CELL with exhaust mod
Race Director
Joined: Dec 2013
Posts: 12,502
Likes: 3,629
The OEM cats are not robbing you more than a few HP,
If you remove or punch them out, the car cabin is going to stink of un-burnt fuel at every stop light instead.
As for if you do punch them out, then the car will throw codes.
Hence the only purpose of the rear O2 sensors is to see if the cats are working correctly. So no cats, means not differential reading between the pre and post cats, and it going to throw codes.
Hence even when adding headers (which increases HP since they scavenge the exhaust better than the stock exhaust manifolds), you still run high flow cats (lose again of about 3HP, but since the cats are moved farther way from the head, the high flow cats are not as effective as the OEM cats/will not end up with a big enough differential reading, then the rear cats are tuned out instead.
So again, front 02 sensors used for the engine to run correctly in closed loop, and the rear 02 sensor only used to tell if the cats are working correctly only (and nothing more).
Bluntly, understand how headers work in the first place and why you gain HP with them, the fact that the oem cats are not robbing you of any major power to being with, and you have no reason to either remove or punch the cats out if you are running the stock exhaust manifolds.
If you remove or punch them out, the car cabin is going to stink of un-burnt fuel at every stop light instead.
As for if you do punch them out, then the car will throw codes.
Hence the only purpose of the rear O2 sensors is to see if the cats are working correctly. So no cats, means not differential reading between the pre and post cats, and it going to throw codes.
Hence even when adding headers (which increases HP since they scavenge the exhaust better than the stock exhaust manifolds), you still run high flow cats (lose again of about 3HP, but since the cats are moved farther way from the head, the high flow cats are not as effective as the OEM cats/will not end up with a big enough differential reading, then the rear cats are tuned out instead.
So again, front 02 sensors used for the engine to run correctly in closed loop, and the rear 02 sensor only used to tell if the cats are working correctly only (and nothing more).
Bluntly, understand how headers work in the first place and why you gain HP with them, the fact that the oem cats are not robbing you of any major power to being with, and you have no reason to either remove or punch the cats out if you are running the stock exhaust manifolds.
Last edited by Dano523; Jun 24, 2016 at 01:55 PM.
Race Director
Joined: Dec 2013
Posts: 12,502
Likes: 3,629
No worries, and explains why headers make more power, verses something like just wider exhaust pipes, or punching out/removing the cats may not instead (not enough to matter, and the losses outweigh the gains isntead).
As for Long tube headers, on a LS2 or 3, around 25HP gains with tune, while the OEM Z06 shorty type headers, around a 15hp gains with tune isntead.
OEM z06 exhaust manifolds, and you can see the collector sections in them.
As for Long tube headers, on a LS2 or 3, around 25HP gains with tune, while the OEM Z06 shorty type headers, around a 15hp gains with tune isntead.
OEM z06 exhaust manifolds, and you can see the collector sections in them.
