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The 40 series are smaller 2 chamber mufflers. The 50 series are larger 3 chamber mufflers. The 40's are louder with slightly less back pressure. I use 3 chamber 50's on my car and they are still pretty loud. The 40's would be slightly higher performance than the 50's but I doubt you could tell the difference. FlowMaster does make a 70 series that are larger still and probably quieter. As Ryan said, there would lots more info on the FlowMaster web site.

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