decided to append to this existing thread, but there many other threads all more or less having the same theme ..... I hope this will move the discussion away from mufflers to tailpipes and branch resonators (or other resonators -- please comment with your experience) .... or this may fall into the "total crap" bucket ... but I thought it was all worth noting ....
not to keep poking the skunk here, but add me to the ever growing list of owners being sick and tired of the dreaded "drone" ..... still seems that a solution to this problem (if you consider it as such) remains ellusive .... since I have some time this wintry AM to do some research, I stumbled across some interesting info that may shed light on the subject and provides some insight into possible cures .... first, "drone" semms to be all about exhaust system resonance, so you can imagine the variables that go into figuring it out ..... but it seems the basic problem is the tailpipe, specifically the length of it from the mufflers on back ..... found an interesting line of discussion at this site (
http://forums.corral.net/forums/general-mustang-tech/496793-exhaust-drone-completely-gone.html ) that pretty much sums up what I think we've all been experiencing .... I also found this article (
http://www.enoisecontrol.com/related_articles/engine_exhaust_sound_control_barrier_wall.pdf ) that backs up the previous one with a fair amount of "engineering" (aka equations) .... seems that the "2200 RPM drone" is highly related to the roughly 72 inch tailpipe length found on typical over-the-axle Mustang exhausts .... true enough, there are other factors that influence the severity (such as muffler design), but this gets real close to the core issue .... slide 35+ in the pdf file addresses this issue and solution exactly .... and it turns out the 6 ft tailpipe is precisely the wrong length of pipe for 2200 RPM (your specific experience may vary) as it makes a pipe organ out of the exhaust .....
both references discuss adding 1/4 wave branch resonators to the tailpipe as one way to significantly mitigate the resonance (acts as a notch filter) ..... all of this backs up the anecdotal evidence presented here by many that muffler dumps get rid of the drone (no "organ pipe" following the muffler) ... also noted was that different muffler designs/products have demonstrated different levels of drone -- probably do to how the output of the muffler provides resonance excitation to the tailpipe ..... and of course H- and X-pipes influnce the resonance inducing characteristics of the entire system ....
now as far as practical solutions go, unfortunately I have nothing to offer beyond what is out on the web ..... the notion of adding 1/4 wave branch resonators to the tailpipes sounds interesting but has practical implications as noted in the first reference ..... I guess technically the 1/4 wave branch resonator can go anywhere in the exhaust circuit so as to reduce the excitation source frequency (which for the typical tailpipe are harmonics of roughly 34 Hz) .... running straight pipes is also good as then the "organ pipe" would have a much lower resonance frequency due to the extra length ....
HTH -- 67GTFB
