Two means to achieve a purpose. Aircooled, radial-engined Harvard/Texan/AT-6/SNJ runs up while the water-cooled Rolls-Royce Merlin V12 in the Mustang idles.
The thrust from the rearward-facing exhaust stubbs added the equivalent of 150 BHP to the propulsive effort on the Merlin: Ref: pp 53, "Not much of an Engineer" Airlife Autobiography of Dr.Sir Stanley Hooker, Rolls-Royce mathematician. Assisted by Bill Gunston. ISBN 1 85310 285 7.
What effect does the Harvard's big exhaust have on the airflow over the wing?
The standard crew in the Harvard was two, student in the front, instructor in the rear seat. It was powered by a 440kW Pratt & Whitney R-1340-AN-1 engine. It's top speed was 208 mph (335km/h), with a ceiling of 24280 ft (7400 m) and a range of 745 mi (1200 km). It could be armed with two 0.50 inch machine guns for target practice. Later models also mounted dummy bombs for army cooperation drills.