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Old 11-17-2011, 04:58 PM   #1
Scooby Newbie
Member#: 254590
Join Date: Aug 2010
Location: Costa Rica
1998 V4 Impreza STI
EJ20K engine

Default Gutted Downpipe/Upipe? Emissions issue?


I've been reading about gutting the Uppipe and Downpipe on my car (completely stock JDM EJ20K V4 STI). I believe both pieces include a cat on them.

However, I'm worried about the fact that I could get a "cat inefficent CEL" after doing this?

My other worry is about emissions. Here in Costa Rica I need to pass HC and CO emissions, with a test the includes measurements at idle an at 2500 rpm.

Emissions need to be within the following numbers:

At idle:

CO less than 0.50% (last test on my car showed 0.02%)
HC less than 125 ppm (last test on my car showed 60ppm)
CO2 : higher than 10% (last test on my car showed 12%)

At 2500 rpm

CO less than 0.30% (last test on my car showed 0.01%)
HC less than 100 ppm (last test on my car showed 25 ppm)
CO2 : higher than 12% (last test on my car showed 14%)

I believe that on top of those cats (uppipe and downpipe), car has an additional 3rd one. Would I be able to pass emissions with only that 3rd one? Or would I fail?
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Old 11-18-2011, 07:19 PM   #2
Scooby Newbie
Member#: 257043
Join Date: Sep 2010
Chapter/Region: VIC
Location: Vancouver BC
1993 JDM WRX Wagon


If you are concerned about passing emissions my suggestion would be to find out what your car is doing right now before you remove any cat's from the system. The previous test results are a benchmark.

And yes typically there is a cat on the up-pipe downpipe and midpipe. If your going to do anything do the up-pipe as a search will yield a few people whom had the up-pipe cat come apart and murder their turbo. Then if your other two cats are up to snuff you pass emissions and later-on install a turbo back exhaust. Also dont gutt anything. Get a catless up-pipe and if your fail emissions put it back no-harm no foul.
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Old 11-18-2011, 11:17 PM   #3
Scooby Guru
Member#: 88618
Join Date: Jun 2005
Chapter/Region: SCIC
Location: New Zealand
1994 Legacy
"The Silver Slug"


There is no cat in the up pipe. Only one in the entire system is downpipe.
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Old 11-19-2011, 05:43 AM   #4
Scooby Newbie
Member#: 29145
Join Date: Nov 2002
Location: Woking, home of Mclaren
1998 Version 4 STI
White, gold wheels


If you have a stock JDM V4 STI i_c_the_light is correct. I have one myself. There is only a cat in the downpipe. The uppipe is catless and the midpipe just has a resonator.
Hence it would be some kind of miracle if you passed emissions with a catless downpipe!
I would not recommend gutting your downpipe. I have done that before on a bugeye, but that has a cat in the midpipe.
I would recommend you get a bellmouth downpipe which starts at 3 and tapers to 2.5 inches. This will mate up well with the stock exhaust system. Then you can pass emissions with the stock catted downpipe and switch to the catless one for the rest of the year. I have this one on my car
If you can get it imported, I can vouch that it is not hard to fit up to the rest of the stock system. It has the bracket half way along, you may have to bang the thing it attaches to with a hammer to bend it a little, but the bracket does function and the downpipe works very well.
You won't get the full advantage from the downpipe without an aftermarket computer and a tune. If you run high octane petrol (I use Shell 99 octane) you should be OK, but no-one can guarantee that as the ECU is aggressively tuned and is expecting a massively obstructive cat in a downpipe with a flat flange over the turbo's internal wastegate.
Hope that helps, Tom
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