Can I go catless on my 1992 C4?
01-13-2018, 07:00 PM
#21
Team Owner
OBD2 cars (1996+) also have a second set of O2 sensors after the cats. These only exist to allow the PCM to test the function of the cats: it compares what's coming out of the cats with what's going in and uses that info to assess whether the cart is working well or not. The PCM does not use these post-cat or "downstream" sensors to manage the fuel mixture. If you remove or hollow out the cats on an OBD2 car, it will quickly through a trouble code because the PCM knows the cat isn't functioning anymore. If you remove or disconnect the downstream O2 sensors it will do the same thing. In order to avoid the CEL, you have to either modify the PCM tune to turn off that diagnostic test, or somehow trick the rear sensors into giving the PCM the signal it's looking for. Doing it in the tune is the proper way to achieve this. This is the step that c4cruiser was referring to in his post.
You can remove the cats or whatever, and the ECM won't know the difference.
As to whether that's a good thing to do or not, I'll let others address that question!
You can remove the cats or whatever, and the ECM won't know the difference.
As to whether that's a good thing to do or not, I'll let others address that question!
Agreed
Depends on whether you think spending the money on replacing something emissions related is meaningful or not. I don't but if someone wants to, they can knock themselves out doing it. I'd spend the money helping poor girls get through college at the strip bar before the emissions but that's me.
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Sworn (01-14-2018)
01-13-2018, 07:11 PM
#22
Zen Vet Master Level VII
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DykstraMotorsports (01-22-2018)
01-13-2018, 10:11 PM
#23
Team Owner
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DykstraMotorsports (01-22-2018)
01-14-2018, 10:14 PM
#24
Instructor
Thread Starter
Yeah, I was reading c4cruiser's post and though the same as you, "Whoa!" So just to be clear, 1996 is the only OBD2 C4 they made. All prior years were OBD1. In probably all EFI-equipped cars of all years, there are O2 sensors before the cats, which the PCM uses to properly maintain the air:fuel ratio at all times in closed-loop operation (closed-loop is any time the car is warmed up and not running at or near WOT). You do not want to remove or delete these sensors in real life or in the software tune. If you did, you'd have a car running in open-loop all the time.
OBD2 cars (1996+) also have a second set of O2 sensors after the cats. These only exist to allow the PCM to test the function of the cats: it compares what's coming out of the cats with what's going in and uses that info to assess whether the cart is working well or not. The PCM does not use these post-cat or "downstream" sensors to manage the fuel mixture. If you remove or hollow out the cats on an OBD2 car, it will quickly through a trouble code because the PCM knows the cat isn't functioning anymore. If you remove or disconnect the downstream O2 sensors it will do the same thing. In order to avoid the CEL, you have to either modify the PCM tune to turn off that diagnostic test, or somehow trick the rear sensors into giving the PCM the signal it's looking for. Doing it in the tune is the proper way to achieve this. This is the step that c4cruiser was referring to in his post.
They key is this: any 1984-1995 C4 is OBD1, and (to my knowledge) won't even have the downstream O2 sensors. They have no way of testing their cat function, so they can't tell if it's missing or hollowed out (or on there and working well). So there is nothing to disable or worry about. You can remove the cats or whatever, and the ECM won't know the difference.
As to whether that's a good thing to do or not, I'll let others address that question!
OBD2 cars (1996+) also have a second set of O2 sensors after the cats. These only exist to allow the PCM to test the function of the cats: it compares what's coming out of the cats with what's going in and uses that info to assess whether the cart is working well or not. The PCM does not use these post-cat or "downstream" sensors to manage the fuel mixture. If you remove or hollow out the cats on an OBD2 car, it will quickly through a trouble code because the PCM knows the cat isn't functioning anymore. If you remove or disconnect the downstream O2 sensors it will do the same thing. In order to avoid the CEL, you have to either modify the PCM tune to turn off that diagnostic test, or somehow trick the rear sensors into giving the PCM the signal it's looking for. Doing it in the tune is the proper way to achieve this. This is the step that c4cruiser was referring to in his post.
They key is this: any 1984-1995 C4 is OBD1, and (to my knowledge) won't even have the downstream O2 sensors. They have no way of testing their cat function, so they can't tell if it's missing or hollowed out (or on there and working well). So there is nothing to disable or worry about. You can remove the cats or whatever, and the ECM won't know the difference.
As to whether that's a good thing to do or not, I'll let others address that question!
I always kind of thought the later C4's from 94 or so and up were OBD2.
01-15-2018, 01:21 PM
#26
Le Mans Master
So the thing about 94/95 is they use a new PCM design that was essentially OBD2 ready, and they even use the newer OBD2 diagnostic port under the dash. But they don't have all the required on-board diagnostics to comply with the OBD2 standard. It is clear that GM was getting ready for it, though. The 1996 PCM is fully OBD2 compliant. It physically looks the same as the 94/95 computers. You can even swap a 94/95 PCM into a 1996 (you'll get the "sys" indicator, but it works fine). If you look at the tables of a 94/95 PCM in Tunercat and the tables of a 96 PCM in Jet DST (same software but enabled for the OBD2 system), I think all the running parameters are the same (I haven't compared line by line yet, but I will be). The 96 just has extra diagnostics enabled.
