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      12-09-2011, 10:49 PM   #1
Ateam
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Drives: M3
Join Date: Sep 2011
Location: USA

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A surprising extra benefit of ACM exhaust

I cannot speak to many of the other ones out there, but I am sure most of them are a "straight pipe" to some degree with varying amounts of absorption material and different size/shape hemoltz resonators however the ACM mod is 2 straight pipes basically through the hemoltz resonator. This is such a more direct path than the zig zaggin tight turns of the OEM exhaust. SO obviously its nice and loud and sounds good BUT

I actually picked up 2 mpg on the freeway-was able to hit 27mpg going 70mph on the same road I go on almost every day, same temps out, same gas etc. I have driven over 25 times with this car and always try to see how much economy I can get on the long stretch of 20 minutes on the freeway. Consistently 25 is the highest i hit and usually end up at 23-24. I never saw 26 even for a moment, let alone 27!

I would like to take a longer trip and watch the mileage. I only went about 10 miles and then a HUGE long tunnel was coming up and had to have some fun so dropped the mpg down to 16 quickly!

Anyway anyone else get better mpg with the exhaust? Since I assume that means much less back pressure and more engine efficiency, I am curious if there is some decent hp gains with this (decent I mean 4-6hp for a rear which is good!) I know MOST cars do not make much power with a rear only but the zig zag stock design really must do a number on exhaust velocity at the end of the muffler system and cause some degree of back pressure that is alleviated with 2 straight pipes.