EGR
i.e. Exhaust Gas Recirculation. One of the most beneficial air pollution devices installed on our modern cars. It does a signifigant amount of reducing the oxides of nitrogen in the modern car.
HOWEVER, Early designs without proper passage flow can cause more harm than benefit over a period of time.
The valves on early model cars(and I include Triumphs in this category) do not have height controllers, so it's usually full on, or full off. The passageways that are in the head/intake manifold are not evenly cast/machines, usually dumping exhaust in one or two cylinders... So what does this do for the other cylinders? Doesn't help them reduce the oxides of nitrogen.
Does increase carbon buildup in the intake manifold, head leading to excessive carbon buildup(especially if your system has been running rich) that will cause sticking valves, loss of proper air induction, loss of performance, mileage, etc.
If you keep the ignition and fuel system "dialed in" , especially with a multispark system(MSD) you can see as good as, if not better oxide of nitrogen reduction on a 6, without EGR.
When I was in California, I was a smog tech, with 4 gas machines available(when I left 5 gasses were just coming out). And I did readings on my 6, with egr on, off, PaceSetter exhaust, Crane/MSD ignition/ adjusting the needles on the Strombergs, and eventually replacing the Strombergs with SK's.
The cleanest my car ran, was without EGR, with Crane, Pacesetter and Strombergs(couldn't get the fuel pump discharge cams I wanted for the SK's) but the difference between the Strombergs and the SK's was not that much different, just richer on tip in, but still below state requirements. This was back in 88/89