AussieZ06

Hi all
Saying hi from Australia and hoping you are all well and enjoying driving our awesome cars when we can.

I may have to change my stock Cats in the near future and would like to ask a question.

My LS7 is stock except for Haltech air intake and Akraprocic titanium cat back exhaust.

Question - do high flow cats change the exhaust sound, if yes in what way?

Turbo6TA

You may not get a good answer for your question ... I say that because the 'high-flow' cats that get used on these cars come with the non-stock intermediate pipe (x-pipe) that is needed when a set of long-tube headers are installed.

Changing to long-tube headers along with the required x-pipe and high-flow cats will make for a louder exhaust tone at wide-open throttle but not change the exhaust tone much during normal driving.

However, just replacing the stock cats with aftermarket high-flow cats might not change the sound at all.

Like I stated; You might be hard pressed to get a good answer for your question ... Nobody I know of just changes the cats on there otherwise stock C6 exhaust system.

Turbo6TA

Completely removing the cats on your stock exhaust system might make the sound louder ... but that is not your question. You talking specifically about replacing stock cats with high-flow cats.

BTW ... Your not going to find a HP gain by just replacing your stock cats with 'high-flow' cats (if that's the reason your thinking about doing this).

And most people have reported only about a 4 HP gain by just removing the cats .... not enough to make it worthwhile + the exhaust will stink after removing the cats on these cars.

.

AussieZ06

Thanks mate
Yes your answer seems right that probably no one has just changed cats as I did a thorough search.

I’m not looking for hp, I definitely want cats as I don’t want the smell and it’s a legal requirement here.

Thanks again I appreciate your 2 posts

Regards
Richard

American Heritage

If you dont want exhaust smell then 100% go high flow cats.
Yes high flow cats will quiet down the exhaust quite a bit. There is no way around it, cats make the exhaust less quieter.
What is louder non high flow cats or high flow cats??? I would say high flow cats are louder than non high flow cats.
I have heard a otherwise stock c6z06 that the customer removed and installed high flow cats on. It was louder than it was with the stock cats (at least thats what I and two of our techs thought).


https://americanheritageperformance.com/

grecoz06

Wondering if you could answer a question along the lines of this post. I have a C7 z06, already had corsa npp exhaust. Just did a stage 2 cam, ported heads, arh 2 ich headers and catless xpipe. Long story short, I made the car too loud and the smell is too strong. I'm going to add green high flow cats xpipe back and in also considering going back to the stock exhaust. But, for my question. Tempted to try just the green cat xpipe first but I'm worried it will still be too loud.

Keeping the Corsa, stage 2 cam how much will adding the green cars quiet it? Say my set up is a 10 on that loudness scale, what would it be after green cats?

grinder11

........'cats make the exhaust less quieter'? To be clear, cats make the exhaust quieter. IDK how the "less quieter" came about. Maybe a typo? Just trying to clarify, not being a wise ***....

American Heritage

If you currently have no cats and your sound level is at a 10 now, adding high flow cats (green cats) would drop your sound level down to a 7.5. Adding cats helps out alot when it comes to DB Levels as well as exhaust smell.

LOL no worries I think I had a typo or two there.
What I meant to put was that cats make your car quieter.
Removing the cats makes the car ALOT louder.
If you had two cats you were comparing, one std flow and the other high flow the high flow cat will be substantially louder than the std flow cats.

https://americanheritageperformance.com/

grinder11

That's what I thought, just trying to be sure. Yeah, Ive been there. This latest "update" to my phone has changed punctuation, on my phone, at least. Like missing apostrophes, and sometimes capital letters. Note the missing apostrophe in Ive.........TWICE now!!!😳😳😳

American Heritage

LOL tell me about it... I think my phone has learned all the wrong spelling and punctuation's and now it actually suggests bad grammar to me instead of the correct words and punctuation's. Ah yes the advancement of modern tech, LOL


https://americanheritageperformance.com/

grecoz06

Thank you for the reply!! I appreciate it!

That's a big drop for sure. But, may not be enough. I'm getting this insane howl-drone at around 2k rpm. Not pleasant sounding and hurts my ears. Corsa typically does not drone. How do you think the green cats will impact that?

