Measuring stock exhaust restriction

[FWombat]

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)

  
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.

[doctorstupid]

Thanks for taking the time to do this. I'm suprised what a large difference in made in the header, I never would have guessed the difference would be that significant before the turbine.

[Erick]

interesting.. I have a DP & no cats.. and plan on adding a "Y" pipe to run to passenger side exhaust.. and help free up the flow. Of course sound will go up as well.

[RobBeck]

Thanks for doing this test, it should be clear now to all of those who said the stock catback was good up to a much higher power level, that they were incorrect. I always figured just by looking at it that it was a POS.

Rob

[VR-4ever]

cutout.


Good research FWombat.

[FWombat]

Originally Posted by RobBeck

Thanks for doing this test, it should be clear now to all of those who said the stock catback was good up to a much higher power level, that they were incorrect. I always figured just by looking at it that it was a POS.

That was my main motivation in doing all this - refuting the old myth floating around that the stock catback is good. I figured that as long as I had the sensor and logging capability to do something like this (thanks AEM!), I may as well take some before and after measurements along with making my new exhaust.

Up top at 7000 RPM with the stock exhaust, I actually had as much pressure in the exhaust system as I had in my intake manifold (13 PSI). That's just nuts. I still can't believe there was that much backpressure.

[Multiades]

Good info.

I'm a little surprised too that the stock exhaust was that much of a problem.

Apparently flowbench measurements with a standard ATR cat-back versus the style you made (one that Tim made, with ONE 30-degree 36d 3" bend) there was 58% more airflow at the same pressure.

I like one-bend exhausts.

-Chris

[bluemax_1]

So there we have it folks. Definitive proof that the stocker is not only one heavy-ass POS. It's also a POS air-flow wise. I like my ATR single-shot.


Max

[Multiades]

Originally Posted by bluemax_1

So there we have it folks. Definitive proof that the stocker is not only one heavy-ass POS. It's also a POS air-flow wise. I like my ATR single-shot.

Not that the ATR isn't nice, but as I just mentioned, it doesn't compare favorably to the minimalist one-bend exhaust my friend has been producing.

The only possible compromise with this one-bend exhaust is noise cancellation versus weight. The largest (longest) possible muffler I could find weighs about 17lbs, but is very, very quiet. If you want to shoot for light weight, you could get a 2.5-3lb burns stainless muffler, but it would be REALLY LOUD. Either arrangement should flow equally if you use a straight-through muffler (stainless wool around a perforated core), but one will be louder and lighter, while the other will be quieter and heavier.

My friend used a Magnaflow muffler (as did Brian) that is 36" long overall (it's HUGE), and made out of 409 ferritic "stainless" steel unlike the ATR which is a much nicer 304 SS. The Magnaflow is a bargain and can be found for under $100 shipped.

-Chris

[bluemax_1]

Kewl. Were those airflow differences measured vs stock ATR dual or single shot? And aside from weight, do you know of any airflow differences between the ATR dual and single? I got mine from DR when it went on sale about 2-3 years ago and I figured at the time, if Matt could hit 10's with it, should be enough for me. Don't think anyone else really had something that was proven to be better back then (that I was aware of anyway) aside from maybe the crazy 4"(or was it 4.5") JIC (Pitroad?) full titanium that cost and arm, a leg and left nut. Super light and big enough for a cat to crawl into but uh... money better spent elsewhere.

So you guys just got a shop with a mandrel bender to put the 30 degree bend in a pipe and tacked on any muffler you liked? Interested to see where the bend is. There's not a whole lot of room to route the pipe under there.


Max