This issue came up in another thread about spec-U exhaust resonance - I started a new thread to avoid cluttering that thread. A 1/4 wave resonator is a pipe of a specific length that is closed on one end and welded into the exhaust at a 90 degree angle on the other end. Since it is perpendicular to the exhaust system, it does not affect flow or performance at all. It is just a sound chamber. It allows sound waves to travel down the pipe, bounce off the closed end and then intersect the exhaust flow again. Based on the length of the pipe, the sound waves cancel out certain frequencies when they re-intersect the exhaust stream. If you want to know more, it's explained much better than I can (with all the supporting math) in this thread on a Mustang forum:
http://forums.corral.net/forums/general-mustang-tech/496793-exhaust-drone-completely-gone.html
I added a 30" long 1/4 wave resonator to my Spec-U exhaust to try to tone down the low-frequency noise that occurs from about approx 2,300 to 2,900 rpms. I hesitate to call it a drone, it's less than that, but the spec-U is louder (at a low frequency) just in that rpm range. The 1/4 wave resonator was VERY effective - the low frequency sound in that rpm range is GONE. In addition, it quieted down the entire exhaust considerably. I don't know what a stock exhaust on an XR sounds like (mine came with a 3" Magnaflow cat back), but I'd bet that this is as quiet or quieter than stock unless you are really in the throttle. My exhaust is practically silent at 70mph in 6th gear (2,500 rpm) - no more headaches. I love the effect, but I'm on the wrong side of 40, so I enjoy the quiet. If you like the attention you get from your loud exhaust, this is not for you.
To avoid any confusion, this mod does NOT affect the high-frequency buzzing or resonance that a few people (me included) have experienced with the Spec-U exhaust. That resonance is present under load (approx 50% throttle or more) at any rpm. The resonance that the 1/4 wave resonator cancels out is just the low frequency resonance in the 2,300-2,900 rpm range.
I'll attach a few crappy pics - the front of the truck it to the left in the pics. I just pulled off my supercharger and shipped it out to have it rebuilt, so I won't be able to post sound clips until I get it back in 3-4 weeks.
http://forums.corral.net/forums/general-mustang-tech/496793-exhaust-drone-completely-gone.html
I added a 30" long 1/4 wave resonator to my Spec-U exhaust to try to tone down the low-frequency noise that occurs from about approx 2,300 to 2,900 rpms. I hesitate to call it a drone, it's less than that, but the spec-U is louder (at a low frequency) just in that rpm range. The 1/4 wave resonator was VERY effective - the low frequency sound in that rpm range is GONE. In addition, it quieted down the entire exhaust considerably. I don't know what a stock exhaust on an XR sounds like (mine came with a 3" Magnaflow cat back), but I'd bet that this is as quiet or quieter than stock unless you are really in the throttle. My exhaust is practically silent at 70mph in 6th gear (2,500 rpm) - no more headaches. I love the effect, but I'm on the wrong side of 40, so I enjoy the quiet. If you like the attention you get from your loud exhaust, this is not for you.
To avoid any confusion, this mod does NOT affect the high-frequency buzzing or resonance that a few people (me included) have experienced with the Spec-U exhaust. That resonance is present under load (approx 50% throttle or more) at any rpm. The resonance that the 1/4 wave resonator cancels out is just the low frequency resonance in the 2,300-2,900 rpm range.
I'll attach a few crappy pics - the front of the truck it to the left in the pics. I just pulled off my supercharger and shipped it out to have it rebuilt, so I won't be able to post sound clips until I get it back in 3-4 weeks.
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