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Update: The bass boom is starting to annoy me again. I measured it - it happens around 85-90hz. I had my trunk loaded up with a couple of suitcases and I noticed the drone was almost gone. Then when the exhaust got hot it came back again. Based on the temperature dependence and the trunk-loading making a difference, I'm fairly sure the exhaust + cabin is resonating at 85-90hz. My current theory is that when the exhaust gets hot, its length changes and it makes the muffler put tension on the exhaust hangers. To cancel this frequency out with a j-pipe, it would have to be a meter long. A helmholtz resonator tuned to 87hz is doable but it would be complicated to fabricate and might require iteration to get it tuned right. But I'm optimistic the issue can be mitigated by using mass-loaded vinyl on the trunk, or clamping a weight to the exhaust, or by replacing my exhaust hangers.
In the mean time my remus mufflers are for sale.
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