Help me make the right choice

[Jibberific]

So it's time for me to order some more parts for my car. I'm going to buy an N75 Race valve which i hope will take care of the surging i'm experiencing at high boost, and also i'm getting a test pipe. I need the TP pretty badly, as my catalytic converter has developed a very loud rattle that is not only very annoying but embarrassing as well.

Here's where i run into a problem. I'm not sure what size to buy, 2.5" or 3". I have a non-resonated 2.5" catback exhaust on the car right now and i like it a lot. Is it okay to go with the 3" TP, or will i lose power due to the exhaust being too free-flowing? Should i get the 2.5" because the rest of my exhaust is that size? Right now i'm running the stock k03 but in the future i might upgrade the turbo, but nothing bigger than an eliminator setup.

I looked and looked but i can't find any info regarding the size of the exhaust outlet on the k03. If i knew that, it would make this decision a lot easier, but the only way for me to find out would be to take of the cat and measure it myself. All you guys with test pipes, post up and let me know how your setup is working out for you!

Thanks in advance

[mike-2ptzero]

You are best off buying one that is 3" and tapers down to 2.5" so it matches your down pipe. ER makes one and it is around $190-$195, if you need one let me know.


ER test pipe

[djwimbo]

will i lose power due to the exhaust being too free-flowing? Should i get the 2.5" because the rest of my exhaust is that size?

Losing power b/c of your exhaust being too free flowing doesn't really happen on turbo cars. Less restriction after the turbo helps spool.

2.5" is perfectly fine. If you want to spring for the 3" tapered to 2.5" that's great. If you do upgrade to an eliminator turbo you won't need to redo your exhaust at that point. Even a 2.5" TP on a GTRS is decent setup. The 3" in theory would help, but I don't know if it would be noticeable or not on that size of a turbo.
Correct me if I'm wrong, but a 3" DP/TP on GT30 and GT35 framed turbos is still a good size.

Personal experience...
Don't buy the Ebay testpipe. The angle of the bend isn't 100%. It put my downpipe too far to the right(passenger) side of the tunnel. There's an exhaust hanger on the side of the trans that doesn't line up now either. I know it was the testpipe not the DP b/c I prefitted the DP in to make sure it was right. NTM it's a Techtonics DP.

[Jibberific]

Thanks for your help guys.

I was considering either getting the 034 three inch test pipe or the tapered ER one from mike. The only 2.5 inch one i found was the VMR test pipe on ebay, and if it doesn't fit correctly i will avoid it. Is there a company that manufactures a quality 2.5" TP?

[beejohn]

i have an n75k dahlback racing valve i'm trying to get rid of. pm me if interested, hardly used it for even 5k miles.

[mike-2ptzero]

Losing power b/c of your exhaust being too free flowing doesn't really happen on turbo cars. Less restriction after the turbo helps spool.

2.5" is perfectly fine. If you want to spring for the 3" tapered to 2.5" that's great. If you do upgrade to an eliminator turbo you won't need to redo your exhaust at that point. Even a 2.5" TP on a GTRS is decent setup. The 3" in theory would help, but I don't know if it would be noticeable or not on that size of a turbo.
Correct me if I'm wrong, but a 3" DP/TP on GT30 and GT35 framed turbos is still a good size.

Personal experience...
Don't buy the Ebay testpipe. The angle of the bend isn't 100%. It put my downpipe too far to the right(passenger) side of the tunnel. There's an exhaust hanger on the side of the trans that doesn't line up now either. I know it was the testpipe not the DP b/c I prefitted the DP in to make sure it was right. NTM it's a Techtonics DP.

Some people see a slight loss in bottom end tq when decreasing back pressure on the internal wg turbos because it is also decreasing resistance at the wq outlet, this means more exhasut will flow thru the wg once it cracks open to control boost so the turbo might have a slight delay in spool. But since there is less back pressure at high rpms the turbo will spool even more giving a gain in hp.

Less back pressure really helps the ext wg setups because only the back pressure at the back of the exhaust wheel has been decreased allowing more flow thru the turbo while the wg has not changed.

Yes the 3" dp is still the correct size for a GT30 or GT35 which can come with tend to come with a chioce of discharge of IIRC 2.75" or 3".