Bigger cats = more flow but will it throw CEL?

I did some measuring on the exhaust a while back and found a few spots on the pipes between the cats and resonator that are crimped down to less than 2", so I figured running 2.5" pipe to get rid of those restrictions should improve the exhaust flow. I'm also thinking that swapping out the V6 cats for the larger GT cats might take advantage of any increased flow from the smoother/bigger pipes.

Thanks to the helpful peeps in the 5.0 forum I have some measurements for the V8 cats to compare to our v6 version. I also have a lead on possibly getting a set of GT cats for a good price, so now my next step is to figure out what an exhaust shop will charge to weld everything together and if my plan is actually worth the effort.

I know the GT cats are roughly 2" bigger in diameter, but I also found out they are 1" shorter, which got me wondering about O2 sensor placement. Just how critical is it to avoid getting a cel, does the sensor in the cat have to be precisely the same distance from the top O2 sensor or will the computer be OK with it if the distance changes by an inch or so from the stock location?

My goal would be for it to function without requiring a tune to eliminate a CEL, so if I ever need to get the car serviced I can return the tune to stock and it'll still run fine without throwing codes.

I haven't heard of anyone else trying GT cats on a V6 before, so I don't know if the 2" bigger size is enough of a difference in flow to matter, or what other obstacles might come up. In my research I haven't seen anyone post before and after dyno numbers or time slips for cat-delete pipes or hi-flow cats, so I'm not even sure how much potential gain there is in this part of the exhaust system, and I have my reasons for not going with those other options but I welcome your input and if you have any suggestions for the sensor issue or know of anyone who has already tried this please chime in :ford:

There's a sticky thread at the top of the forum. The GT catbacks literally swap right over to the V6 and don't cause any kind of CEL.

Not sure on any gains, but it does sound a bit better for mine.

I'm going to keep an eye on this

Nagare said:

There's a sticky thread at the top of the forum. The GT catbacks literally swap right over to the V6 and don't cause any kind of CEL.

Not sure on any gains, but it does sound a bit better for mine.

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I already have the GT cat-back and even posted my opinion of that swap in the sticky you referred to, however my questions are about swapping the GT catalytic converters (aka "cats"), which definitely are not a simple bolt-on, but something I think might be a beneficial swap for the same reason the GT cat-back swap is popular, the used take-off cats can be found fairly cheap. I would never consider doing this if it meant buying new cats at retail price!

One of the two GT cats is one piece with the header. The other cat is in the mid pipe.

There are other options. For example, I have a 2.5" cat back exhaust and I got the lethal performance 2.5" mid-pipe and had a 2.5" cat welded in. No CEL problems.

Another option is the solo performance 2.5" catted mid pipe. They are bolt on - and should work with your GT catback exhaust. Found here: https://solo-performance.com/ultra-high-flow-converter-assemblyv6-ford-mustang-2015-present

These options are not CARB compliant so they would not work in CARB states. But, they do pass emissions and keep the CEL off.

Rick#7 said:

I did some measuring on the exhaust a while back and found a few spots on the pipes between the cats and resonator that are crimped down to less than 2", so I figured running 2.5" pipe to get rid of those restrictions should improve the exhaust flow. I'm also thinking that swapping out the V6 cats for the larger GT cats might take advantage of any increased flow from the smoother/bigger pipes.

Thanks to the helpful peeps in the 5.0 forum I have some measurements for the V8 cats to compare to our v6 version. I also have a lead on possibly getting a set of GT cats for a good price, so now my next step is to figure out what an exhaust shop will charge to weld everything together and if my plan is actually worth the effort.

I know the GT cats are roughly 2" bigger in diameter, but I also found out they are 1" shorter, which got me wondering about O2 sensor placement. Just how critical is it to avoid getting a cel, does the sensor in the cat have to be precisely the same distance from the top O2 sensor or will the computer be OK with it if the distance changes by an inch or so from the stock location?

My goal would be for it to function without requiring a tune to eliminate a CEL, so if I ever need to get the car serviced I can return the tune to stock and it'll still run fine without throwing codes.

I haven't heard of anyone else trying GT cats on a V6 before, so I don't know if the 2" bigger size is enough of a difference in flow to matter, or what other obstacles might come up. In my research I haven't seen anyone post before and after dyno numbers or time slips for cat-delete pipes or hi-flow cats, so I'm not even sure how much potential gain there is in this part of the exhaust system, and I have my reasons for not going with those other options but I welcome your input and if you have any suggestions for the sensor issue or know of anyone who has already tried this please chime in :ford:

Click to expand...

As long as the top O2 is in the correct place the second one does not matter. The downstreams only measure catalyst efficiency. Of course all of this assumes the larger but shorter cats still do their job( no reason they wouldn't)

However like I said in the other post you will not likely see any gains outside of placebo. a cat is a cat is a cat. short of removing them for straight pipe there is no appreciable gains to be made.

NedMcMahon said:

One of the two GT cats is one piece with the header. The other cat is in the mid pipe.

There are other options. For example, I have a 2.5" cat back exhaust and I got the lethal performance 2.5" mid-pipe and had a 2.5" cat welded in. No CEL problems.

Another option is the solo performance 2.5" catted mid pipe. They are bolt on - and should work with your GT catback exhaust. Found here: https://solo-performance.com/ultra-high-flow-converter-assemblyv6-ford-mustang-2015-present

These options are not CARB compliant so they would not work in CARB states. But, they do pass emissions and keep the CEL off.

Click to expand...

