xr4tic

mainly increased spool-up and turbo efficiency.

The downpipe and catalytic converter are the two main restrictions. You have to start at the front, and work your way back. A 3" dp won't help much with the stock cat.

The exhaust after the DP isn't that big of a restriction (unless it's stock), 2.5" is good. Bigger is better, but I'd bet you only gain around 5hp from going from 2.5" to 3" tubing (assuming you still use a muffler and resonator) whereas a dp and cat can gain upwards of 15-20hp, and have a noticeable affect on spool-up.
When the exhaust gas comes out the turbo, it's extremely hot and turbulent, larger piping gives it room to expand, cool, and straighten out.
Because the exhaust gas has had a chance to cool down and become less turbulent, bigger tubing after the dp isn't as critical.

If you have a decent 2.5" dp-back system, don't bother upgrading to 2.75". If you're looking to upgrade, get the biggest you can fit.

I have seen some S4 exhaust that are single 3" systems, it probably wouldn't be too hard to adapt it to a 3" dp on an A4, you'd have to make one small section of pipe to replace the two-into-one section.

As for my daily driver, I had a 99 Sonoma, but the lease ended in July. Then I drove my XR4Ti (with a bad turbo) for a couple weeks.
I've been driving my A4 (finally) in limp mode for the past 3 weeks (rhe trunk is covered in black soot). It's running pretty good now that I've swapped in smaller injectors and the K03 GIAC chip.

SameerP

I have, unfortunately, a 2.25" Borla exhaust with my PES T28. So ive been kicking around ways to increase it short of spending $900 on a Milltek. I know the stock cat is a huge block...so i need to tackle that as well... I suspect alot of power can be unlocked via a cat +2.5-3.0 exhaust...and it seems a logical place to improve heat issues as well.

One thought was buy a 2.5" DP for $250 (SPP selling Pictureboy's) and then buy a Remus (2.5") and then go to a quality exhaust shop that can bend me a mid-pipe. This really does not save me too much $$ from what i see on pricing. Alternatively, considering the downpipe is the only semi-hard piece to make, i could just take a SPP 2.5" DP and walk into a good shop which could make me a magnaflow exhaust with a resonator.

Option two is to look for a hi-flow cat and DP combo, and perhaps just use that with my existing 2.25 exhaust.

Option 3 is to do the entire turbo back replacement. I have been looking around at hi-flo cats and don't really know where to go...Random's current ones seem pretty good as used by APR, Magnaflow has a great rep, and i don't know where else to look. It's also imperative the hi-flow cat be smaller (thinner) than my OEM, im having some rubbing issues.

In an ideal world id love to mate a 3.0" DP to a 2.5"-2.75" exhaust...3.0" seems overkill if guys like johnAWD is putting 500+hp through 3.0" tubing, and S4s use 3.0 as well.

Finally, since ive bugged you immensely already, what validity is there to the argument that large diameter kills the low end power? I know this is valid for NA cars, but isn't a pressure difference a good thing for a turbo? So would running around with a 3.0" DP and a largish exhaust make my car even more anemic from 1-3.5k? Is it a good tradeoff? My T28 feels like a dump truck hit me at 3500, i can't imagine what yours will feel like when the tuning is finished.

Thanks again, its nice to bouce ideas off someone who actually turns his own wrench

Cheers,
Sameer

xr4tic

there are places that will fab up a complete (tip to turbo) mandrel bent 3" exhaust for around $400-$500.

Pictureboy is working on some cats, maybe you could be a testbed for him (seriously, contact him)

The exhaust isn't going to really limit power too much after the DP, most people won't be able to feel a 5hp gain.

You can't really look at it from a diameter point of view, look at it from an area point of view.
Look at the cross sectional areas of the following pipes:
2.25: 3.98 sq in.
2.50: 4.91 sq in.
2.75: 5.94 sq in.
3.00: 7.07 sq in.

The 3" is almost double the size of 2.25" Each step is approx. a 20% increase in area.

My guess is JohnAWD has a big-*** turbo that doesn't spool until 4000rpm (at least) and has a much higher redline than our 7000rpm limit. He's not going to see that much of a gain (hp and spool-wise) from bigger exhaust then we would.

I'd say option 3 is the best bet, but you'd see bigger gains by moving the cat away from the turbo, and under the car. This would give you more room as well.

A large exhaust will only help your car make power sooner. The turbo is the main backpressure source for the motor, and going to less backpressure may make the motor see slightly less backpressure (pre-turbo) at low RPMs (off-idle to 2000RPM maybe) but I bet you'd see at least a 300rpm decrease in spool time with a 3" downpipe and larger exhaust (2.5" min) The stock cat/downpipe is super restrictive.

SameerP

you rock John, gave me lots to chew on, thanks alot.