American Heritage

I know what your talking about when you talk about that drone at 2k.
The green cats will help out with that alot in my opinion with the drone @ 2k but they wont make it totally go away. It will reduce it enough to make it tolerable and make it so you could hold a conversation with someone in the car though. I would add the green cats to your setup if I were in your shoes.

https://americanheritageperformance.com/

grecoz06

Thank you, again! Yes sir on the green cats. I'm debating on also putting the stock exhaust back on. I could try the green cats first, but I'm a bit tired of messing with the car and just want to enjoy it. Also I think I'll still be begging for a cop to mess with me here in So-Cal. I also now know why most people who do headers keep the stock mufflers lol

I'm leaning towards green cats, stock exhaust. Keep the Corsa and give ​the corsa a try again with the green cats when we move to TX later this year. I'll have more patience to experiment after having a break from modding.

grecoz06

Just left the shop, fun 45 minute drive home. Smell is resolved. Sound is much more enjoyable, much of the raspy sounds is improved. That drone around 2k is still there, just like you said. Much better. Driving at higher rpm on the freeway I love it and would keep it as is. I think the drone around 2k may still be too much when driving around town casually. it's hard to stay out of that rpm zone when in mild traffic under 30-50mph. I picked up a used set of the stock mufflers, I'm going to ceramic coat them to get them black to match the rest of the car and try them out. It'll be a few weeks.

American Heritage

Great feedback and thank you for the follow up to the thread. I'm sure others will find this thread very useful as well.
Good stuff!

https://americanheritageperformance.com/

R&D

Have you ever played around with different cell count cats? I know 400, 200, 100, and even 50 cell high-flow cats are made (the later likely just to satisfy class rules). Obviously one would think the lower the cell count, the more power can be made and the louder they are, but at the same time allow more exhaust smell to pass by. I'm about to install LT's and currently have only deleted the mufflers from the factory system on my C6Z. While the LT's and x-pipe I have don't have cats, I really don't want the car (or myself!) to stink like exhaust, but at the same time I don't want to kill to much power or sound... although I'm sure removing the factory cats and installing the LT's w/o cats would make it too loud without installing some type of mufflers back on it. I'm really tempted to weld in some 100 cell race cats to kill most of the odor and a little noise, but I'm not sure at what cell count they really become significantly effective at removing the smelly gasses. Any experience with this?

449er

"High flow cats" will give you no performance gains & no cats at all will gain minimal increases....if you want a louder exhaust, go with a catback system like Corsa

grecoz06

Thank you! I've been on a few more drives and had more time to digest, I don't think it's too loud. It's the sound profile I don't care for. Whether it's "drone" or something else I can't tell. I had a shop tell me a few years ago his personal opinion was he hated the way corsa sounded with headers. Perhaps that's it. Really hoping the stock mufflers fix this. I'm sure my other mods such as stage 2 cam/ported heads impact all the sounds and I went with 2 inch instead of 17/8 inch headers so someone with a stock cam will probably get a different sound that I'll get. However, I think I know why "most" people that do headers leave the stock exhaust. I have an appointment scheduled for next week, also the cats are hitting the underplate so we have to sort that out too. Will report back to the thread.

American Heritage

I have not played around with different cell counts cats before but its an interesting idea for sure.
If your not running cats then just lean the heck out of the tune at idle to keep the smell down as much as possible. I like to run mid 14s (AFR) at idle just to keep the smell down and that really seems to help out alot.
If you do end up playing with the different cell count on cats be sure to document it and post it up here so we can all learn something about the options for aftermarket cats.

https://americanheritageperformance.com/

R&D

Thank you for chiming in! There's no doubt testing various cell counts would have merit, although being a "little" guy doing DIY stuff, I don't have the time, money, or resources to properly test these. Ideally one would be able to test them with a sniffer, dyno, and db meter, all on the same vehicle and tune.

And speaking of tune, yes, I was already planning to set idle at 14.7:1 to start with and go from there. If you feel the smell is minimal with just the lean(sih) idle, maybe I could get away with just a pair of 50 cell count race cats? Although, I probably should play it safer and go with at least the 100 cell count so I'm less likely to be dissatisfied with the smell, especially off idle.