Interesting test...since rich mixtures reduce the tendency to ping. Think about this logically for a moment: the idea was to clean up the exhaust. Cleaning up NOx emissions meant reducing peak combustion chamber temperature. This was done with EGR, which dilutes the air/fuel mixture -- not just the air supply, not just the fuel supply -- headed into the cylinders. If you find that logic kind of hard to follow, then go look up the jetting details of carbs used with vs. without EGR. They are in almost every case the same. There is no rejet required and the engine will not run rich by disabling the EGR.