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      08-17-2012, 07:46 PM   #1
ThatGuyMikee
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people with oem exhaust mod please step in

i have a question...poeple who did the mod did you feel any loss of power? i dont want to do the mod and be unhappy with it if it lost a crap load of tq... any input would be great... i was thinking of just taking off the axle back and getting custom straight pipe axle back instead so if i dont like the sound or need to brig it back to bmw for warrenty stuff i can put the stock back on
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      08-17-2012, 08:45 PM   #2
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Option 1. No power loss. You aren't affecting back pressure in any way as far as I can tell.

Option 2. Youll probably regret this option.
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      08-17-2012, 08:50 PM   #3
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Absolutely no power loss, amazing exhaust sound, the best mod I have ever done to my car, could not be happier with the result, only regret is why I didnt do it earlier.

I used this method: http://www.m3post.com/forums/showthread.php?t=320052

This is the video of my car after the mod

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      08-17-2012, 09:15 PM   #4
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I don't have the stupid loud mod. I love it. No loss of power. Great start sequence. quiet in the neighborhood. Awesome on the street. There is no downside.
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      08-17-2012, 10:53 PM   #5
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Ok good im gettin custom straight pipe axle back made soon
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      08-17-2012, 10:58 PM   #6
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No power loss...no power gains..just a nice throaty sound
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      08-17-2012, 11:25 PM   #7
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Quote:
Originally Posted by tibra1 View Post
No power loss...no power gains..just a nice throaty sound
+1

Love the sound at the cold start. Nice and quiet after about a minute. Great sound anywhere after about 3000 RPM.

Like the others have said, best bang for the buck!!!
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      08-17-2012, 11:33 PM   #8
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Quote:
Originally Posted by ThatGuyMikee
Ok good im gettin custom straight pipe axle back made soon
Just so you know. What you're doing is not the oem mod.
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      08-17-2012, 11:41 PM   #9
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Quote:
Originally Posted by Crimson92
Quote:
Originally Posted by ThatGuyMikee
Ok good im gettin custom straight pipe axle back made soon
Just so you know. What you're doing is not the oem mod.
+1
Take a look at the thread link I posted above.
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      08-18-2012, 10:37 AM   #10
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Quote:
Originally Posted by Crimson92 View Post
Just so you know. What you're doing is not the oem mod.
Quote:
Originally Posted by Prince_of_Persia View Post
+1
Take a look at the thread link I posted above.
Not really. The ACM mod is the same as running just axle back straight pipes. Same noise, same premise, but less weight and looks better.



I have done/seen both.
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      08-18-2012, 11:11 AM   #11
ThatGuyMikee
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^^ exactly its the same exact thing as running straight pipes lol what do tou think they do when they cut open the muffler
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      08-18-2012, 12:31 PM   #12
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Take another look at what ACM is really doing: http://www.m3post.com/forums/showthread.php?t=609950

They're only bypassing the absorption chambers in the OEM exhaust, while keeping the Helmholtz resonators intact to maintain the M3's sound and minimize drone. If you unbolt the entire axle-back section and put up straight pipes instead, you will sacrifice the Helmholtz chambers. You'll likely have a drone monster on your hands at that point.

Even when Subw00er and Periokid did their mods they changed the plumbing post-Helmholtz.

I have the ACM style mod on my car and I love it. There's a reason that ACM, Dinan, and our other forum members saw good reason to keep Helmholtz chambers in place.

(Below images courtesy of Dinan.) Best drawings I've seen that show how much time and money BMW-M spent on designing our original exhaust to A) meet BMW expectations and B) meet U.S. federal requirements allowing them to sell these cars on our shores.

Dinan's explanation: http://www.dinancars.com/university



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      08-18-2012, 12:50 PM   #13
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Yea but no1 who does the oem mod keeps them they delete all te baffles and everything which pretty much makes them straight pipes lol
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      08-18-2012, 01:21 PM   #14
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Just had the "perforated pipes replaced" method installed this morning and I am very satisfied. It it not stupid loud, but is noticeable over the stock OEM.
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      08-18-2012, 01:27 PM   #15
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Haven't gotten my car Dyno'ed for true results but I didn't feel any different. Everyone has their opinion but I rally love the mod. I had a Magnaflow axle back exhaust before I did the mod. Magnaflow claimed an + in HP and lightweight but I couldn't take the drone anymore. So I sold my Magnaflow and bought a spare OEM exhaust, did the mod and still had $400 extra in my pocket. I really love the sound and the best part is when your cruising it's quiet but once you start adding throttle it really comes alive. I have gotten more compliments with the OEM Mod then when I had my Magnaflow.
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      08-18-2012, 01:30 PM   #16
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If you're talking about the walls (highlighted in red) that separate the six different chambers in the OEM exhaust, then no, they don't remove them. Those walls are also structural parts of the exhaust system.

Chambers 1 and 2 (the Helmholtz resonators) are never altered in the OEM mod.

ACM does their work inside chambers 3 and 4 only. Subw00er's "Bates valvetronic" flapper mod used these chambers as well.

Periokid and most of the early OEM exhaust modders did their work in chambers 5 and 6.

Either way, OEM exhaust mod does not equal straight pipe only.

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      08-18-2012, 01:56 PM   #17
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I think people are mixing some terminology. IMO the "oem mod" refers to replacing the two large perforated pipes with straight pipes then closing it back up. The "ACM mod" from what i can tell bypasses the center chamber and is essentially a straight through design.

Sounds like the op is asking for a comparison of the ACM mod to straight pipes which according to most should sound identical.
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      08-18-2012, 03:03 PM   #18
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^^ yes they should sound exactly the same... which is amazing lol .. i just dont want to cut up my oem muffler ...so to loose to weight and keep my oem muffler im going custom straight axle back with tips..still debating to keep quad tips or not
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      08-18-2012, 03:20 PM   #19
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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.
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      08-18-2012, 04:14 PM   #20
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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
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      08-19-2012, 12:15 PM   #21
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Has anyone put a straight pipe design on there e92 m3 and checked power dif. on a dyno?
Just curious to see power gain/loss.
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      08-19-2012, 12:17 PM   #22
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Im looking for a loud exhaust mod. Not to concerned with drone or anything as the M3 is my weekend racer/driver. Don't take to many long trips in it.
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