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.

 

[Warpspeed]

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.

 

[GregLocock]

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.

 

[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, 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.

 

[Warpspeed]

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.

 

[evelrod]

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

 

[TravisR]

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?

 

[TravisR]

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 !

 

[evelrod]

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

 

[dgallup]

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.

 

[evelrod]

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

 

[MikeHalloran]

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

 

[Warpspeed]

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.

 

[evelrod]

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

 

[Warpspeed]

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.

 

[TravisR]

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?

 

[GregLocock]

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.

 

[Warpspeed]

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.

 

[TravisR]

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?

 

[GregLocock]

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.