Folks,

The stock exhaust of my '12 MR drones quite strongly in several RPM ranges. Strangely, the drone peaks at around 1700 RPM or so. Just to disambiguate, by drone I mean a very low frequency sound plus inaudible (sub-20Hz) vibration. Further, I am not bothered by the overall SPL level of the engine, and I am OK with it getting louder, if it helps. The drone is driving both me and my wife nuts, which is a great pity, as, otherwise, Evo is a supremely enjoyable car!

I have done my homework:
1) Lots of research on this forum and on google
2) Phone conversations with several aftermarket shops
3) Phone conversations with exhaust manufacturers (Works, RRE)

Sadly, while I gathered lots of information, its value is dubious because much of it is self-contradictory, exacerbated by peoples' vastly different levels of tolerance for noise, vibration, etc.

For example: the exhaust systems that seem to get the most praise for being quite and/or drone free are:
1) Road Race Engineering Stealth catback
2) Tanabe medallion touring
3) Cobb SS-3 cat-back exhaust
4) Varex remote-controlled (though I don't understand how this can work well in concert with a particular CPU mapping)

Well, the supremely nice and helpful gentleman I spoke to at RRE told me that his system is definitely not going to be quieter than stock, and actually recommended I go the cabin-sound-insulation route, as he believes that no aftermarket exhaust mod can help me decrease the drone. On the other hand, a (seemingly very competent) mechanic (he routinely works on very high-end cars) said that the drone is clearly caused by the design of the OEM muffler, and that a simple muffler swap ought to do it. There were also suggestions of putting in a longer 24" resonator pipe. Finally, there are lots of comments to the effect that designing drone-free exhaust system is an extremely complex undertaking.

To state my goals more precisely:
1) I want to eliminate drone *at all RPMs*, preferably leaving room for power increasing mods in the future.
2) I am willing to live with overall louder sound if it helps to defeat the drone.
3) I am willing to take a (hopefully minor) performance hit, especially if it can be compensated by other mods.
4) Budget is quite flexible.

Huge thanks for your suggestions!

 

How sure are you that the drone is below 20 Hz? At 1700 rpm, the engine is at 28 Hz, so exhaust pulses are at 56 Hz or so. If you're sure that the drone is at 20 Hz or so, then it's not the exhaust; it's the engine, itself. The only other explanation for a sound that low is a leak in one runner of the exhaust manifold.

 

I am not at all certain about the 20Hz figure -- I was just guessing, since it would appear to be accompanied by vibration-like sensation. I would not be surprised at all if it's actually around 50Hz. Further, it's not just 1700 RPM -- it often drones at slightly over 2K as well. I would say that the drone is distributed mainly between 1300 RPM and 2200 RPM, with peak around 1700 RPM. Many thanks!

 

Drone was terrible in my Mazdaspeed3 stock exhaust.
At 2,500 RPM (the drone zone for mazdaspeed3's) it was hitting 90DB. (get a free DB meter for your iphone/android)
I dynamatted the whole trunk. After that it was peaking at 85 DB



I bought a Magnaflow 14616 resonator online and had a local shop swap the existing terrible resonator that essentially amplified the resonant frequencies of the exhaust.



After the resonator was installed DB peaked at 2,500 RPM around 80DB
Just driving with the wheel noise and no engine at all, on the highway is around 70 to 75DB to give you an idea.

The overall sound was still throaty and nice, but the "drone" was gone.
I would wager to guess doing something like this on the Evo could fix it up nicely. You'd have to get the right resonator for an Evo diameter exhaust, and find somewhere to fit it though.

Hope this helps.