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.

 

[SomptingGuy]

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.

 

[GregLocock]

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.

 

[SomptingGuy]

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.

 

[Rob45]

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

 

[SomptingGuy]

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?

 

[Warpspeed]

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.

 

[Rob45]

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.

 

[Rob45]

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.

 

[Warpspeed]

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.

 

[NormPeterson]

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

 

[carnage1]

http://naca.larc.nasa.gov/

http://naca.larc.nasa.gov/index.cgi...set=0&mode=simple&order=DESC&keywords=muffler

these are some very informative articles from the naca server on muffler design. Also search for david vizard's magazine article on the web, it has some interesting info as well including a backpressure vs power graph.

 

[GregLocock]

http://superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/index1.html

Some of the statements in that article seem a bit odd.

Cheers

Greg Locock

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

 

[carnage1]

That is the vizard article I was refering to.
here is a link to the beginning.

http://superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/

Greg any particular parts of the article seem odder than others? I have had a hard time finding a lot of exhaust info and not all of it seems to agree.