellet, and make one nice long static discharge into the atmosphere. As described before the initial hit of the air from the cracking of the exhaust valve causes a wave, and that wave really doesnt make sound until it hits the atmosphere... in which case it causes the accoustical vibrations..  I think drone is caused by too much exhaust system volume and too little flow.  THat is the accoustical waves begin generating much sooner because atmospheric is reaching farther into the pipe..  I'm exploring this area as well..  We'll see if i get slaughtered or not ;)

RE: A little exhaust design indecision.

Temperature has nothing to do with it. If when you start up a stone cold engine on a frosty morning, it is no louder or quieter than when it has achieved full normal running temperature. There may be clouds of condensing water vapor for a while, but it sounds no different.

RE: A little exhaust design indecision.

Quote:

If when you start up a stone cold engine on a frosty morning, it is no louder or quieter than when it has achieved full normal running temperature

Eh?  The speed of sound in the exhaust changes during warming, which has a pretty noticeable effect on the resonant frequencies.  It's a pretty important thing to capture when trying to predict exhaust noise.

RE: A little exhaust design indecision.

Quote:

The main reason a turbo reduces noise is the conversion of a lot of waste heat into intake boost.

Not true.  Both parts of a turbo (compressor and turbine) are very inefficient at transmitting pressure waves.  Bizzarely (in my mind) Some OEMs are actually dabbling with "sound pipes" these days.  The basic idea is to route noise from the high-pressure side of a turbo (either intake or exhaust) to a site where it can be heard inside the car.  From memory, BMW seem to like this idea at the moment.

RE: A little exhaust design indecision.

(OP)
T=Time your trying to cut the wave up into small segments where the expansion into atmosphere isnt as violent or concentrated..Similiar to using assymetric tread designs on tires,  lots of small individual noises of different frequencies amounts to nothing, but if all the tread tiles were the same the tire becomes quite loud.  

Is it the pressure wave that makes the sound or the mixing of extremely hot gas with cool atmosphere... I'd imagine a combination of both seeing how one determines the other PV=NRT buttt...    we should be able to break down the ratio of which causes more.

RE: A little exhaust design indecision.

SomptingGuy, measure the speed of a turbo. You'll see that it absorbs the firing order pulses, and accelerates and decelerates in response to them. It would be interesting to measure the exhaust noise with turbos with different inertias. The aeroelastic coupling may be quite important as well.

That's why a turbo's exhaust can be basically a low pass filter, the low frequency resonators aren't needed.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: A little exhaust design indecision.

I guess turbos do let a fair amount of exhaust noise through (and yes, the speed does vary through the engine cycle).  But compressors are really effective at supressing virtually all intake noise - not good if you want a car to sound sporty when you jump on the throttle.

RE: A little exhaust design indecision.

hello,im a new contributor ,so hi to all!
running a series of experiments on a 2 litre propane powered 8 valve astra engine,that was dynoed at 135 bhp at 5500.this i equated to approx 150 on 97 octane petrol eg minus the oft quoted figure of 10 percent loss due to the propane displacing air and its lower energy per cc ,i used a exhaust system that was 2.5 inches dia,one straight through absorbtion type silencer 20 inches long and 6inches wide ,then a hedman turbo muffler 2.5 inch inlet .this ws then tested for back pressure .and at 6500 gave a max of 1.5 inches water.this was within acceptable limits.in my mind having donew considerable computer modeling and design work ,oh ic engines ,i se the air flow through the engine as bieng like a stream ,it flows in the air filter ,and out the exhaust .any restriction or back pressure is  bad .any usable pulse tuning in the ex whether organ pipe pulse effect or inertia  effect ,is a good thing if it creates a better cyl fill .but the average back pressure should be as low as poss .i wonder wether these ex presures of 12psi etc ,are pre turbine ?
thanks
robert

RE: A little exhaust design indecision.

oh incidently,the prime influences on  exhaust note for tone and volume ,are the opening point of the exhoust valve,in relation to the point of combustion , the compression ratio and the primary pipe diameter .generally the later the ex valve opens ,and the lower the cr ,the quieter the exhaust.imho.!

RE: A little exhaust design indecision.

As Greg says 4-6 psi without significant noise would be a worthy achievement on a legal road car. I have measured three times that on a normally aspirated engine, and even more on a turbo engine (after the turbo).

Using Donaldson's figure of 2psi for 120Kw with a 2.5" pipe, and your engine being around 100Kw, are you sure you dont mean 1.5psi not 1.5 inches of water ??

RE: A little exhaust design indecision.

yes im sure .i think maybe donaldson is referring to a diesel engine in those sizings.a diesel flows a lot more air than a petrol engine to get the same power thats why one uses a t25 on a 2.5 diesel ,and the same size turbo would suit. around about 1.8 on a petrol.my main point was to give some practical data about the possibilty of a reasonably quiet exhaust with almost no back pressure and its dimentions for the topic originator to try on his miata.

RE: A little exhaust design indecision.

