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As with the e90/92 PE v.s. non-PE I suspect there is very little difference between the two. There are numerous threads with pictures of the stock and PE 335i muffler cut wide open to expose these differences.
Essentially it boils down to perforation of the pipes going through the box and more sound deadening material in the case of the stock box and solid, unperforated pipes and less material in the case of the PE.
Those that have modified the 335i stock box to have it resemble the PE setup seem to have had great success in making it sound almost identical (or better) than the PE. Would love to see someone with a stock 135i exhaust give it a go.
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