if OP bought a y8 manifold, would it bolt up to his oem catback and essentially delete the cat without having to use the test pipe?
Is there any reason you want to equip this test pipe on a stock car?
No it will not just bolt right up. The ex/si style header stops much closer to the engine. The reason it stops shorter is so there is room for the catalytic converter. That exhaust piping the OP has is meant for an ex/si style header setup with the use of a cat or test pipe in mind. That space has to be addressed. I wouldn't suggest a test pipe, you can buy OEM style catalytic converters
(Just an unnecessary breakdown of how it goes together because I'm bored and I take too many pictures)
The OP has the cat here up front where the O2 sensor is
The ex/si header or aftermarket ex/si style header with move the catalytic converter location to under the engine, just behind the oil pan like...
A test pipe or catalytic converter will fill in the gap between where the bolt on exhaust and new header setup sit. It will have a 2 bolt flange on one side and a 3 bolt flange on the other for a seamless install.
This is my old test pipe
Ultimately, it will all bolt together like this underneath the car
(this example is with my K series, but it's the same setup just a very minor length difference for the catalytic converter/test pipe)
If there are any more questions.....then start back at the top of this thread and read it again