For a long time, I held the assumption that the stock VR-4 active exhaust, in sport mode, was not terribly restrictive. I think I picked up the idea in when I first joined the Team3S mailing list years ago, and carried it over with me to 3SI. Whenever I would think about upgrading the exhaust, I would always decide not to, and upgrade something else...after all, it's not that bad. Right?
Wrong. With the aid of an extra pressure sensor and some copper piping, I took some measurements of exhaust pressures. I was a little shocked at what I found. Even at relatively tame power levels, and in sport mode, the stock exhaust is a MASSIVE restriction.
Methodology:
I measured exhaust pressure before the turbo in the header (EGT bung in merge section of my front header), and after the turbo (O2 bung 2-3" downstream of rear turbo). I did a series of 2nd gear pulls with the sensor at each location while the stock exhaust was still on the car (data below are all averages). I fabricated a 3" exhaust with straight through muffler, and repeated the same measurements. I had to do some boost control readjustment after putting on the new exhaust, but I got a very similar boost curve for all test cases. Boost peaked at ~15 PSI, falling to 13 PSI by redline. I used a fixed-duty wastegate curve in order to avoid pressure fluxuations.
Modifications of consequence:
TD05 headers
14B turbos, in Bullseye Power exhaust housings
2.5" O2 housings (no precats), recirculated wastegates
ATR downpipe modified with flex section.
No main cat, no precats.
Stock '94 active exhaust in sport mode for first test set. Custom 3" exhaust for second test set.
Tests done at a peak boost of ~15 PSI, tapering to 13 PSI at redline.
Data
All right, let's get to the data, shall we?
(All pressure measurments in PSI)
Code:
| Header Pressures |
|
|
|
| RPM |
Stock Exhaust |
3" Exhaust |
Difference |
| 4000 |
14 |
12 |
2 |
| 5000 |
30 |
26 |
4 |
| 6000 |
42 |
32 |
10 |
| 7000 |
44 |
32 |
12 |
| O2 Housing Pressures |
|
|
|
| RPM |
Stock Exhaust |
3" Exhaust |
Difference |
| 4000 |
2.5 |
0.75 |
1.75 |
| 5000 |
6.5 |
2.5 |
4 |
| 6000 |
11 |
4 |
7 |
| 7000 |
13 |
4 |
9 |
Conclusion:
Yes Virginia, the stock exhaust really IS that bad.
As you can see, the stock exhaust is a wee bit of a bottleneck.
It's important to note that we're not dealing with outlandish power levels here. According to a piece of dyno software I wrote for the AEMLog program, I was making around 360 AWHP with the stock exahust, and around 410 AWHP with the 3". The car picked up a ton of power getting rid of the stock exhaust.
Random notes:
- I had a 2.25" exhaust gasket at the end of my 2.5" rear O2 housing, so the post-turbo pressures for the 3" exhaust may have been even a bit lower without that extra restriction.
- My rear manifold had very significant cracks and leaks during this testing. The front manifold (where I took measurements) was not cracked, but the leaking from the rear manifold may have caused the front turbo to do more of the boost-making work, and so header pressures may be a bit higher than otherwise expected across the board. The pressure difference between tests should still be valid, though.