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Again she's going to pieces, just to be rebuilt stronger...
There's been a lot of though put into the new exhaust system and now it's time to see if it works as well in steel as on paper...
Below is the old dual 3" exhaust. It was just what the doctor ordered for the 700 rwhp system and stock engine. Certainly flowed enough for that. Two cross-over tubes, on ahead of the mufflers and another close to the tailpipes would have probably made it a bit quieter.
For what it's worth, my current state of thinking about silencing the turbo-back exhaust is the following:
- One should have separate muffling strategies for high-frequency, medium frequency, and low-frequency noise.
- Placing the mufflers as far back as possible is the best.
- Since there's not an infinite amount of space at the back, some choices have to be made. Because the pipes and mufflers can generate whistling sounds themselves, the high-frequency muffler should go to the tail last. The medium-frequency and low-frequency mufflers can go further to the front.
- Absorptive "glasspack" mufflers take out high-frequencies, simple chambered mufflers take out medium frequencies, and complex chambered mufflers with Helmholtz resonator chambers take out low frequencies; so that's the order in which I'd place them starting from the tail pipe.
- One should not use two similar type mufflers in the same exhaust system. So the second chambered muffler in series, especially if the same exact make and model, probably doesn't make sense.
- Exhaust cools and becomes much denser as it flows in the pipe. Therefore, I think that the pipes should be fat close to the engine and the tailpipe muffler(s) right at the rear of the car can (but don't have to) be skinnier.
- Main determinants of muffler effectiveness are (1) the case volume and (2) the ratio of the pipe cross-sectional area to end plate area. That comes from the math on expansion chambers. This reinforces the idea of placing mufflers as far back possible, because cooler gasses allow one to use smaller pipe size in the rear. Holding the case dimensions constant, the smaller pipe will be muffled better.
- In terms of the pipe section lengths, short pipe sections that are interrupted by mufflers are better than long, uninterrupted, and droning pipe sections. The magic number seems to be 2/3 in terms of keeping the sections close to 2/3 of each other lengths and not close to low-order integer multiples of each other. If one is to nitpick, the length should be adjusted by the relative speed of sound which depends on the exhaust gas temperature.
- A crossover is beneficial every time the pulses in the two pipes are not identical. So, right after the turbos one should have a crossover such that some of the noise cancels. Also, after the unequal pipe length sections and the resulting minor pahse shift in the rear, one also should have a crossover.
So you'll take $50.00 for the "old" system then? LOL. Just kidding. That's a thing of beauty, even if won't support more than "only" 700 hp. :-)
I think it's been sold to a good home. It's got cats and test pipes that can be swapped with v-band clamps. Welds look awesome, both inside and outside.
The one tweak I'd make to it if running it with the 700hp system is the following: Add two crossovers, one upstream and one downstream of the rear-axle mufflers.
The first crossover could be integrated to the cat and test-pipe elements. The second cross-over could be integrated near the tail pipe hanger. This is simply to alter the sound to my liking.
They cover the basic resonator shapes and styles and show both theoretically and experimentally how they attenuate different sound levels. The document is long and comprehensive.
This document has multiple muffler blueprints with the associated sound results at three rpms and three load levels.
On the practical side of things, the new downpipes will start at 3" like the current ones, but then expand with a cone to 3.5" right before the wastegate dump pipe merge. The wastegate pipes are increased to 2" diameter, not so much because of flow capacity but for sound attenuation purposes. When the wastegate is closed, the wastegate dump pipe that merges to the main pipe will act as a quarter wave resonator. The two sides have slightly different wastegate dump pipe lengths, and depending on the exhaust gas temperature they'll likely significantly attenuate two frequencies in the 200-300 Hz range, basically for free in terms of incremental back pressure.
Turbos (with wastegate closed) will take out frequencies above 400 Hz pretty effectively. The absorption of muffler packing will really help with frequencies above 300 Hz. The 12" long muffler cases also work as expansion chambers, and take out frequencies in the 200-400 Hz well. As mentioned above, when wastegates are closed, the wastegate dump pipes attenutate a couple of frequencies in the 200-300 Hz range. All this said, I think we'll be well covered in terms of frequencies above 200 Hz. The y-pipe merge will help drive the natural engine frequencies higher, where we now will have the muffling to kill it. The large pipes slow down the exhaust velocity and reduce the flow-generated noise. Furthermore, all pipe sections are relatively short and unequal length, so we shouldn't be producing much "drone" resonance at below 200 Hz. There's room for a Helmholtz or side-branch resonator to take out one problem frequency under 200 Hz later if there is one after it's all said and done.
Here's an excellent article by BMW about how they muffle turbo cars: http://www.bmwblog.com/2014/04/30/bm...xhaust-system/ It looks like all the competently designed modern sport-car exhausts have electrically activated valves, and those BMW systems are not exceptions.
And here's something little from the Handbook of Acoustics by Malcolm J. Crooker. How much does a turbocharger reduce the exhaust noise at each frequency?
Here are some estimates:
Octave Frequency Band (Hz), Unmuffled Exhaust Attentuation (dB)
31, 5
63, 9
125, 3
250, 7
500, 15
1000, 19
2000, 25
4000, 35
8000, 43
A-weighted, dB(A): 12
So what's the moral of the story here? The turbocharger muffles the high frequencies very effectively. It doesn't do much for low frequencies. Therefore, one should spend the available space to muffle low frequencies in a turbo car. Pure absorptive mufflers, such as small case diameter glass packs, are mostly a waste of space on a turbo car, other than to take out flow generated noise.
Last parts are here. John now has all the components he needs to build the bigger downpipes. The main pipe transition is very gradual because it has to flow well. The wastegate dump pipe goes to 2" as early as possible to make it as large quarter wave resonator as possible.
Molds being made of the stainless steel components to give John the option of switching to a different material:
The mold can be used as a fixture for stainless parts. It could also theoretically be used to cast ceramic plugs for making parts from other materials.
05-09-2017, 06:35 PM
Disassembly starts again
