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      08-18-2012, 04:14 PM   #20
m3an
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Drives: M3
Join Date: Oct 2010
Location: South Florida

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Quote:
Originally Posted by Xuphious View Post
Sorry it took me a couple times reading this, but now I understand your point of view, so I think I can equate my terminology.

I associated "OEM mod" with any form of cutting open the stock unit and messing around inside. So I thought of it as a single "OEM mod" with two different means to the same end.

I understand "straight pipes" to mean simply that. A straight (or properly bent, in our case, to go around the differential) piece of pipe with absolutely no purpose other than to provide safe pathway for the exhaust gases past the rear end of the car. Similar to test pipes that completely eliminate emissions controls.

Either way it's done ("OEM mod" or "ACM mod") the Helmholtz chambers are left unmolested and the stock muffler unit is returned to the car to maintain the appearance as if no tampering ever occurred.


For ThatGuyMikee:

If you are asking about completely removing the entire stock axle back unit where it connects to the mid-pipe and replacing it with straight pipes only, then I'm fairly certain the sound won't be exactly the same as the OEM/ACM mods and you will have more drone.

The key is in the Helmholtz chambers. http://en.wikipedia.org/wiki/Helmholtz_resonance
Their job is to generate specific frequencies that mold our cars' unique sound while also cancelling out drone.

while you're right that the ACM mod (what we're talking about here) does maintain the heimholtz chambers they are essentially worthless here. Heimholtz chamber length is calculated using DBs. The OEM chambers are designed for OEM volume. Even Dinan gets away using a small set as it is a quiet exhaust. Being that the ACM mod "is" essentially straight pipes (bypassing the middle section altogether) the volume is so great that it really would require a longer heimholtz chamber. The little ones are overpowered by the DBs produced. I have done both, and I have a set of straight pipes in my garage. I sometimes swith em out for my gintani race as they really do sound good and I can attest that there is no drone. Yes it's loud, but sound is identical to ACM mod while losing the hideous stock muffler and shedding quite a bit of lbs too.

My gintani drones like crazy whereas the straight pipes/ACM mod are loud "behind" the car. It really doesn't intrude into the cabin much other than the loudness.

Remember, drone is essentially the same as blowing over the top of a glass bottle. That noise produced is drone and the heimholtz chambers just cancel out the resonance created. Different lengths for diff volumes required.

The synopsis= straight pipes= ACM mod and you can save your OEM muffler and some $$ to boot