I've been trying to figure out what to do about my cats when I upgrade my exhaust (PPE headers, full dual 2.25" exhaust).

Our (front) cats are 10" long, 4" wide body, I believe they're 600 cell. Does anyone have any more data than that? CFM? Metallic vs ceramic substrate? Confirmation of the number of cells?


When I spoke to PPE, they recommended going with 2.5" cats to match the collector of the headers. I don't have any problem with this, except that I live in California, so even just getting my hands on non-carb cats is a project (it's illegal to SHIP them to CA). I'm not really concerned with inspection since the headers aren't carb certified anyway, nor the x-pipe, so I'm going to have to deal with that anyway.

If I still lived on the east coast, I'd probably buy these:
2.5" inlet/outlet
8.75" length
5" wide body
400 cell

https://www.magnaflow.com/products/9...ytic-converter


It's hard to imagine that a 2.25" oem cat can't handle 2.15 liters (half the engine) and there would be much if any performance gain. But that's just conjecture, and I can't rest until I have some data to prove or disprove that.

Are there any experienced exhaust builders on here? or anyone with detailed information on our cats?

If there really is any performance to be gained from replacing the cats, Arizona is a short drive away, lol. I just want to make an informed decision.

Also, before anyone suggests it, I'm not interested in straight pipes: CEL, pollution, and loss of torque on a naturally aspirated engine.

 

I’m sorry that I can’t answer your question, I’m new to the car community. Since you mentioned that a straight pipe loses power, do you reckon that just doing a third Cat delete and a muffler delete would have any negative impact on the sc430. I heard taking of the muffler released back pressure and that is essentially good. And I also heard that removing cats gives it a raspier tone. I’m only removing the third one so I’m guessing it would’ be too bad. What’s your opinion? I don’t have money to get a performance exhaust btw, just a high school student lol. Any tips?

 

You should be fine removing the 3rd one, just leave the 2 main cats in place.

A good budget setup (based on all the reading I've been doing on here) is removing the rear cat, replacing the resonators with straight-through resonators (you can delete them but it won't sound very nice from what everyone says), and replace the crushed section of pipe in the rear. Maybe some free-flowing magnaflow mufflers or something like that.

Anything more than that and you're looking at big bucks for headers to get any significant performance gains.

Here's a pic of the stock exhaust from the front cat-back so you can see what I mean.

 

Thank you! I’ve been doing my fair share of reading as well, and there was this one individual who stated that he ended up liking the sound of a muffler delete (2.5 in pipes all the way to the exhaust tips). So you would suggest against that? I actually want my car to be loud, but sound good obviously. So instead of doing a muffler delete, you think it would be best to do the setup that you recommended above? I was leaning towards a simple third cat and muffler delete. You guys know best I just want to know before I make an investment. Cheers!

 

I would strongly recommend against 2.5" pipes. Literally everything I've read says that they're too large for our engine and will result in a loss of torque.

Our stock pipes are pretty much the right size (I believe they're 2.25" or pretty close to it).

I've seen posts on here where people did muffler deletes and kept the resonators, just removing the 3rd cat and the crushed pipes. If you want it loud and are on a budget, that'll probably work. And if you keep the resonators, it will likely be loud but not a nasty tone. If it's not loud enough at that point, then just replace the resonators with free-flowing ones.

 

My mistake it was 2.25! Thanks for the correction I wouldn’t have caught on otherwise. Thank you for the help my friend! Good luck on your search as well, hopefully someone knows more than me and can answer your questions 🙏🏽
Ohhhh btw, wdym by the crushed pipe? Lol don’t want to sound dumb but I would rather learn what it is.

 

post 21 shows the before and after exhaust mod...post 22 shows the work performed.
https://www.clublexus.com/forums/sc4...magnaflow.html

On my 2007, I went with Magnaflow 16917 cat back.
picked up the Magnaflow for $597 shipped, about a year ago...
https://www.hottexhaust.com/Magnaflo...7#review-title

 

To the OP - Before you invest in Magnaflows, you night want to read about my experience and the customer support I was given.

https://www.clublexus.com/forums/is-...w-no-more.html

Lou

 

wow, 23k, that's terrible!


Unless I can find some information that sways me otherwise, I think I'm just going to keep the stock cats.

 

Can anyone confirm the inlet and outlet diameter of the stock front cats? I thought they were 2.25" but I've rear a few different things now.



EDIT: The stock cats have a 60mm inlet and a 54mm outlet. And PPE confirmed they will bolt up to the headers.


For what it's worth, if you decide to change cats, PPE recommends a GESI cat with a 2.5" inlet and 2.25" outlet:
https://www.verociousmotorsports.com...tic-converters

Though, they said it should be fine with the stock cats. I've heard a lot of good things about GESI cats but man are they expensive, especially for the EPA approved ones.

I'm going to stick with the stock cats, we only have like 150hp worth of air on each bank and we're talking 2.37" vs 2.5" (0.13" difference) inlet and 2.125" vs 2.25" outlet (0.125" difference).

If anything, I'll see if the shop I have do the the exhaust can just smooth out the entry into the cat so it's not a tiny "step."

 

I couldn't stop thinking about this... and after I learned a little more about measuring airflow in my throttle body thread (LINK), I now have the math to answer my own question.

In that thread, I calculated the theoretical max flow rate for our engine to be 455cfm @ 20" H2O, with the real-life flow rate (75% VE) being around 350cfm.

Looking at these comparisons of catalytic converters, the OEM 1999 Camaro/Firebird cats flow 268-300 cfm (@20" H2O) each:
http://installuniversity.com/install...n11.122001.htm

Now, obviously we have a different cat than the Camaro from the 90s and ours has a 2.4" inlet and 2.125" outlet. Although looking at the design, ours appears to be a much shorter/more modern design.

Without flow testing our cats, we can't know for sure how many CFM they support, but even if you round down on the lowest [20 year old] OEM cat in that test, say 250cfm each just to be safe, that would still be 500cfm for the pair, higher than our theoretical max by a nice margin, and way higher than our likely real life maximum.

With this in mind, I would say our stock cats would probably not restrict flow in any meaningful way on any bolt-on naturally aspirated application (ie. no forced induction, no built engine).


Here are some other cat flow tests. These are all aftermarket 300-400 cell cats (oem is 600 I believe), but I found it useful:
https://honda-tech.com/forums/all-mo...rters-1128725/
https://honda-tech.com/forums/all-mo...d-2-a-1323923/


As a side note, I came across this test of Miata catalytic converters on a dyno which was interesting (and also showed negligible gains from an aftermarket cat). However, what I wish they would have included was torque numbers. While a test pipe doesn't gain/lose much hp, my experience on naturally aspirated cars is torque loss.

https://grassrootsmotorsports.com/ar...nverter-face-/