The pipe is tuned to the engine. Next you must tune the prop and fuel to the engine-pipe setup.
9x7 will certainly be too large to get on the pipe, which seems to be tuned for ~24000 rpm. At least a quickie I tested will not.
I removed the thicker coupling from the pipe, and used the standard silicone coupler method combined with a rear exhaust stack. That allowed me to tune for lower rpm.
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