Gutted precats = flow eddies = poor performance?
Thread Starter
Racer
Joined: Oct 2012
Posts: 311
Likes: 128
Gutted precats = flow eddies = poor performance?
Hey all,
In my ongoing effort to determine why my car feels slower, I'm posing this question. I just cut open my front y-pipe and gutted the disintegrated precats entirely, then welded it all back up. I swear the car is slower now. I mean maybe it's just psychological now that the exhaust is louder and more dramatic while the performance has not changed to match it but I do wonder about the effects of those two large empty chambers right off the manifolds. Having those big low pressure areas certainly isn't going to help flow. What I'm wondering is how much they might actually be hurting performance? Anyone have an opinion on that? Do I need to cut them out and put some standard pipe sections in?
In my ongoing effort to determine why my car feels slower, I'm posing this question. I just cut open my front y-pipe and gutted the disintegrated precats entirely, then welded it all back up. I swear the car is slower now. I mean maybe it's just psychological now that the exhaust is louder and more dramatic while the performance has not changed to match it but I do wonder about the effects of those two large empty chambers right off the manifolds. Having those big low pressure areas certainly isn't going to help flow. What I'm wondering is how much they might actually be hurting performance? Anyone have an opinion on that? Do I need to cut them out and put some standard pipe sections in?
Team Owner
Joined: Sep 2001
Posts: 62,153
Likes: 1,731
From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
If they're gutted then you're flowing more, how much power you may have gained back is debatable. The main cat is gutted as well? Did you look hard at it when you had the front Y off?
Melting Slicks
Joined: Mar 2002
Posts: 2,876
Likes: 764
From: Sligo PA
I know a lot of people prefer to weld tubes inside the cat or just replace them with a straight pipe if they live in an area they can get away with it. I was always told that the open chamber would cause turbulence that would make the exhaust sound bad and could cause low rpm performance loss. I think if that's true it's probably very minimal if its happening at all. So if your "seat of your pants" dyno is telling you the car feels slower it's probably in your head. But you could crack those cats back open and weld a pipe in. Do the main cat while you're at it. I'm not sure of your year so I don't know if you have a rear O2 sensor or not. But if you do you can make an O2 signal generator or better known as a "O2 simulator" for less than $10. I plan on doing this same thing to my 2011 Silverado when I take the bed off to clean up the frame.
Thread Starter
Racer
Joined: Oct 2012
Posts: 311
Likes: 128
There's definitely something going on with my engine that isn't in my head. You guys have convinced me that it's not being caused by whatever flow impedance those open chambers are causing (I'm certain it does degrade flow to some degree, even if it's small) so I'm looking elsewhere.
Current symptoms are: sluggish WOT performance, higher running temps of oil and coolant, and the displayed instant fuel mileage values are way low.
I suspect my O2 sensor is shot. I should have replaced it or at least tested it when I had the y-pipe apart. Duh. I recall that it did have a white coating of residue all over the probe. Given that I had an unburned fuel issue resultant from a disintegrated HEI rotor, I feel it's likely my O2 sensor is shot. Back to the jackstand dance!
Also, FWIW, I lean toward retaining all the factory emissions stuff rather than tearing it out. I like having a clean running car! And at 245 HP, I'm not really worrying about all-out performance. In an age when a common minivan of any make has a higher HP rating, it's the turns where this car shines, not the straights...
Current symptoms are: sluggish WOT performance, higher running temps of oil and coolant, and the displayed instant fuel mileage values are way low.
I suspect my O2 sensor is shot. I should have replaced it or at least tested it when I had the y-pipe apart. Duh. I recall that it did have a white coating of residue all over the probe. Given that I had an unburned fuel issue resultant from a disintegrated HEI rotor, I feel it's likely my O2 sensor is shot. Back to the jackstand dance!
Also, FWIW, I lean toward retaining all the factory emissions stuff rather than tearing it out. I like having a clean running car! And at 245 HP, I'm not really worrying about all-out performance. In an age when a common minivan of any make has a higher HP rating, it's the turns where this car shines, not the straights...
Last edited by ThickLizzyVetteswerv; Apr 27, 2024 at 05:12 PM.
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Pro
Joined: Oct 2003
Posts: 506
Likes: 12
From: Savannah GA
You’re way over thinking it.
the car is probably emission exempt anyway
one of my 87s precat came apart and plugged the main cat
I pulled the whole exhaust out and busted all the guts out without doing anything else
performance, didn’t do anything
sound, don’t need a high priced exhaust
walker dynomax (bang for buck) and no cat serious sound with no cat
the car is probably emission exempt anyway
one of my 87s precat came apart and plugged the main cat
I pulled the whole exhaust out and busted all the guts out without doing anything else
performance, didn’t do anything
sound, don’t need a high priced exhaust
walker dynomax (bang for buck) and no cat serious sound with no cat
Thread Starter
Racer
Joined: Oct 2012
Posts: 311
Likes: 128
I'm definitely not overthinking it. There's something going on with the engine, for sure. I've moved past suspecting the exhaust, though.
My car, along with every car newer than 1975, absolutely is not emissions exempt where I live.
My car, along with every car newer than 1975, absolutely is not emissions exempt where I live.
Team Owner
Joined: Sep 2001
Posts: 62,153
Likes: 1,731
From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
An O2 is difficult to diagnose but if your MPG is actually dropping after this, then its an indicator. They do not throw codes until they are completely toast. I actually got 2-3mpg back from adding headers and gutting the main cat.
Pig rich running will be often sluggish.
Pig rich running will be often sluggish.
Pro
Joined: Jun 2018
Posts: 532
Likes: 104
Hey all,
In my ongoing effort to determine why my car feels slower, I'm posing this question. I just cut open my front y-pipe and gutted the disintegrated precats entirely, then welded it all back up. I swear the car is slower now. I mean maybe it's just psychological now that the exhaust is louder and more dramatic while the performance has not changed to match it but I do wonder about the effects of those two large empty chambers right off the manifolds. Having those big low pressure areas certainly isn't going to help flow. What I'm wondering is how much they might actually be hurting performance? Anyone have an opinion on that? Do I need to cut them out and put some standard pipe sections in?
In my ongoing effort to determine why my car feels slower, I'm posing this question. I just cut open my front y-pipe and gutted the disintegrated precats entirely, then welded it all back up. I swear the car is slower now. I mean maybe it's just psychological now that the exhaust is louder and more dramatic while the performance has not changed to match it but I do wonder about the effects of those two large empty chambers right off the manifolds. Having those big low pressure areas certainly isn't going to help flow. What I'm wondering is how much they might actually be hurting performance? Anyone have an opinion on that? Do I need to cut them out and put some standard pipe sections in?
That said, if you are running stock manifolds I doubt there will be any significant change if you replace the now empty cats with a straight pipe. It will sound a lot better though.
Last edited by auburn2; Apr 29, 2024 at 07:12 PM.