Cat questions

[kbuskill]

Ok, so as some of you know I recently purchased a set of titanium ceramic coated JBA stainless "cat4ward" (shorty) headers for my '08 Burb 5.3L.

This discussion is not about short VS long tube headers as all of that has been hashed and rehashed before.

I know that with long tubes most people run an off road (cat less) Y-pipe and disable the downstream O2 sensors in their tune.

With that being said. Do you think there would be any real advantage to gutting the cats or removing them completely and retuning the ECM to ignore the downstream O2 sensors???

Also if I did gut/remove the cats would there be any advantage to installing the upstream O2 sensors in the down stream location by using extensions on the wiring???

Of course I would put a plug in the upstream O2 sensors bung to block it off in the above scenario.

I have been pondering this for a bit and thought I would run it by the good folk of this forum to get their thoughts/opinions.

My reasoning for thinking about gutting the cats is this...

The idea of ceramic coated headers is to keep the exhaust gases hotter. Hotter gases move faster than cooler gases. The hotter, faster moving gases in each individual header pipe create a suction/vacuum effect on the other header pipes/cylinders (scavenging) thereby causing more of the spent exhaust gases to be removed from the cylinder and allowing more room for fresh air and fuel into the combustion chamber which creates more power.

Now with that being said, my thoughts are that with the cats in place right behind the shorty headers it would cause turbulence and possibly the pressure waves to bounce back toward the engine causing the headers to not scavenge as much as they possibly could. So by removing them you would be keeping the gases moving as fast as possible.

Again I may be way off base here so that is why I'm asking the opinion of the forum.

I have already sent an email to Lew from Diablew since he is the one who tuned my truck. I will let you guys know what he says.

The idea of moving the upstream O2 sensors to the downstream location just came to me since the rear O2 sensors wouldn't be in use. I really have know idea what effects or implications there would be from doing this, if any. I know that guys running long tubes often relocate their upstream O2 sensors further back in the pipes because of the headers.

Any thoughts on this as well... pros or cons???

 

[kbuskill]

How about a bump...

@swathdiver
@randeez
@Rocket Man
@Tonyrodz
@THE YETI


And anyone else that may be running headers or just have an opinion.

 

[Tonyrodz]

Well, this is already all beyond me--but--in my case I wanted to run cats because NJ has a visual for inspection, plus I'm getting older and I didn't want to deal with the fumes--which I have to because I don't have my cats on. Back in the day we would hollow the cats for better performance, but I don't think that really matters today. I don't know about the velocity of the gases or anything remotely close to that lol, so sorry, I can't answer that. As you said too, my rear O2's are tuned out. I think my front O2's are before my collectors, but it's been awhile since I've been under there. Do you have emissions in your state?

 

[kbuskill]

Well, this is already all beyond me--but--in my case I wanted to run cats because NJ has a visual for inspection, plus I'm getting older and I didn't want to deal with the fumes--which I have to because I don't have my cats on. Back in the day we would hollow the cats for better performance, but I don't think that really matters today. I don't know about the velocity of the gases or anything remotely close to that lol, so sorry, I can't answer that. As you said too, my rear O2's are tuned out. I think my front O2's are before my collectors, but it's been awhile since I've been under there. Do you have emissions in your state?

Not any more.

 

[randeez]

i think what youre confusing scavenging with is known as "reversion"... the exhaust valve stays open long enough for the piston to start traveling back down creating vacuum. the shortys will help but i think with VVT its somewhat by design for GM as a replacement of an EGR system.

with all that said, gutting the cats and placing the o2s in the 2nd hole shouldnt change much, they would probably still be closer than they would be on long tubes (in the collectors) I dont think it would make much difference though.

 

[swathdiver]

I think the argument that gutting the cats does not offer a performance increase is bogus. I don't think they'll cause any scavenging problems like you mention but the whole thing would flow better if the honeycomb was gone. That being said, all engines need some backpressure and in my racing days saw cars fall flat on their face because the conventional wisdom (which was wrong) said to use 2.5"-3" duals. I ran my sleeper with the stock exhaust and still made over 400 rear wheel horsepower (Buick GN).

The only benefit I could see with moving the O2 sensors is if the pipe is larger on the downstream side of the cats than before. Might be good for 1 horsepower! You could test all that out with your Torque Pro app.

I say leave the cats and make the runs and do the things you can easily/inexpensively undo. You can't undo a gutted cat or catless Y-pipe with spending some money.

 

[Rocket Man]

I’m running cats because we have visual inspections but I would have gutted the cats if they weren’t welded in where I couldn’t really get to them. My downstream sensors are tuned out. I do know in mine the wire connectors are different between down and upstream so keep that in mind.

 

[kbuskill]

i think what youre confusing scavenging with is known as "reversion"... the exhaust valve stays open long enough for the piston to start traveling back down creating vacuum. the shortys will help but i think with VVT its somewhat by design for GM as a replacement of an EGR system.

with all that said, gutting the cats and placing the o2s in the 2nd hole shouldnt change much, they would probably still be closer than they would be on long tubes (in the collectors) I dont think it would make much difference though.

I understand what your saying about reversion but I am referring to the scavenging theory that the exhaust pulses going into the collector create a vacuum effect on the other primary tubes of the header.

Also my truck is an '08 so no VVT on my truck.

Thanks for responding... I didn't really think moving the O2 sensors back would make any appreciable difference but since the idea popped into my head I figured I would pick everyone else's brain to see what you all thought.

 

[kbuskill]

I think the argument that gutting the cats does not offer a performance increase is bogus. I don't think they'll cause any scavenging problems like you mention but the whole thing would flow better if the honeycomb was gone. That being said, all engines need some backpressure and in my racing days saw cars fall flat on their face because the conventional wisdom (which was wrong) said to use 2.5"-3" duals. I ran my sleeper with the stock exhaust and still made over 400 rear wheel horsepower (Buick GN).

The only benefit I could see with moving the O2 sensors is if the pipe is larger on the downstream side of the cats than before. Might be good for 1 horsepower! You could test all that out with your Torque Pro app.

I say leave the cats and make the runs and do the things you can easily/inexpensively undo. You can't undo a gutted cat or catless Y-pipe with spending some money.

I have also considered the possibility of losing too much back pressure. I would rather lose a few HP up top than to lose any TQ down low where I am trying to gain as much as possible.

These shorties are 1 5/8" primary tubes instead of the 1 3/4" or 1 7/8" primary tube headers so they should be better for keeping a little more back pressure and not losing the bottom end I hope.

 

[kbuskill]

I’m running cats because we have visual inspections but I would have gutted the cats if they weren’t welded in where I couldn’t really get to them. My downstream sensors are tuned out. I do know in mine the wire connectors are different between down and upstream so keep that in mind.

We don't have any inspections here any more so I am seriously considering cutting the cats out and having pipe welded in and just keeping the cats in case I ever decide to put it all back to stock.

When I was talking about moving the O2 sensors back, I was talking about getting extension leads to extend the forward O2 harnesses back to where the rears are. I had no intention of trying to plug the upstream O2s into the downstream wiring.

If I do end up removing the cats then I will have Lew tune out the rear sensors.