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As stated on the Exhausts page under the “Terms and Definitions” section of this website, Wardale defines the exhaust system as “thermodynamically the heart of a locomotive which must therefore be as good as possible within practical limitations”.
The development of locomotive exhausts took a leap forward in 1926 when André Chapelon developed his Kylchap exhaust which incorporated a Kyala spreader (second stage nozzle) and third stage cowl between the blastpipe (first stage nozzle) and chimney. The name “Kylchap” recognizes the contributions of Chapelon and the Finnish engineer Kyösti Kylälä who developed the Kyala spreader.  The Kylchap exhaust is perhaps best known for its association with the LNER A4 Pacific “Mallard” which broke the world speed record for steam traction in 1938 with the assistance of a double Kylchap exhaust.
Ing. L.D. Porta subsequently improved on the Kylchap exhaust by developing the “Kylpor” exhaust which he successfully demonstrated on the 2-10-2 Santa Fe locomotives of the Rio Turbio Railway – see Hugh Odom’s Ultimate Steam Page for details.
Porta subsequently developed the Lempor exhaust which he claimed to be superior to the Kylpor.  The theories behind both systems are complex and beyond the scope of this website to explain.  However Porta wrote a paper in 1974 which provides the mathematical foundation of the Lempor, titled “THEORY OF THE LEMPOR EJECTOR AS APPLIED TO PRODUCE DRAUGHT IN STEAM LOCOMOTIVES“.
This paper can also be found on Martyn Bane’s website from where the above copy was sourced.
The paper is also available through Hugh Odom’s Ultimate Steam Page which also offers a link to a spreadsheet for designing a Lempor exhaust system (see http://home.ca.inter.net/~mguy/lempor_ejector_calculator_page.htm).
Hugh Odom’s page also includes a list of corrections to Porta’s Lempor Theory paper as provided by Dr. Jos Koopmans in January 2006.  It should be noted however, that David Wardale disputed some of Koopmans’ claims, expressing his views at the Modern Steam conference held at York in December 2006.  At Wardale’s request, his documented views have been posted onto this website where they may be found in the FAQ section, together with associated correspondence between himself and Koopmans.
Note: Jos Koopmans is the author of a book titled “The Fire Burns Much Better … ” (see also http://www.thefireburnsmuchbetter.nl/).