Kooks Green Cats on Stock headers?

[2017-RS3]

Hey Guys, I have a 2017 Stage 3 Roush and i think one of the cats might be going out. From what i have read, the stock cats dont like boost or horsepower above 600whp. I live in an emissions state and I'm wondering if 2.5" Kooks Green Cats can be used with the stock headers. I'm guessing they will have to be welded in but can that be done without removing the headers? I've only been under my car for general maintenance and to install an upgraded heat exchanger but from pictures it looks like the passenger side header has a flange to bolt to but the drivers side doesnt or is there a flange higher up? Trying to avoid the cost of aftermarket headers and install. Thanks in advance for the help. Pictures attached for reference.

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[Cordero1]

https://www.makperformance.com/2015-2023-Mustang-GT-MAK-Performance-Pipes_p_299.html

 

[2017-RS3]

https://www.makperformance.com/2015-2023-Mustang-GT-MAK-Performance-Pipes_p_299.html

I considered that option but they cost almost $1600+ tax and for that type of money I would probably just replace the headers.

 

[Cordero1]

Yeah that's a ridiculous price. I didn't price it out. Though it was in the high 600s

 

[2017-RS3]

Yeah that's a ridiculous price. I didn't price it out. Though it was in the high 600s

At first glance it is until I realized the lower price is for Naturally Aspirated and as soon as you choose the boosted option, the price triples.

 

[Cordero1]

At first glance it is until I realized the lower price is for Naturally Aspirated and as soon as you choose the boosted option, the price triples.

Yeah that's highway robbery. I'm sure the kooks green cats would he an East weld in, or the ones LTH sells which I believe are also the ones that make the kooks ones

 

[jayman33]

the cats alone retail for 800, not including the cost to cut and weld a new cat on. The shop will most likely have to remove the stock header in order to properly cut and weld and that’s if you can find a shop in your area willing to cut a cat off. I’m not too sure how strict shops are in your area. I’d assume a shop will cost you a hefty penny as well.

Personally, I would run headers with gesi cats (green cats are a form of gesi cats). We’ve still had customers pop a check engine light for their O2 with these cats so I would look at some vibrant O2 bungs as well.

headers are also going to slightly lower your boost level by about .5-1 lb of boost typically as you’re lowering the restriction. You’ll still gain a solid amount of power.

just some food for thought.

 

[GregO]

I live in an emissions state and I'm wondering if 2.5" Kooks Green Cats can be used with the stock headers.

Something to consider, supercharged exhaust flow is very destructive to metal substrate catalytic converters.
Finding a shop that will remove your OEM cats could prove to be impossible.
Exhaust emission tampering days are over for all but the underground community.

 

[2017-RS3]

the cats alone retail for 800, not including the cost to cut and weld a new cat on. The shop will most likely have to remove the stock header in order to properly cut and weld and that’s if you can find a shop in your area willing to cut a cat off. I’m not too sure how strict shops are in your area. I’d assume a shop will cost you a hefty penny as well.

Personally, I would run headers with gesi cats (green cats are a form of gesi cats). We’ve still had customers pop a check engine light for their O2 with these cats so I would look at some vibrant O2 bungs as well.

headers are also going to slightly lower your boost level by about .5-1 lb of boost typically as you’re lowering the restriction. You’ll still gain a solid amount of power.

just some food for thought.

Thank you for the info. It looks tight in there so I figured they might have to remove the header and if that's the case I'll probably just go with some shorty's so it doesn't change the cat location and I don't fail visual inspection. The reason I was considering the green cats is for the Horsepower rating and because they may be less likely to throw a code. I'm definitely open to other high flow cats but I just want to make sure they will hold up as I plan to be right at 800whp and on e85 before summer.

 

[GregO]

Thank you for the info. It looks tight in there so I figured they might have to remove the header and if that's the case I'll probably just go with some shorty's so it doesn't change the cat location and I don't fail visual inspection. The reason I was considering the green cats is for the Horsepower rating and because they may be less likely to throw a code. I'm definitely open to other high flow cats but I just want to make sure they will hold up as I plan to be right at 800whp and on e85 before summer.

Then go straight to OEM GT500 5.2 Predator cats. Those are flanged.

 

[TexasMetallic5.0]

I considered that option but they cost almost $1600+ tax and for that type of money I would probably just replace the headers.

 

[Cory S]

My Kooks "green" cats were smoked within 10K miles. You will also still throw a catalyst code with them unless you manipulate the O2 sensor metering/spacing.

 

[engineermike]

It's actually really easy to pull the driver's side stock header/cat assembly. You get the front 3 bolts from the top and remaining 5 from the bottom. Unhook the steering column (1 torx bolt), the exhaust ball-joint, then it comes right out. You just have to get the car high enough to get the whole thing out. Once it's out, you can weld in a cat of your choice.

That said, many (including myself) install a flange on the driver's side manifold. The Gen1 and 5.2's have this flange already, but I believe they are oriented differently from each other. In other words, you can't just install a Gen1 manifold with a GT500 cat. Also, the 5.2 headers won't bolt to Coyote heads. You could install a Gen1 header or add a flange to your stock header, then make a catted mid-pipe to connect to the rest of the exhaust if you want.

 

[2017-RS3]

My Kooks "green" cats were smoked within 10K miles. You will also still throw a catalyst code with them unless you manipulate the O2 sensor metering/spacing.

Damn. Good to know! How did you know they went out. Was it just visual or did your car have different behavior?

 

[2017-RS3]

It's actually really easy to pull the driver's side stock header/cat assembly. You get the front 3 bolts from the top and remaining 5 from the bottom. Unhook the steering column (1 torx bolt), the exhaust ball-joint, then it comes right out. You just have to get the car high enough to get the whole thing out. Once it's out, you can weld in a cat of your choice.

That said, many (including myself) install a flange on the driver's side manifold. The Gen1 and 5.2's have this flange already, but I believe they are oriented differently from each other. In other words, you can't just install a Gen1 manifold with a GT500 cat. Also, the 5.2 headers won't bolt to Coyote heads. You could install a Gen1 header or add a flange to your stock header, then make a catted mid-pipe to connect to the rest of the exhaust if you want.

Very insightful. Thank you for the info!

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