A motor vehicle type engine has an exhaust gas crossover passage connecting exhaust gases to the intake manifold below the carburetor and past a selectively operable, valve type sonic flow metering control that provides very accurate and reproducible measurements of the exhaust gas flow at all times, the control comprising a variable area convergent- divergent nozzle that is so designed and constructed as to provide sonic flow of the gases through the metering area over essentially the entire operating range of the engine manifold vacuum regardless of the change in metering area, thereby providing a constant flow rate for each changed area of the nozzle, with variances in the flow rate being in direct proportion to the change in metering area.
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