DIY: Gutting your Pre-Cats (make your own Piggies) + FAQ

[SprintBlueWorld]

New to me b7 s4 cab. Looking to gut the pre cats. Is there a gasket between the exhaust mani and the DP?

No, there is no gasket where the exhaust manifold meets the downpipes. When you get to that point of the reinstall it is imperative that you take the time to verify the surfaces are together flush and evenly bolted together. Otherwise you'll end up with an exhaust leak and have to go back for adjustments.
Only other thing is be careful with your sensors, as uless you take them completely out, they will not freely spin which stresses the shielding of wires.

[patastinky]

New to me b7 s4 cab. Looking to gut the pre cats. Is there a gasket between the exhaust mani and the DP?

No

[ptatohed]

There is also the B5 DIY which should be very similar as far as gutting.

http://www.audizine.com/forum/showth...ies-(For-6spd)

[CdnaudiS4]

How much should a used set of oem downpipes cost? Not gutted.

Edit: For a B7 S4 Tip

[AUD1OS]

$300 to $400 bucks I believe

[CdnaudiS4]

$300 to $400 bucks I believe

Alright thanks! I'm just looking for a set locally and trying to figure out a price.

[E.A.S customs]

Hey guy's first great write up thank you. I have an A6 2.7 and just started getting the rotten egg smell couple weeks ago. Would that be from pre cats or the main ones? Second question is i have a stage 2 tune will the jhm tune effect that in anyway and do i have to send out my computer again like i did for my stage 2 in order to keep CEL light off.

[Cottonmouth]

I think I will be doing this but won't be getting a tune any time soon so I was wondering what spacer you guys have been using? I would like to source one locally so as to not spend too much.

What length has everyone been using?

[Sickws6]

I think I will be doing this but won't be getting a tune any time soon so I was wondering what spacer you guys have been using? I would like to source one locally so as to not spend too much.

What length has everyone been using?

What works best spacers or relocating 02 to rear of main cats?

[brad65ford]

What works best spacers or relocating 02 to rear of main cats?

+1 wanting to do this mod myself though i'm not a good welder. Would love to get some help with this too.

[SprintBlueWorld]

Shouldn't need a welder just spacers like these or make your own.

https://www.europaparts.com/034motor...-111-z002.html

[brad65ford]

Shouldn't need a welder just spacers like these or make your own.

https://www.europaparts.com/034motor...-111-z002.html

You the man, thank you for the link. Just order, hopefully that will work well. Plan to gut the pre-cat soon.

[Silver Streakin]

You can't use straight spacers or they'll hit each other since the sensors will cross at some point, which doesn't take much. Think of them making an X. It doesn't work.

You need at least one side or both to be angled or a J so they don't hit each other. Check Amazon.

Edit: at least that's what happens on a B7 S4. I have some spacers already to go, if you want them since I learned this the hard way. And then bought JHM tune.

[brad65ford]

You can't use straight spacers or they'll hit each other since the sensors will cross at some point, which doesn't take much. Think of them making an X. It doesn't work.

You need at least one side or both to be angled or a J so they don't hit each other. Check Amazon.

Edit: at least that's what happens on a B7 S4. I have some spacers already to go, if you want them since I learned this the hard way. And then bought JHM tune.

No one has comment on this regarding a RS4 though you could be correct.

[brad65ford]

No one has comment on this regarding a RS4 though you could be correct.

Just to note the spacers have worked perfectly so far, also no cel so far. Fingers crossed.

[brian_216]

Curious why spacers are being mentioned if you are only gutting the pre cat. I gutted the pre cats on my RS4 and left the main cats intact. I did this prior to getting my JHM tune and never got a check engine light.

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[REVOofRustler]

Curious why spacers are being mentioned if you are only gutting the pre cat. I gutted the pre cats on my RS4 and left the main cats intact. I did this prior to getting my JHM tune and never got a check engine light.

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I'm wondering this as well. I'm planning on gutting my pre cats on friday while my motor is out. I guess I'll see if I get a CEL from the main cat o2 sensors. Spacers shouldnt be too hard to throw in after the fact. I'm running a JHM tune, so the pre cat sensors shouldnt be an issue.

Also, what solvents are you guys using to dry out the downpipes after flushing them out?

[brian_216]

Jhm tune you shouldn't get a code for piggies with or without spacers. If you log the car it looks like the rear o2s are coded out. The value does not change.

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[pchrist]

Piggies on a convertible yes or no? What would be the reasonable price to have it done? Would it be easier to cut out and weld in straight pipe?


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[S4orgy]

Diy... its not that hard. Time consuming

[S4orgy]

Get a drill and drill out the inside

[pchrist]

Anyone have any clips of the sound difference? I


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[snowboardgod]

Hate to bump an old thread, but this write up was awesome! Grabbed the exact tools mentioned and a second set of downpipes, took me 20-25 min per pipe to gut the ore cats. Can't wait to swap them in next week, waiting for lift access.

[SSSSS5]

Only had a long ass 3/8" spade bit in my tool bench. started swiss cheese-ing the honeycomb until it started breaking up, then used a claw tool to remove some chunks. 30 minute job each. Should have used a mask though, blew my nose after and it was all gray.
use something like this = its a great tool to have anyway https://www.harborfreight.com/24-inc...ool-94162.html

Removing these bastards is a different story. 2013 DSG s5 total PITA

[vw_man99]

B7-S4 removing the precats when I have the engine out.
Audi parts site shows a manifold to precat triangle gasket (8E0-253-115-C) but some here say there is no gasket (?)

http://www.audipartsca.com/oem-parts...bC12OC1nYXM%3D

I plan to cut out the precats, not gut, so will be using flange with 3" flare to 2.5" reduction and get muffler shop to bend some pipe in place of cat. Considering the price of replacement OEM cats there is value to save/resell these.
When gutting there could be some resonance in the empty precat barrel which the replacement pipe would avoid - just my 2 cents :-)