Test pipes flow more freely and will give more top-end hp that stock cats or hi flow cats. Definitely the way to go if you aren't concerned with emissions.
I use Berk hi flow cats so I can pass VA inspection without being shady, and they provide pretty decent power over the OEM cats. I just got my 7at tuned at just under 319 whp with the Berks. I probably could have gotten a few more hp out of test pipes, but i like the result.
For test pipes, consider the following
1. Internal diameter: some test pipes (I want to say berk and invidia) have an internal diameter of 60mm (2.36"). If you pick up a 2.5" cbe, you'll see the best result from test pipes with the same 2.5" diameter (like fast intentions or AAM). However, I haven't seen this explicitly tested for power differences.
2. Resonated v non-resonated - Resonated test pipes can cut down a bit on the drone of your exhaust, but are more expensive. At another level, ART pipes have a separate chamber that branches off the pipe and makes the sound much better (in my opinion), but these are the most expensive.
3. o2 bungs - extended o2 bungs reduce the chance of throwing a CEL. This would be a moot point if you get it tuned. You can also pick up bung extenders from vendors like Z
For hi flow cats, look at the internal diameter, and the cell count. The lower the cell count, the better your exhaust will flow. Common cell counts are 200 (Berk, AAM), or 300 (Fast Intentions).
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