Downpipe FAQ: Read if you are thinking of buying one!

[kris142b]

Originally Posted by Unabomber

We have no idea. That's a question for:

a. The place where you are buying the stuff.
b. You. When you get both pieces, it will be 200% obvious if a flat gasket will work or if you need a donut.

Makes sense. I got a better look at my CB, looks line the inside of the flange is a little coned / rounded for a donut. Ebay, here I come.

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[AllOuttaBubbleGum]

Hey all, new to the forum. I am not new to cars and engine tuning. Quick question. Got a bone stock 2002 WRX auto. Got a Cobb Accessport V3 and flashed to Stage 1 91 ACN works great. Now looking at Stage 2. But here in CA I am a bit concerned about a mirror-visual inspection and was thinking of getting an OEM downpipe and knocking out the cats before flashing to Stage II.

I know no cats is stinky but am wondering:

1. If anyone has tried this with the pre-cat still in but the DP cats out. Still too stinky?
2. If I only knock out the second cat, would you think the back pressure would be reduced enough for me to use the Stage II flash? I do not race and would not get on the engine for more than a 5-10 seconds at a time anyway, if that.

Thanks!

Steven

[Legacy]

What is the typical lifespan of an aftermarket DP ? Or perhaps better said, how does one know when an aftermarket DP needs replacing ?

Reason for asking: My HG needs replacing so the motor is coming out. I've had a Cobb DP installed on my daily since 2010, typical average mileage. Its a simple setup 07 Forester 2.5 XT with Cobb DP, 3 inch exhaust, VF48 , STI TMIC and protune.

Is there a way to physically inspect the DP while its off to check if the cat is clogged ? I'd rather not spend the $600 on a new Cobb DP but if there's ever a time to do it this is it. The car still runs well and makes boost.

[TurboWagon-Chris]

Originally Posted by Legacy

What is the typical lifespan of an aftermarket DP ? Or perhaps better said, how does one know when an aftermarket DP needs replacing ?

Reason for asking: My HG needs replacing so the motor is coming out. I've had a Cobb DP installed on my daily since 2010, typical average mileage. Its a simple setup 07 Forester 2.5 XT with Cobb DP, 3 inch exhaust, VF48 , STI TMIC and protune.

Is there a way to physically inspect the DP while its off to check if the cat is clogged ? I'd rather not spend the $600 on a new Cobb DP but if there's ever a time to do it this is it. The car still runs well and makes boost.

Well I can say that once your cat gets clogged up you will notice. My stock downpipe/ cat(s) got clogged up (130,000 miles) and I was only making 7psi. The more I floored it, the more it felt like the power was cut. The Cob down pipe should have a decent quality cat that will last 100-150,000 miles.

[TurboWagon-Chris]

Originally Posted by AllOuttaBubbleGum

Hey all, new to the forum. I am not new to cars and engine tuning. Quick question. Got a bone stock 2002 WRX auto. Got a Cobb Accessport V3 and flashed to Stage 1 91 ACN works great. Now looking at Stage 2. But here in CA I am a bit concerned about a mirror-visual inspection and was thinking of getting an OEM downpipe and knocking out the cats before flashing to Stage II.

I know no cats is stinky but am wondering:

1. If anyone has tried this with the pre-cat still in but the DP cats out. Still too stinky?
2. If I only knock out the second cat, would you think the back pressure would be reduced enough for me to use the Stage II flash? I do not race and would not get on the engine for more than a 5-10 seconds at a time anyway, if that.

Thanks!

Steven

In general It is not safe to use a Cobb off the shelf tune unless you have the exact modifications listed in the map notes.
However the early 02-03 wrx is the only one that you can run a down pipe and Intake without a tune (better gains with a tune of course). You might be better off gutting the down pipe and keeping with the stage 1 tune for safety. If you log the car and keep an eye on knock and AFRs you should be good.

[MGrizzard]

Originally Posted by geekxism

Will my CEL come on even if my downpipe is catted? Everyone seems to be running catless, but I was thinking of ordering the turboxs catted dp.

I have the same question.

