Testing exhaust back pressure is simple. remove the pre cat. oxygen sensor from the pipe or exhaust manifold and adapt in a vaccum/pressure guage. Start the engine and give it a couple of good revs while watching the guage. Look to see if the guage goes over 4psi. I have seen a plugged cat go as high as 30psi. normally a restricted cat. ends up around 10 to 15 psi. Twice in the 17 years as a tech I have seen an exhaust so
plugged the car would not start. Most tool trucks sell an exhaust back pressure tester kit with all the adapters needed to perform the test. Your car may build some back pressure but if it was excessive it would be low on power. Of course with a properly tuned exhaust I am sure there is a lot of power to be gained.
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