(OP)
I definitely want to keep a cat in the exhaust...   what i was thinking of doing is buying a cat with a very large intake and exhaust pipe (and a hopefully proportionatly larger inner matrix) for minimum back pressure.  I know there is no such thing as a high performance cat (well not one for under a thousand) but it would seem that cats with large intake and exhaust side would perform alot better.  

On a side note after reading Four stroke performance tunning  A. Graham Bell cited some imperical evidence that alot of flow loss comes from poor housing desing, entry and exit angles that are too high, does anyone else back this statement?

RE: A little exhaust design indecision.

David Vizard is a proponent of fitting long tapered modified sections to catalytic converters too, his flow testing suggests that it is well worth the trouble.


 

RE: A little exhaust design indecision.

According to ASHRAE tables, long cones would be much less restrictive than the short cones and steps that are usually fitted.  But fitting a six-foot cat under a car is a challenge, and recent cats are fitted _very_ close to the engine to speed lightoff.

Mike Halloran
Pembroke Pines, FL, USA

RE: A little exhaust design indecision.

(OP)
With the Miata its not an issue, the cat is rearward fitted (stock location) and in the long gaulley between the engine and the rear wheels.  I'm excited to start work on it thank you all for your inputs.

RE: A little exhaust design indecision.

Isn't some amount of backpressure desirable to discourage erosion caused by blow-by (erosion) past the exhaust valve seats?

RE: A little exhaust design indecision.

Has anybody heard of a variable muffler? Where in there is a floating baffle with some spring pre-load.

My understanding is that at low throttle the spring keep the baffle "full closed" and like any regular muffler, and as throttle is increased, the increased flow would push the baffle open to reduce back pressure and increase performance.

I have heard of this being used on a small two stroke, but would the effect be the same on a 4 stroke?

RE: A little exhaust design indecision.

It's a fairly common device for getting sporty vehicles through type approval (noise) tests.  The pre-load is designed so that the muffler is more resrtictive and quieter at pass-by conditions, but opens up at high load/speed to give a "better" sound.

The justification is somewhat flakey (in my view).  Manufacturers state that it's a fuel economy device - lower back pressure at higher speeds/loads means less fuel used.

RE: A little exhaust design indecision.

Toyota use something similar in the Camry I think.

Now, the bizarre bit is that the external noise test is at full throttle. There is also a stationary noise test, but in my experience it is a formality.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: A little exhaust design indecision.

Quote:

Now, the bizarre bit is that the external noise test is at full throttle.

True, but the entry conditions and test length (at least for the European pass-by tests) are such that the valve stays shut during the test.

RE: A little exhaust design indecision.

As an exhaust system engineer, I can tell you there are sure a lot of misconceptions in this thread!

First, about backpressure:  as was pointed out above, bacckpressure has little effect on engine performance except at high rpm and WOT operation,  and even then it is dependent on a number called the "valve overlap factor" (VOF) which is defined as the area under the valve lift vs crankshaft rotation plot in the region of exhaust valve closing/intake valve opening.  That is,  the greater the valve overlap factor,  the greater the engine's backpressure sensitivity,  or loss of power with increasing backpressure.
I have stashed away somewhere a fascinating study done at Ford in the late '80's where power loss was plotted vs VOF for a variety of passenger car and light truck engines, done as part of the MN-12 (Thunderbird) program.  It showed clearly that 4-valve engines (case in point was the 5.0L Porsche 928 S4 rated at 300HP) typically have very  low valve overlap factors, compared to 2-valve engines,  and very very little backpressure sensitivity.
I later confirmed this in testing done on the Lotus-designed ZR-1 Corvette's LT-5 engine,  for which I designed a variable-backpressure muffler.

One other quick point at this time is that few if any aftermarket mufflers show any evidence whatsoever of attention to reducing noise at the driver's ear.  You're right to note that the OEM's have vastly greater resources to devote to this task.

More to follow.
 - R

RE: A little exhaust design indecision.

Aftermarket mufflers are not (in Europe) subject to type approval (in terms of pass-by noise).  Hence the big market for aftermarket motorcycle exhaust syetems.  However, original equipment must pass European noise legs.  And squeezing your vehicle through them isn't always squeeky clean.

I'm not sure about other countries, but there's no noise test in our annual vehicle checks.  Police can stop you if your vehicle is too (subjectively??) loud though?

RE: A little exhaust design indecision.

Very true Rob45.

The late model production four valve engines generally have zero (or absolutely minimal) valve overlap. But if an aftermarket sports camshaft is subsequently fitted to one of these engines, there can then be significant valve overlap.

The exhaust reversion problem will be far more extreme than with a two valve head with similar valve timing because of the very large valve curtain area.

The variable area muffler is a clever idea, I have only read about them and seen pictures, but never actually tested one.

On my own turbocharged road car I have had some considerable success at reducing noise with an automatic exhaust butterfly located near the rear of the exhaust system. I developed this idea myself and am very satisfied with the results.

This consists of a very simple centre pivoted homemade butterfly valve fitted into the exhaust pipe just before where the pipe goes up over the rear axle.