[subiXTnewbie]

wow such excellent information here.

My question is I have a 2004 forester xt with a stock WRX turbo installed. I was going to go with a catless downpipe and catback exhaust this year but not sure if I am ready for a tune. Was hoping to get away without it for this year until more mods next year. Does anyone know where I can learn more about this or has suggestions for me? I have been reading things on COBB and on here but I am new to this all.

[84GLPROJECT]

Hello, first time posting in a long time. I recently bought an invidia downpipe from a dismantled to replace my stock one because the cat got plugged. My question is what I will need to bolt the downpipe to the stock exhaust, I don’t know the diameter of the exhaust pipe on it currently as I bought it used from Craigslist and it does have the Sti exhaust tip. So to actually get to my question what would I need to do if it is the stock 2.5” exhaust pipe? What gaskets will I need and how would I go about connecting the 3” down pipe to the 2.5” exhaust. Thank you in advance

[ifearnoevo]

Originally Posted by 84GLPROJECT

Hello, first time posting in a long time. I recently bought an invidia downpipe from a dismantled to replace my stock one because the cat got plugged. My question is what I will need to bolt the downpipe to the stock exhaust, I don’t know the diameter of the exhaust pipe on it currently as I bought it used from Craigslist and it does have the Sti exhaust tip. So to actually get to my question what would I need to do if it is the stock 2.5” exhaust pipe? What gaskets will I need and how would I go about connecting the 3” down pipe to the 2.5” exhaust. Thank you in advance

https://www.rallysportdirect.com/par...iABEgK3SvD_BwE, that's assuming that your exhaust is 2.5 inch. You would also need a 3 inch exhaust gasket, a 2.5 inch exhaust gasket and or donut gasket, and the turbo to downpipe gasket. Please fill out your profile so that we know what car you have to better assist you.

[SirBrass]

Originally Posted by 84GLPROJECT

Hello, first time posting in a long time. I recently bought an invidia downpipe from a dismantled to replace my stock one because the cat got plugged. My question is what I will need to bolt the downpipe to the stock exhaust, I don’t know the diameter of the exhaust pipe on it currently as I bought it used from Craigslist and it does have the Sti exhaust tip. So to actually get to my question what would I need to do if it is the stock 2.5” exhaust pipe? What gaskets will I need and how would I go about connecting the 3” down pipe to the 2.5” exhaust. Thank you in advance

Please fill in your car and location information so we're better informed about what you have.

[SirBrass]

Originally Posted by MGrizzard

I have the same question.

It depends on your cat, however, because most of the aftermarket catted downpipes use a high flow cat, it will still not satisfy the parameters the rear O2 sensor is looking for and thus you will still inevitably throw a code.

The reason some of us still opt for the cat is in order to reduce the evidence that we've messed with the emissions system and in order for an inspector just looking into the engine bay to see that there' still a cat in the system.

Gas stink is unneighborly .

[SGIrix]

I assume an aftermarket downpipe + a tune will put the warranty in danger. Correct?

[SirBrass]

Originally Posted by SGIrix

I assume an aftermarket downpipe + a tune will put the warranty in danger. Correct?

On your engine, yes. On everything else, no.

[singularity88]

Concerning the topic of divorced vs bellmouth, I found these gems recently (credit to where it is due, not my work) and figured I'd post since I haven't found much other than hear-say.

CFD analysis results compared with some real-world testing (different platform, mind you):
https://www.cummins.com/sites/defaul...egate-flow.pdf

Take that into account with this:

Wastegate Valve Simulation - YouTube

Wastegate Valve Simulation - YouTube

My thoughts (let the flaming begin):
-The angle that the wastegate flow is re-introduced does affect efficiency
-Since there is no divider on our turbos, and more of the flow from the wastegate comes from the edge opposite the arm, then it is reasonable to believe that there would be SOME effect given our wastegate orientation.

That being said, I've only ever had a bell-mouth style downpipe (both catted and non), and have had zero issues.... so either way, run whatever makes your conscience happy.
The above is probably for those who want that last tiny little bit from a stock location setup.