A little exhaust design indecision. 1

[TravisR]

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.

 

[TravisR]

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

 

[turbocohen]

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

 

[GregLocock]

No it isn't.


Cheers

Greg Locock

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

 

[TravisR]

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

 

[Warpspeed]

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.

 

[SomptingGuy]

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.

 

[SomptingGuy]

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.

 

[TravisR]

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.

 

[GregLocock]

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.

 

[SomptingGuy]

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.

 

[ivanheow]

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

 

[ivanheow]

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.!

 

[Warpspeed]

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 ??

 

[ivanheow]

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.

 

[TravisR]

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?

 

[Warpspeed]

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.

 

[MikeHalloran]

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

 

[TravisR]

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.

 

[qform]

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

 

[curious125]

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?