If you take out the sound resonator from the TRD exhaust,you will get a louder noise from it.
Exhaust Resonator:Internal combustion engines usually make a lot of noise while they are running, and this noise can be annoying and exhaustive to your ears, especially when it is heard for a long period of time. Thus, mufflers were created to reduce the noise generated by the exhaust system. Exhaust resonators, meanwhile, further reduce the noise by making sound waves that cancel noises out.
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There were only four different model numbers for the TRD Sport Mufflers (aka by third-party venders, the "TRD Performance Muffler" or "TRD Performance Exhaust system"):
'00-'05 Celica GT:
PTR08-20000-05 Muffler
PTR08-20000-08 Muffler for use with extend tip for use with rear valance (such as TRD Action Package)
'00-'05 Celica GTS:
PTR08-20000-06 Muffler
PTR08-20000-07 Muffler for use with extend tip for use with rear valance (such as TRD Action Package)
The extended length variation was recommended for persons with body kits installed. It is what I have on my Celica.
The main exterior differences between the GT and GTS TRD mufflers are the piping locations and the hanging bracket location. TRD Exhaust systems are "bolt-on" replacement exhaust parts.
Other than the HANGER positions and the length of the exhaust TIP, all four variations are basically the same for output.
The TRD Kazuma exhaust (aka "TRD Racing exhaust) is basically the same thing as the standard TRD version, except it has the resonator lessened/removed, and because of this, your engine does not work as hard to push out its exhaust, and you have more power... and noise (the sound makes it not street legal in USA -- for off-road use only). Its the difference between 12hp (TRD) and 14hp (TRD Kazuma), IIRC. Something very minor. There are numerous posts regarding this very same topic.
https://celicahobby.com/forums/ubbthreads.php/topics/372581/exhaust-facts-and-misunderstandings.html