Open pipes just make louder sounds. I don't think they would affect emissions, which are created in the combustion chamber. The motorcycle manufacturers should spend research time to get the emissions within limits while getting the best performance. Unfortunately the UK industry of the late 1960's didn't have the money to do that. I was hired by N-V specifically to use my test instrumentation experience at British Aerospace (as an apprentice) and at a machine tools company in Coventry, to build a mobile instrumentation lab for the Commando and subsequent programs.
When I actually went to work at N-V, I discovered that the funding that had been set aside for the project had been spent with the PR outfit that dreamed up the "Green Globe" logo. That got canned shortly after the uproar that deleting the "Norton" logo brought down on their heads, wasting the $30K (approx.) that N-V had paid for it. I ended up as one of the test riders on the Commando program and subsequently on the AJS Stormer. All the development effort was based on rider opinion, no recorded engineering data.
It was so crude a method, with no chance of getting the idea back on the table and funded, I accepted a Boeing job offer and emigrated. Later, it felt like I'd stepped off a sinking ship onto a rescue boat without getting my feet wet. We're still in Washington State, enjoying retirement after 28 years with Boeing.
