Catalytic Converter Placement

[rcarson25]

Looking to add cats to my 1978 Firebird LS3 swap in order to reduce sound and emissions a bit as this will be a street car. I have the Holley/Hooker ls swap kit, including exhaust. I may be able to squeeze the cats up by the exhaust manifolds, but I have plenty of room behind the X in the X pipe just before the mufflers. Would there be any negative to placing the cats this far downstream in the system?

[BonzoHansen]

I believe they put them as close to the exhaust manifold as they can so they get hot enough

[rcarson25]

In an OEM application that needs to meet EPA requirements that makes perfect sense. They want to get the cat as hot as possible as quick as possible for efficiency. I am not really questioning that logic for cat performance. I am wondering if there would be any negative effect to running them further down stream because of fitment issues. Ie cats don’t ever get up to temp to do anything or they cause some kind of flow issue, Back pressure build up down stream issues etc.

[Snowcatter]

I have run them further downstream without any issues..

[rcarson25]

Thanks, I needed up putting these in the normal spot just after the exhaust manifolds. They fit well and plenty of room for the O2 sensors before the cats.

[68Formula]

Good to hear I'm not the only one thinking of adding cats to my swap.:)

So which ones did you go with?

[rcarson25]

Went with stainless magnaflow 2.5” high flow cats. I liked that the had heatshields like stock cats and got a good price for a pair on EBay.

[cdrod]

I'm installing 2 Dynatech cats right after the header collectors on my '72 Olds 442 convertible. I'm running an old school Olds small block engine, stroked to 375cid with a FiTech EFI setup. I chose the Dynatech cats because they have superior flow rates (400 cfm) and use metal substrate cores which will not melt down like a ceramic core can if they get too hot. They're available in 2-1/4", 2-1/2" and 3" pipe sizes. Here's an Amazon link with pricing and more info:

https://www.amazon.com/DYNATECH-779-40025-Power-Catalytic-Converter/dp/B005JV7AWQ

Rodney

[65 drop top]

I'm installing 2 Dynatech cats right after the header collectors on my '72 Olds 442 convertible. I'm running an old school Olds small block engine, stroked to 375cid with a FiTech EFI setup. I chose the Dynatech cats because they have superior flow rates (400 cfm) and use metal substrate cores which will not melt down like a ceramic core can if they get too hot. They're available in 2-1/4", 2-1/2" and 3" pipe sizes. Here's an Amazon link with pricing and more info:

https://www.amazon.com/DYNATECH-779-40025-Power-Catalytic-Converter/dp/B005JV7AWQ

Rodney
Are they 400 cfm or 400 cell count?

[rcarson25]

So I got the cats in, broke them in and went for a little drive. All seemed good car drove fine not a hickup. Parked the car overnight, went to drive it today and idle seemed a little off. Drove at low speed through the neighbor hood and car felt way down on power and was running rough below 3,000 rpm. After 3k rpm the car woke up. Almost seems like the cat was clogged at low rpm but not a high rpm. I have had a cat go bad before and slowly bring power down, but never had one choke the engine at low rpm. No check engine lights etc any thoughts on what may be going on cats are brand new.

[cdrod]

Are they 400 cfm or 400 cell count?

They’re 300 cells, 400 ctm

[65 drop top]

They’re 300 cells, 400 ctm

Thanks

[NovaSF]

I have a set of MagnaFlow mini-cats (#59956) between my headers and In front of the x-pipe. Choose them cause they are tiny (8 inches long) and hi-flow (200 cell). I like em. Kills the fuel smell. Running them behind a LS3 “hot cam” and a MagnaFlow turbo muffler system. Not super loud but still sounds good.

[68Formula]

So I got the cats in, broke them in and went for a little drive. All seemed good car drove fine not a hickup. Parked the car overnight, went to drive it today and idle seemed a little off. Drove at low speed through the neighbor hood and car felt way down on power and was running rough below 3,000 rpm. After 3k rpm the car woke up. Almost seems like the cat was clogged at low rpm but not a high rpm. I have had a cat go bad before and slowly bring power down, but never had one choke the engine at low rpm. No check engine lights etc any thoughts on what may be going on cats are brand new.

If they were plugged, it should affect WOT, not just low speed. Check for exhaust leaks and look at your LTFTs to see if the PCM thinks your lean.

[rcarson25]

Thanks for the input. I have not had the issue since, the pcm was holding a code saying no communication with the 02 sensor on one side. I cleared the code and have not had an issue since

[showdog75]

Fwiw I just put some of the low profile high performance cats on my LS swap Chevy II wagon and they made a huge difference in the fuel/exhaust smell. I got the Stainless Works cats. I was very space limited and the o.d. is 3" for these 2 1/2" cats. Metal substrate.