1952Mike-
 


        I would decide how much fuel I want to assume is in the pipe.  The most conservative assumption is that the air-fuel ratio in the exhaust system is the same as it is in the intake.


        Then I would assume that all the fuel in the pipe burns at constant volume.   Use the ideal gas law to calculate the final pressure.

    Since volume is assumed to be constant, & the ideal gas law is assumed,  then  P/T  is the same both before and after the fuel burns.

Actually,
If you return to
http://www.eng-tips.com/viewthread.cfm?qid=130088&page=1
and see my posting (third posting after yours), you will notice that I replied to my question basically using your method here.

Thanks anyway.