My Plus 8 is at around 45,000 miles, some 26 years old. Last week I spent a few days working on the base idle, and setting up the air flowmeter. At the MOT the CO at idle and fast idle was 0% (yes he did check that the engine was running), and the HC was 11 ppm (limit 200 ppm). I have heard of Plus 8s getting through with bypass pipes, but I have a friend whose GEMs Plus 8 only just scrapes through using the 'in service' limits, so I don't think it is easy to predict if a 200 cell would be good enough, on my 4/4 I ran a 200 cell during the year but needed a 400 cell for the MOT.
