Subaru WRX Forum banner

WARNING FOR BUYING CATTED DOWNPIPES!!!

135K views 103 replies 40 participants last post by  78wrx78 
#1 · (Edited)
If you live in an area that does advanced smog testing.. don't waste your money on a catted downpipe.

I bought a catted downpipe a few years back, and had to go this month for my emissions testing.

At Advanced Smog Testing Facilities they test HC (Hydrocarbons) CO2 (Carbon Dioxide) and NOx (Nitrogen Oxide)

My Catted Downpipe allowed me to pass HC and CO2. But I failed NOx miserably!!

Here's the important part:
I spoke with the state emissions inspector (very nice guy btw) and he advised me that on WRXs... we have 2 cats in the downpipe because we don't have an EGR system. The two cats are used to break down the NOx. One cat would be sufficient to break down the HC and CO2 (just as mine was). But only our stock exhaust is able to break down the NOx.

I asked him how to reduce NOx levels without putting a stock exhaust back on. He said that NOx is produced by high EGTs (exhaust gas temperatures). Our turbo charged cars run a higher EGT than the standard car (1400degrees is common).

The only ways to decrease NOx are with alcohol/water injection, an alcohol fuel source (E85) or increasing the fuel / retarding the timing.

Retarding timing / increasing fuel would allow less fuel to be burned increasing the HC output. Alcohol / Water injection would lower the NOx level, but not enough. E85 would also lower the NOx level... but not down to levels acceptable for a 4cyl car.

Solution? Stock downpipe for emissions testing. And Catless for stage 2+
 
See less See more
#77 ·
So I just moved to Denver from Colorado Springs (which doesn't have emission testing) and I was planning on get a catted DP just to pass emissions. It would seem that is a waste of time and money. I'm now wondering if I even want to get a DP. I'm now just thinking about an intake and a tune from a local shop. My question is: Is a intake and a tune even worth it? Will I see any noticeable power gains? I own a 2010 WRX.
 
#78 ·
You'll see some power, but obviously not like going to Stage 2. Stage 1 tune with the intake will increase your boost some as well as just smooth out the powerband nicely. It will improve the driveability quite a bit IMO.
 
#81 ·
It can... But at altitude we are actually less prone to boost creep because we have a harder time hitting our boost targets.

We are around 12psi for 1 ATM (atmosphere) even though the standard is 14.7psi (at sea level at 59 degrees). Because our air is less dense by approximately 80%, that means the turbo has to work harder to compress that lower atmospheric pressure to hit the same goals.

When my car was on a vf39 I had 19psi at sea level (California), and up here I had only 17.5psi.

In Colorado I haven't heard of boost creep on a factory turbo before. It may be possible, but I haven't seen it.
 
#95 · (Edited)
Currently 2000 + vehicles in California don't get tailpipe due to new DAD system. Obd2 check/visual/smoke test. If Obd don't pick up any Nox problems than Nox is not a worry. Not to mention the change recently where your only allowed 1 monitor down and evap only one allowed down.99 and lower still get smogged on bar 97.know cause I'm a CA smog tech.
 
#96 ·
Sorry for bringing this old thread back up but I was wondering if I just change the downpipe back to stock oem then will it pass smog? Also does it fit without brackets of any kind? I think the car I just bought has a 3" exhaust system. If anyone could help I'd greatly appreciate it. I just bought a 06 wrx wagon.
 
#98 ·
We have a different but related issue: '04 WRX with aftermarket Borla header and exhaust. It has always passed CA smog before, but now the CA Air Resources Board is apparently cracking down - aftermkt equipment has to be CARB certified and smog shops have to check. Although my system fits any WRX from 2001-2016, it is only certified by CARB for 2001-2003. Borla couldn't offer much help. Any ideas? Not looking to spend a lot or make major mods to a 14 yr old car, even one this good.
 
#99 ·
We have a different but related issue: '04 WRX with aftermarket Borla header and exhaust. It has always passed CA smog before, but now the CA Air Resources Board is apparently cracking down - aftermkt equipment has to be CARB certified and smog shops have to check. Although my system fits any WRX from 2001-2016, it is only certified by CARB for 2001-2003. Borla couldn't offer much help. Any ideas? Not looking to spend a lot or make major mods to a 14 yr old car, even one this good.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top