"Two-box" muffler from Blair

[Poweron]

Hi all,

I'm Erik, currently designing the intake/exhaust systems for Columbia University. This forum definitely has some great info over the past few months that I've been reading posts.

More to the point, I've been reading Blair's "Design and Simulation of Four-Stroke Engines" (an excellent resource) and am unable to find what Blair is talking about in chapter 7 when he mentions a so-called "two-box exhaust silencer" (page 743 if you have the book). This specific muffler can apparently reduce the dB output significantly and is second in torque output only to a straight pipe, essentially exhaust without a muffler. I've only been able to find "two-stage" mufflers available from various suppliers and would like to know if anyone has an idea of what Blair is referring to and what manufacturers make these so that I could reach out to get more details on their design. Getting better performance and less noise in one package doesn't seem to logically go together, unless Blair's comparisons are unfair due to dimensional differences in the specific mufflers he compares.

[Mbirt]

I've been through this and hopefully my experience will be helpful here. First, read a little deeper into a few things. Are the noise-related graphs scaled by dB, dBA, or dBC? If I recall correctly, unweighted dB is used, but SAE standard noise tests use dBA and FSAE may be using dBC in 2015. Weighting decreases the significance of low-frequency noise and this might make a purely absorptive silencer of equivalent volume as effective at attenuating important frequencies as the reflective designs. Second, were the silencer canister volumes identical across all compared designs? Total area under the transmission loss curve roughly correlates to silencer volume regardless of tuning used to shift the attenuation with respect to frequency.

Our first noise-legal team-constructed single-cylinder silencer was a two-box design with an absorptive element followed by an expansion volume per Blair's two-box silencer. I didn't get to test it for engine performance back-to-back with future purely absorptive straight-through designs, but there is no doubt that the discharge coefficient at the tailpipe causes a pressure loss. The silencer's main saving grace is that the second half of the volume was not filled with 100-150 g/l fiberglass to save weight. It was an all-stainless construction with an automotive-sized oval shell.

Future silencers were slightly heavier full absorptive designs of similar volume with greater reliability, lower skin temperature (fiberglass insulates the shell from exhaust flow), and better sound quality (behaves as a low-pass filter with no holes in its attenuation at mid-high frequencies).

[Poweron]

To follow up, I even mentioned this to our design judges at FSAE West and it seems like they weren't following Blair either sure either. I've decided it was some jargon for another type of muffler, but our team ended up using a typical racing muffler with glass pack. The real conundrum is designing the muffler without much clue as to what the noise output will be in the first place. The only way to move forward in that respect seems to be getting a baseline noise measurement with a specific muffler and engine/exhaust setup, then conducting trial and error tests based on various modifications. At any rate, there seems to be nothing a bit of steel wool can't solve!

[Jay Lawrence]

For what it's worth, the observation I've made with regards to noise is that a significant portion of it can come from the engine itself (i.e. not just out of the muffler). There's lots of little vibrations that go on, especially with the engine hard mounted to the chassis, and you end up hearing a lot more induction noise and just general engine noise (bearings, cams, etc.) than anticipated. I remember one year with a N/A engine where we were trying to pass noise with bungs etc. and you could hear the difference from the exhaust outlet, but the actual sound level didn't change that much because of all the other noise going on. This was for a 600/4, and I imagine singles would behave slightly differently in this regard.