Don Terrill’s Speed-Talk

j-c-c wrote: ↑Thu Nov 02, 2017 3:27 pm That hemi panter thread was interesting and worth reading, on everything but the muffler.

Sound-Shaping is Like Walking an Acoustical Tightrope

“Every four-stroke engine has an acoustic excitation where the base frequency is half the engine order of 1.0,” explained Martin Unbehaun, Faurecia’s acoustic analysis engineer. “Therefore a single four-stroke cylinder has an engine order of 0.5 (one excitation every two revolutions of the engine), so every car exhaust note comprises multiples of the engine order 0.5 (1.0, 1.5, 2.0 and so on), depending on the number of cylinders. When a second cylinder is added to the first, the exhaust pulse comes between the two pulses of the first cylinder so all half engine orders are cancelled by the second cylinder, leaving just the full engine orders.”

“Compared to an engine operating on just four cylinders, a V8 engine has an exciting character with tonal pitch at approximately 6,000 rpm, because there are twice the number of cylinders,” Unbehaun added. “If the exhaust system is symmetrical and the mixing is good, most of the ‘half’ engine orders are cancelled, leaving mainly engine orders four and eight to give a V8 sound reminiscent of race cars. At the other extreme is an engine with asymmetric manifold and poor mixing, which results in many half-engine orders. Even though the engine may be identical in design, the exhaust sounds completely different, giving a harsh rumbling note much like a muscle car. Neither of these extremes are what we want for a luxury vehicle.”

“The design of the TFSI exhaust manifold was defined and fixed, as was the routing of the pipes and available space for mufflers,” Unbehaun shared. “The basic layout consists of a twin exhaust system with two front mufflers, a center muffler and crossover pipe and two tail mufflers containing valves to vary the system performance according to the engine mode.”

“However, despite the fixed design constraints, we found it was possible to experiment with the mixing and with the internal geometry of the mufflers using Ricardo’s WAVEBuild3D Primitives CAD modeling software. Compared to other alternatives, WAVE was faster, the file sizes are smaller and once the models have been created, they can be easily inserted into simulation software, which generates crucial savings for an OEM.”

Implementing an Acoustical Mindshift

Three layout variants were designed and built using WAVE software, then simulations were run to finalize the optimal exhaust design. The virtual components included:

• Different front muffler sizes and mixes.
• Cross pipes were evaluated at a range of sizes to achieve an optimal solution.
• Various mid-mufflers were also assessed with more or fewer perforations, with and without mixing in the center, or with Helmholtz resonators.
• Rear mufflers were tested with both straight-through pipes and resonators.

“The first version featured symmetrical downpipes, a mid-muffler with absorption and equal length tailpipes; the sound produced had more rumble than was desirable,” Unbehaun noted. “In this system, the sound produced was mainly in the range of 50-60 Hz, with little fourth order content but with a strong contribution from the first and 1.5 orders. Four-cylinder mode exhibited too much boom, even with the valves closed.”

“The second layout was a more sporty version with asymmetric downpipes, no mixing in the cross-pipe, very few perforations in the mid-muffler baffles, and straight-through tail muffler pipes. Adding a resonator in the rear muffler reduced the low frequencies and emphasized the high frequencies, but this was still too loud and aggressive for an Audi.”

“The Faurecia team decided on a final layout that had asymmetric downpipes, a mid-muffler with Helmholtz resonator, and unequal length tailpipes,” Unbahaun stated. “In this configuration engine order 4 was emphasized, booming at around 60 Hz was reduced, and the asymmetric downpipe and full mixing produced enough rumbling for the powerful character desired in eight-cylinder mode. In four-cylinder mode the sound was dominated by the second engine order with the valve open, but with the valve closed the system produced a mellow, unobtrusive sound — exactly what the Audi engineers were looking for during low load cruising.”

“To fine tune the sound, the Faurecia team then built and added a small number of sets of tail and middle mufflers,” Unbahaun continued. “The front silencers were carry-over parts from existing models but were fitted asymmetrically, with a larger silencer on one side and smaller ones on the other. When final simulations were complete, the exhaust systems were built, fitted and tested on a chassis dynamometer to carry out the fine-tuning over a period of several days. Then tail pipe and interior noise were measured in vehicles driven on the road in real-world conditions. After these initial assessments, minor adaptations were made to achieve the desired sound.”

MadBill wrote: ↑Sat Nov 18, 2017 8:31 pm I can't get the audio to work!

pdq67 wrote: ↑Tue Nov 21, 2017 5:04 pmI still don't understand why just a shoebox on the front end of a long 2.5" dia., WDT real true smooth perforated inner pipe glass pack won't work?? Hell, pack it with SS rovings instead of fiberglass and go...Talk to me here!!!pdq67

Kenova wrote: ↑Tue Nov 21, 2017 4:36 pm I'd be more afraid of small children getting stuck in there! Ken