Gasoline engine, there is a relationship between, valve size and exhaust pipe size and length which can have a profound effect on the torque curve.
But I can’t see it on a tractor. We used to set the length of the zoom tubes (one tube per cylinder no collectors), by spraying them with cheap spray paint and making a few runs to tune the car. This would burn off the paint where the exhaust gas was still hot, and then you went two pipe diameters past that and cut them off square. If you have some old spray paint around you could try that and see what happens.