Single Cat on I-Pipe Instead of Dual cats on Y-pipe
Thread Starter
Teching In
Joined: Jul 2013
Posts: 10
Likes: 0
From: Dallas, TX
Single Cat on I-Pipe Instead of Dual cats on Y-pipe I've searched and searched for information regarding this and couldn't find a single post about it. I'm building a new exhaust setup for my 1999 trans am with a stock LS1.
My goals: Replace stock manifolds and Y-pipe with the BBK longtube + Y-pipe package (Non catted Y). I want to install an electric cutout followed by a single magnaflow cat on the I-pipe so that i can switch between open header mode and cat+muffler mode.
My concern is, can i still pass emissions with a single cat on the i-pipe rather than dual cats on the Y. (I live in Texas). I would buy the biggest converter magnaflow sells, rated to handle a 7.8L.
Has anyone ever tried anything like this? Anyone think i can pass emissions with this setup and the cutout closed?
My goals: Replace stock manifolds and Y-pipe with the BBK longtube + Y-pipe package (Non catted Y). I want to install an electric cutout followed by a single magnaflow cat on the I-pipe so that i can switch between open header mode and cat+muffler mode.
My concern is, can i still pass emissions with a single cat on the i-pipe rather than dual cats on the Y. (I live in Texas). I would buy the biggest converter magnaflow sells, rated to handle a 7.8L.
Has anyone ever tried anything like this? Anyone think i can pass emissions with this setup and the cutout closed?
Very interesting idea! I'm definitely interested to see how this might end up playing out. It'd be a much more affordable proposition for anyone who needs converters for inspection. I can almost guarantee that the tech doing emissions inspections wouldn't have any idea that the location isn't factory. If they question you, just bring in a picture of a third gen exhaust 
that actually sounds interesting.. you would just need to have an O2 bung welded in after the cat and use a corvette sensor with the 4 or 5 foot long harness to reach the plug.
I've searched and searched for information regarding this and couldn't find a single post about it. I'm building a new exhaust setup for my 1999 trans am with a stock LS1.
My goals: Replace stock manifolds and Y-pipe with the BBK longtube + Y-pipe package (Non catted Y). I want to install an electric cutout followed by a single magnaflow cat on the I-pipe so that i can switch between open header mode and cat+muffler mode.
My concern is, can i still pass emissions with a single cat on the i-pipe rather than dual cats on the Y. (I live in Texas). I would buy the biggest converter magnaflow sells, rated to handle a 7.8L.
Has anyone ever tried anything like this? Anyone think i can pass emissions with this setup and the cutout closed?
My goals: Replace stock manifolds and Y-pipe with the BBK longtube + Y-pipe package (Non catted Y). I want to install an electric cutout followed by a single magnaflow cat on the I-pipe so that i can switch between open header mode and cat+muffler mode.
My concern is, can i still pass emissions with a single cat on the i-pipe rather than dual cats on the Y. (I live in Texas). I would buy the biggest converter magnaflow sells, rated to handle a 7.8L.
Has anyone ever tried anything like this? Anyone think i can pass emissions with this setup and the cutout closed?
If you do want one that will still be somewhat effective then install a magnaflow # 59959 right off the bbk y-pipe and then into a E-cutout exit.. Now if you want a little better flow the you'll need to replace the bbk y-pipe merger with one that has a larger 3.5" outlet, then install a magnaflow #59920 which has 3.5" inlet/outlet, followed by a 3.5" E-cutout into a 3.5" to 3" reducer going back into the cat-back before the over-axle pipe. FYI the single cat won't reduce the overall loudness a lot, but it will give the exhaust a better sound.
Last edited by 99Bluz28; Jul 2, 2014 at 02:51 PM.
Why would you even want to consider doing that ?
Adding a rear O2 sensor behind a converter that's to far back is only going to set a code, since it won't be able to get hot enough to be 100% effective at converting harmful compounds to harmless. So, spending the time and money to install a rear O2 is only going to tell you what you already know.
FYI, it's the same reason that the rear O2 sensor are removed(or disconnected) and either tuned out or replaced with O2 Sims when L.T. headers are installed; converter inefficiency.
Last edited by 99Bluz28; Jul 2, 2014 at 02:24 PM. Reason: corrections
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I have done the same thing, I installed a 3.5" 200 cell cat in the I pipe merge. I did a run test then dropped the pipe and checked the temp of the cat element, I read 1000 degrees. it only takes 400-600 degrees for a cat to light off or start working, so it can work that far down stream, not as well but it will work, the single cat was a bit louder that the two cat in the normal position. I have a y-pipe I build with race merge and removal inserts so I can swap out diff setups.
Thread Starter
Teching In
Joined: Jul 2013
Posts: 10
Likes: 0
From: Dallas, TX
Wow I'm really surprised that some people have already done this. Setup looks great toadzq8. Like I said if it weren't for emissions I would've already done this. Have you guys been able to pass inspection with that setup? This car will be my daily driver.
Bringing this back from the dead.
I'm running:
Pace Setter Coated LT's
Pace Setter Y PS82-1163 and IZM993-2 O2 sensors, with dual DMH cutouts on the Y-pipe
Magnaflow 14419 muffler used inline with I pipe
Walker 54064 (I-pipe)
Walker 53061 (I-pipe)
CME Exhaust tips and elbows
Dynomax, Ultra Flo Welded, muffler #17227
I'm considering putting in a high flow cat 59959 before the 14419. Would this help with exhaust fumes when I have the cutouts closed, or is it not even worth the effort?
I'm running:
Pace Setter Coated LT's
Pace Setter Y PS82-1163 and IZM993-2 O2 sensors, with dual DMH cutouts on the Y-pipe
Magnaflow 14419 muffler used inline with I pipe
Walker 54064 (I-pipe)
Walker 53061 (I-pipe)
CME Exhaust tips and elbows
Dynomax, Ultra Flo Welded, muffler #17227
I'm considering putting in a high flow cat 59959 before the 14419. Would this help with exhaust fumes when I have the cutouts closed, or is it not even worth the effort?
