just bought a used HKS downpipe. I was told it was a 2.75", though when I measured it, seems to be only 2.5". It has a test pipe attached to it as well, and my plan is to retain the stock cat back.

I will be adding a fuel pump, and electronic boost control and getting it tuned, will the 2.5" downpipe hinder the performance I am trying to accomplish with these mods, or should I return it and look for a 3". I like the idea of the 2.75" allowing for a little more torque at lower rpms, but I don't want to be wasting my money on a 2.5" if it's not worth it.

Thanks

 

I would have to say that if you re keeping the stock cat back 2.75 aint gonna make a difference. Your restriction will be in the cat back, not the down pipe.

P.S. I would seriously consider getting rid of the stock cat back. Any particular reason you are stuck on having it?

 

I figured I could make up the diff with the test pipe. I also want to keep it quiet.

 

I think you'll get significant gains with a catless system, dp, and stock exhaust, and still be nice and quiet. Better than putting just a catback on with the cat and stock dp.

 

I believe what everyone is saying is that you need to remove all the significant restrictions before you'll see much advantage. Changing just one of these (dp/cat/stock cat-back) will free up some, but if you keep a restriction downstream of the DP, you won't see any real gains out of a DP change. By changing all of them you'll see much greater improvements.

 

I know, that's what I am trying to do, the setup I bought is a dp and test pipe, so along with the stock system, those two alone should free up alot of the restriction and I will still have a pretty quiet setup.

I was just wondering if anyone had anything bad to say about the HKS 2.5" DP if I didn't plan on changing out the catback?

 

2.5 is better than stock, but it wont help much because the exhaust gets restricted when going back to the stock diameter

id recommend you get a stealth catback like a tanabe exhaust or an exhaust with multiple resonators

i had a 3inch tbe including a buschur o2 housing, and i just recently sold my catback
im now using the stock catback, and i can feel a definite restriction

 

DP Should be fine, I had one that I never used as I had issues test fitting it with acd clearance.

As far as the stock catback is concerned I believe FIMotorsports on here cut low 11's while he was still running it.

 

Well, I appreciate all of the responses. I will go ahead with the install, because as for now, I am not looking to build a 500hp monster. If it feels like a cat back system would help, then maybe I will look into a quiet set up. I also know some selling some HKS cams, 263 and 273 degree setups for a couple of bills. Would these be worth the money for a street set up before the tune if I am simply adding a fuel pump, DP, TestPipe, and electronic boost controller? I am not very well educated on cams
thanks

 

its 2.5 inches only at the flange (allows the downpipe to be bolted to the stock cat), the rest of the downpipe is 2.75