It is very lightly spring loaded, and held closed against a mechanical stop. A 12mm hole is drilled on one side of the butterfly plate to unbalance the pressure. This causes the butterfly to open by itself from upstream pressure, and maintain a low but fairly constant exhaust back pressure.   

At constant road speeds, the reduced pipe area at the butterfly vastly reduces the noise and any tendency to drone. Spring tension alters both the noise level and back pressure. It can be made as quiet or as noisy as you wish. About 0.25psi  to 0.5 psi will work wonders.

A further refinement is to use a very long throttle cable extension to mechanically force the butterfly to the wide open position beyond some set accelerator pedal position.

An easy way to construct this is to fit a pair of welded bolt flanges to the exhaust pipe, these can then either be bolted directly together, or the experimental butterfly plate sandwiched between with gaskets. That makes it very easy to modify or remove.

The drilled hole in the butterfly plate needs to be about half way between the edge and centre of the plate. Using a simple flat, cut onto the edge of the plate to unbalance the pressure (instead of the hole) will not work, it causes the butterfly to flutter, accentuating noise. But with a hole located away from the edge, flow friction around both sharp edges of the butterfly seem to damp out any tendency towards flutter.

Although only made from mild steel, it has been completely reliable and because it is in constant motion and runs quite hot, corrosion or exhaust deposits do not seem to have caused any problems over four years of daily running. It has started to rattle slightly (at idle only) due to wear in the shaft. Fabricating another butterfly plate using better construction and materials is a job I will get around to one day.   
 

RE: A little exhaust design indecision.

OK, one other point: a while back, while working for a major exhaust system supplier,  we decided to see what we could do to improve flow through a muffler without changing its basic design.
We took all the internal pipes for a muffler  and made very simple conical flares at the inlet and outlet ends.  We also made all ports (holes) in baffle plates into extruded holes.
The net result of these simple (but not necessarily inexpensive) changes was a 20% reduction in flow resistance of the muffler.
When we assembled one of these into a production muffler shell (using the adjacent muffler manufacturing plant), and furnished it to an acquaintance of mine running "Showroom stock",  it showed shall we say, a "worthwhile" improvement in performance.
-----------
To get to the point:
a significant amount of the backpressure found in a production performance car exhaust system is often due to mismatches and transitions; for example, when a large diameter system pipe goes into a smaller diameter muffler inlet bushing,  it is often possible to reduce backpressure  by using smaller diameter system piping,  matching pipe diameter to muffler inlet diameter.

There's more.

RE: A little exhaust design indecision.

Warp:
Variable-tuning mufflers have been in (and out) of production for a number of years,  as far back as maybe the late '80's in for example the Nissan Skyline (in Japan only, AFAIK),  and in the '90's in certain Mitsubishi and BMW (3-series) models.
The Nissan at least used a Boden cable with an actuator driven by an RPM sensor, opening at a specified RPM, over a period of about one second,  and closing then at about 250 RPM lower to prevent unnecessary cycling. This operated a butterfly on an auxiliary pipe that short-circuited a portion of the muffler.  I presented a variation on this system to GM for Corvette applications in about 1990.  They wanted it at xero cost, of course...

Then Arvin or maybe it was Walker came up wtih a spring-loaded, backpressure operated valve internal to the muffler;  this was much later,  and probably was licensed to them by a Japanese manufacturer.  I don't recall if this ever saw the light of production.

RE: A little exhaust design indecision.

I believe some Lamborghini and Ferrari models also use various types of butterflies and bypass valves in the exhaust system to keep the exhaust noises down at part throttle.

The idea is certainly not original, just as you say, but the technique is not often known about or copied by the do it yourself at home hot rod people.

Just placing your hand over the open pipe end, and partially closing off an idling exhaust pipe will significantly drop noise levels.  The trick is to have something that will open up and close off the exhaust cross sectional area rapidly, with with changing gas volume. Whatever is used, MUST ABSOLUTELY NOT VIBRATE OR RATTLE, it needs to be quite well damped somehow, or it will just add characteristic noises of its own making things worse.

Another idea I have though about, but not tried is the use of a fairly large turbocharger exhaust wastegate located in the exhaust pipe. If fitted with a suitable light spring, it would open against the spring holding back some small residual back pressure. Just a simple low pressure blow-off valve. I suspect the poppet valve would vibrate fairly violently and chatter on it's seat and probably not work too well. But the pneumatic diaphragm in the actuator could perhaps be filled with some type of light fluid and damp the whole thing hydraulicly ??  That could possibly eliminate all the rattles but still make for a fairly fast smooth operating device.  Not a low cost solution unfortunately, but some of those cheap Chinese external wastegates should be well up to the job.

RE: A little exhaust design indecision.

My '01 Nissan Maxima has such a muffler, and the sales brochure specifically refers to "a special valve made of a heat-resistant alloy used in jet engines".  Unfortunately, it's remained in good condition, and my own curiosity by itself isn't enough to justify cutting it open to see how their approach works.

Norm

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