MikeGLS450

So as a spring project I’m considering an exhaust project. Choice of:

-resonator delete (keep OEM cats)
OR
-catted downpipes

Ive looked at Redstar and RENNtech. Redstar gives a choice of HJS (supposed to be the best aftermarket cats) or GESI (EPA approved). Multiple calls to Redstar and they insist those cats don’t throw a cel but I’ve read on this forum there are some cases where this has in fact happened. What’s more, digging into Redstars FAQ and they admit it’s possible.

Now RENNtech insist there downpipes also won’t throw a cel. They won’t say what cats they used but one of their salespeople told me they are German (has to be HJS). I also read a post where a forum member confirms that.

One tuner said to ask ask the manufacturer if their downpipes will throw a cell with the stock OEM tune. Again both manufacturers say NO.

So what to do?

The easiest/cheapest/safest thing to do is just a resonator delete. Great sound and no cel. But I like to tinker. And I’ve seen pictures of the inside of those primary cats. There has to be even a little bit of horsepower there. Anyone care to opine?

AMG 17GT

Try the delete first and go from there. I think it’s ridiculous How much these companies charge $$$$ for a pipe.

LTHL VPR

What is your objective?
Are you trying to not throw a CEL? I think there are many proven ways to do this with or without cats.
Do you want to pass emissions? In this case, you will CARB approved cats--> depending on the year of your car, you may need GESi2 cats (not GESi1). That should work in 48 of the states...
Do you want the more aggressive sound? Straight pipes
Are you looking for the cheapest? Then just cut out resonators and existing cats and replace with pipes (or at least the cats which would be the easiest to cut and replace). Any competent exhaust shop can cut out the cats and replace).

If it were me.... I'd find an option that does mandrel bends (look factory), high quality stainless and use GESi2 cats. This car deserves this, but to each his/her own...

dav461

What stops you to install catted dp's and in case if CEL pops up, you just need to lower emission standart to EURO2 for example, good tuning company will be able to do it.
cat error light depends on density of new cats (200/300 cels) and which EURO standart they are. car stock is euro6

Acta_Non_Verba

Hi Mike,

I quoted you above and highlighted a few places. Last, first - Completely eliminating the primary CATS (you only want to replace them with aftermarkets) is the second worst mod you can make if you consider $ spent for HP gained. This has been discussed over and over, at least two/three times a year since I've been on here. With a stock setup you gain about 5 to 8 HP (depending on the day and environmental conditions). With an aftermarket CAT, probably a split of the 5 HP or NIL. BTW, the worst mod on this platform, hands down, is an aftermarket intake, Every aftermarket I've seen is either zero HP gain or can actually take you backwards. We really beat this topic DEAD on another thread.

OK, my choice would be your first - Resonator delete (do you mean Secondary CATS?), keep the factory front CATS. I would do it strictly as a matter of making more noise, not a power getter.

So, if you choose that direction the rest of this won't matter, but if you do tinker with the front CATS, the "CEL" is your question. There is more than one way to "skin this CAT" either inside the tune or outside. So, let's do the tune first. This is what a tuner would do if you asked them to eliminate the CEL on a catless downtube:

Depending on the hack SW used - Engine diagnostics > DCTs > P0420 and P0430 > unckeck the two boxes on the right > done.

By the tuner unchecking these two boxes, the EMS still does a startup diag on the rear sensors (which some mfgrs do and some don't) but the ECU is instructed to no longer report a catalyst below threshold. The ECU only periodically does this as a checkpoint anyway. On some platforms I've seen only every 50 to 100 miles.

Another way to eliminate a CEL with a primary CAT delete, and so easy, is to use a O2 sensor extender on the back two sensors.

Like this:


Or like this if room is an issue:


On the GT platform, the first extender would probably work as the back sensors are visible and on an angle form above. So the straight extender that pulls the O2 sensor out of the exhaust stream. Remember what it is doing. It is measuring and reacting to the unburned fuel left in the exhaust after passing through the front CATS (If they're there).

Bad pic, but here is the leftside rear O2 sensor:



So, the problem with the tune option for eliminating the CEL, California will catch it in an emissions inspection, but they're also going to probably catch the resonator delete (second CAT, if that's what you're talking about). The MIGHT miss the O2 sensor extender (at least the straight one), that is if you have front CATS. If not, they're REALLY catch you. Note on the hack SW image, even they are now publishing a warning on the bottom NOT to make changes. In other words, the tuner is on their own.

Hope this helps you with your decision,

Acta

MikeGLS450

Thanks.

Removing the resonators is an easy first choice. Checks off the box for sound and cost. Based on some picks of the inside of those things it to be worth some horsepower. If you told me 10hp I’d believe it.

As for the primary cats I’d prefer to avoid a cel in the first place. It’s my understanding (correct me if I’m wrong) Redstar and Renntech come with the best cats. Redstar comes with a choice of HJS (supposedly the best? Euro6) or GESI Gen2 (EPA compliant). Renntech supposedly (based on the comment that one of their salespeople said they come from Germany) comes with HJS.

Now both companies state that they DONT throw a cel. Again although I’ve read posts were their are instances (1 each for HJS and GESI) where they did.

Seems the prudent approach would be that if I did add the catted downpipes to do it without a tune first and see what happens? I bring this up because if I had to switch back to the OEM tune for whatever reason I’d hate for the downpipes to through a cel and the dealer give me a hard time. What’s the solution for that? An OBD2 scanner like Carly??? I know an O2 spacer or checking off some boxes in the tune is a possibility. Trying to accomplish this without all that.

As for horsepower on the primary cats, again surely a 300 or 400 cell high flow cat is worth a bit more power? 5? 10? I tell my son singles and doubles all day long. Pretty soon your talking real numbers…JMHO.