I'm aware of the drivers side cat being part of the header, since both cats need to be cut off the GT pipes/flange and welded onto 2.5" pipe to do what I'm planning, that's not really an issue.

I don't want to go catless, besides a bit of an environmental conscience one of my goals I stated earlier is for this mod to work without needing a tune to eliminate CEL's so if I ever need to take the car in for service I can return the tune to stock without any issues. AFAIK, cat-delete pipes can't do that without throwing a CEL.

The Solo pipes are kind of the idea I was going for only on a cheaper budget than the nearly $700 they want, i.e. 2.5" pipe combined with cats that flow more than the oem V6 parts for an overall increase in exhaust flow potential. What you did with the Lethal Performance pipes is the same as what I'm doing only I'm attempting it using factory GT cats. I actually already have a set of the Lethal pipes I found cheap enough on Craigslist. Since you already have a 2.5" cat-back, did you notice any difference with the catted Lethal pipes? I would think the less restriction there is down stream the more beneficial this mod would be.

As I mentioned in my original post, I have my reasons for not wanting after-market cats. I won't deny they could flow more than the factory cats, however the oem has much more than just exhaust flow to consider, they build their cats to meet federal requirements for reducing emissions AND being able to maintain a significant percentage of that reduction over the projected serviceable life of the car (which I believe is currently 15 years but I'm not certain on the exact length of time). This is the reason oem cats are so much bigger than the "hi-flow" aftermarket versions. The aftermarket build their catalysts with fewer but larger, freer flowing cells. With fewer, larger cells, there is less surface area for the exhaust to come into contact with. Less surface area means less catalyst (which by the way is made up of very expensive precious metals), so the hi-flow design of fewer cells but larger cells works very well for the aftermarket since it not only increases flow through the part it lowers the cost to manufacture it. These hi-flow cats obviously still make a significant reduction in emissions, but their capacity to keep doing that over time is much less than the oem cats.

I'm just not prepared to spend several hundred dollars replacing factory parts with aftermarket versions that I suspect won't do the same job for the same length of time as the originals, especially when the performance gained from doing it is not a well established fact. Have you noticed that none of the sellers of cat-deletes or hi-flow catted replacement pipes have offered any dyno numbers or shown any kind of testing to support the idea that their parts actually improve power? I knew going into this that I might not see more than a few HP from trying this mod, but I figure that's why you don't see any performance numbers for these options, they'd have a hard time getting people to spend $600+ for something that only produces 5-10 HP, and I'd be really surprised if the performance gain is actually more than that!

Jay-rod427 said:

As long as the top O2 is in the correct place the second one does not matter. The downstreams only measure catalyst efficiency. Of course all of this assumes the larger but shorter cats still do their job( no reason they wouldn't)

However like I said in the other post you will not likely see any gains outside of placebo. a cat is a cat is a cat. short of removing them for straight pipe there is no appreciable gains to be made.

Click to expand...

I appreciate your incite on the O2 sensors, that will help me move forward with this project but I respectfully disagree with "a cat is a cat is a cat".

I firmly believe oem cats have come a long way from the power robbing bricks they were years ago, but everything in designing a mass produced car is a compromise, including the cats. While balancing the production cost with the fed mpg and EPA requirements they still need to meet the airflow needs of the engine, so I have no doubt they make the cats for each model vehicle as small as will be functional for meeting all of these demands.

To continue my thoughts from my post just above, my opinion is that the size of the factory installed cat is purposely made very marginal to keep costs down, and by marginal I mean that it is sufficient for the air flow needs of a 300 HP engine while not hindering the fuel efficiency, but is probably a restriction none the less, and the more air flow you try to push through the engine the more of a restriction it will be. Likewise, the 2" larger diameter of the GT cats was deemed necessary by someone in Ford to support the 435 HP of that engine, so I have no doubt it has a higher flow capacity than the V6 cats and likely flows more than the V6 needs for anything short of adding a procharger, the question is how much is there really to be gained with the increased flow potential at this point in the exhaust system and is it worth the cost/effort to try this for the potential performance increase?

I'll readily admit there probably isn't a lot of HP to be had on a stock engine with this mod, which is why I'm only considering doing it on a very low budget and would never spend the hundreds of dollars it would take to replace oem cats with aftermarket parts, but if I'm at all accurate in thinking the oem cats are marginal at the stock power level I have to believe there is at least a few HP to be gained at the peak and probably more in the mid-range. If nothing else, I'm pretty confident it will have a nice effect on the exhaust sound, and there's lots of people who spend $1000+ on exhaust systems for nothing more than a change in the sound!

You asked if I noticed any difference with the catted Lethal pipes...

I can tell you that the catted mid pipe changed the tone of the exhaust, to a deeper sound. I also like the feel of the pull now, but can't nail that down to any particular component. Basically, your question is tough to answer because of the timing of my mods and the fact that I am still getting it dialed in with a datalog tune from MPT.

Other mods include: Roush CAI with a modified clean air tube expanded out to 80mm at the throttle body. GT 80mm throttle body. Shapeways adapter - ported out to 80mm. Ported upper and lower intake. BBK shorty headers. And the custom 2.5" catted midpipe and the Hooker 2.5" catback.

Short of nitrous/methane/forced induction/or engine build, I think I am as far as I can go.

I will be doing the datalogging over the next week and hope to have the set up dialed in. I may go try a dyno tune, but I understand that some of those places are much less experienced with this particular engine then someone like MPT such that you are 90% there with MPT with low risk.