01-16-2018, 12:27 PM
#27
Race Director
They flow straight through and don't rob horsepower. Most people who say this just want them gone looking for every drop of "go". Next stop will be smooth air tube....or a Tornado! 
They still work even at 66k miles. If you don't hear a rattle or suffer noticeable performance loss, the bricks aren't broken/clogged nor do they need to be removed.
Follow Aklim's advice on where to spend your time/money instead!
They still work even at 66k miles. If you don't hear a rattle or suffer noticeable performance loss, the bricks aren't broken/clogged nor do they need to be removed.
Follow Aklim's advice on where to spend your time/money instead!
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Sworn (01-16-2018)
01-17-2018, 06:38 PM
#28
OBD2 cars (1996+) also have a second set of O2 sensors after the cats. These only exist to allow the PCM to test the function of the cats: it compares what's coming out of the cats with what's going in and uses that info to assess whether the cart is working well or not. The PCM does not use these post-cat or "downstream" sensors to manage the fuel mixture. If you remove or hollow out the cats on an OBD2 car, it will quickly through a trouble code because the PCM knows the cat isn't functioning anymore. If you remove or disconnect the downstream O2 sensors it will do the same thing. In order to avoid the CEL, you have to either modify the PCM tune to turn off that diagnostic test, or somehow trick the rear sensors into giving the PCM the signal it's looking for. Doing it in the tune is the proper way to achieve this. This is the step that c4cruiser was referring to in his post.
They key is this: any 1984-1995 C4 is OBD1, and (to my knowledge) won't even have the downstream O2 sensors. They have no way of testing their cat function, so they can't tell if it's missing or hollowed out (or on there and working well). So there is nothing to disable or worry about. You can remove the cats or whatever, and the ECM won't know the difference.
As to whether that's a good thing to do or not, I'll let others address that question!
Truthfully though - at anything approaching stock HP levels - the factory cats do not cost you much power at all. I highly doubt that you'd be able to consistently see the difference on a chassis dyno - unless the converters are clogged or have internal damage. For a track car - eliminating them makes sense for weight savings, and since you could end up off track - where very hot cats might be in contact with some taller grass - and that might not be a good thing. I will tell you that the cats do also reduce exhaust noise levels a bit (so do turbos).
BOTTOM line - eliminating them is not as easy as you might think (the one place that I knew that made test pipes is no longer around), and will cost you some $$$. (I suppose you could sell the old ones to try to recoup some of the $$$.) So - is eliminating them worth the time and money. Plus - if you do get caught without them (which admittedly is quite unlikely) it will cost you big time...
01-17-2018, 06:45 PM
#29
Team Owner
ABSOLUTELY CORRECT !!! If you don't have the downstream (post cat) Oxygen sensors - you will not turn on a check engine light if you eliminate the catalytic converters.
Truthfully though - at anything approaching stock HP levels - the factory cats do not cost you much power at all. I highly doubt that you'd be able to consistently see the difference on a chassis dyno - unless the converters are clogged or have internal damage. For a track car - eliminating them makes sense for weight savings, and since you could end up off track - where very hot cats might be in contact with some taller grass - and that might not be a good thing. I will tell you that the cats do also reduce exhaust noise levels a bit (so do turbos).
BOTTOM line - eliminating them is not as easy as you might think (the one place that I knew that made test pipes is no longer around), and will cost you some $$$. (I suppose you could sell the old ones to try to recoup some of the $$$.) So - is eliminating them worth the time and money. Plus - if you do get caught without them (which admittedly is quite unlikely) it will cost you big time...
Truthfully though - at anything approaching stock HP levels - the factory cats do not cost you much power at all. I highly doubt that you'd be able to consistently see the difference on a chassis dyno - unless the converters are clogged or have internal damage. For a track car - eliminating them makes sense for weight savings, and since you could end up off track - where very hot cats might be in contact with some taller grass - and that might not be a good thing. I will tell you that the cats do also reduce exhaust noise levels a bit (so do turbos).
BOTTOM line - eliminating them is not as easy as you might think (the one place that I knew that made test pipes is no longer around), and will cost you some $$$. (I suppose you could sell the old ones to try to recoup some of the $$$.) So - is eliminating them worth the time and money. Plus - if you do get caught without them (which admittedly is quite unlikely) it will cost you big time...
https://www.ecklerscorvette.com/diagram/view/index/id/157/
Looking at the diagram, you can cut it off and put in a straight pipe. Worst case, weld in a bung
01-17-2018, 07:11 PM
#30
Race Director
Your location says Ellenton Florida. Florida doesn't require any emission testing at all, even on newer models that may require it in other emissions crazy states like say NY or CA..... so why would anyone be visually checking for cats?
Who was checking and where exactly did this happen?? I'm calling BS on your entire story!!
Who was checking and where exactly did this happen?? I'm calling BS on your entire story!!
Agreed. They are using mirrors to look underneath for drugs, not cats.
01-17-2018, 08:15 PM
#31
Team Owner