Recently installed DEC 400 cell cats on my 4.2 XKR. Didn't see any reviews on these so I was not sure how they would sound or perform. You can skip to the bottom for the actual review.

Also, never took off the exhaust so Ill give some pointers here. I made it a 1-man job on jack stands, but you may want to get someone to help you for a few minutes when re-positioning the exhaust.

This was not a conventional exhaust removal, I knew the CATS were clamped to the exhaust real tight, and judging by the rust on them, I would be laying there hammering them out for hours. So I decided to try and remove the entire exhaust, with the CATS all in 1 shot.

Before you start, spray everything with WD-40, bolts, hangers, O2 sensors, clamps. Makes things a lot easier. Avoid spraying electrical harnesses and connectors.

First I broke the 4 bolts holding the CATS to the exhaust headers. Wanted to make sure I could actually get to them. At this point, you can try to remove the O2 sensors. I left them on since I didn't want to risk stripping them. Also, I deemed the driver side downstream sensor impossible to remove. Which sucks since that's the one that gets in the way when removing everything. Disconnect all 4 O2 electrical sensors and move them out of the way.


You will also want to move this ground cable out of the way. Remove the smaller bolt, loosen the larger bolt and swing it out of the way. Both bolts are on there super tight so get comfy.

Next you can get a jack under the exhaust for support. This will allow you to slide the entire exhaust backwards along the jack wheels. I ended up jacking it on the 2nd muffler, not in the location in the pic.


Following the procedure in the manual, remove the 4 bolts and center support brace. Remove the 2 bolts and center bracket/hanger. Then you can disconnect the 2 middle hangers, leaving only the 2 rear hangers in place.

Disconnect the 2 vacuum lines on each side of the rear muffler. Twist and pull up on the white plastic elbow and they should come out easily.


One at a time, I disconnected the rear hanger and supported the exhaust with bungy cords. Wrap them around twice for extra support. This will give you flexibility to move the exhaust around however you need to.
Be sure your jack is still supporting the exhaust, otherwise you will cause unwanted stress on the still attached CATS. The pic below shows them suspended but I had already removed the CATS, that's why its so low.


Now you can remove the 4 bolts holding the CATS. If you removed the impossible O2 sensor, they should slide down and out. Unfortunately I could not remove the driver side one due to that 1 O2 sensor. Since I was replacing the CATS, I said screw it and just cut it out. Once that 1 was out, the other CAT and exhaust slides down and out. I got lazy (and tired) so I just cut the other one out too. But as you can see it can come out still attached to the exhaust.


You can re-position the exhaust, unclamp, and bang out the pipes still attached. 5-6 hits with the hammer got them out fairly easily.


Here are the old and new CATS (painted, of course)




O2 sensors were left to soak overnight in gasoline (per Google). DO NOT soak the electrical connectors or sheathing in the gasoline, they will get chewed up. Clean the connectors with a can of electrical cleaner.
Inspect the wires at this point also. One of my wires was chaffed so I cleaned it and taped it up tight.


Re-assembly is straight forward. Install the O2 sensors to the CATS, then install the CATS, but do not fully tighten the bolts. Tighten them enough so they are supported, but also give you some wiggle room for the exhaust.

Re-position and lift the exhaust, attach it to the CATS, but do not clamp just yet. Attach the 2 rear hangers, then the 2 mid hangers. Now you can adjust how far in or out the exhaust sits. Once you find a good position, tighten the clamps to secure the CATS. Re-install everything else, clean the electrical connectors and you're all set.

Only issue I ran into: one of the CATS was loose when reinstalling. Seems the pipe diameter was a bit too small for the exhaust, enough for it to bang around a bit. I snapped the clamp bolt trying to secure it. Drove around like that for a few days before I came to a solution. I sealed the gap with some exhaust RTV, and bought one of those twin bolt horseshoe clamps. One end fit perfectly around the lip of the exhaust pipe while the other end fit well around the CAT pipe. Now its nice and secure with no exhaust leak or rattle.

When I first started driving, I got a bunch of codes thrown. Short/long term fuel trims were crazy, like +30 on each bank. I figured it was just the ECU re-learning and/or the catalyst monitor not ready. Cleared the codes, took a 200 mile trip and they have not reappeared since.



As for a review ... at first the sound was very raspy and loud, then I realized it was due to the exhaust leak from the loose pipe/clamp. But now it sounds great! Under 2500 rpm, they still retain the deep Jag growl, but louder. But above 2500 rpm they are noticeably louder and aggressive. Not over the top, but enough to make a noticeable difference. Very satisfied with the CATS. Note that I am running a smaller S/C pulley on this also.

I'll try to get a pic of the clamp setup, as well as a sound bit. When I have time I may cut open the old CATS to see what they look like.

 

Did you notice any decrease in MPG initially? I installed 200 cell cats and got the CEL and got hit with a decreased MPG. Will do ECU tune this weekend.

 

Yes i did at first. Super high fuel trims and maybe 18 mpg highway. But everything is good now, just had to drive it out. Im back at 26 mpg highway.

 

I’m down to 23 mpg hwy. my check engine light is still on.

 

Good job On the effort! There is a good amount of hp to be had with high flow cat install, but only with the proper tune to compensate for the additional exhaust flow, otherwise you are just wasting money because on our cars you will loose plenty of torque down low and your car will be slower, been there done that ,plus most likely you will get a cchek engine light. I can help you with the tune which will add the power back plus plenty extra and remove the cel light . pm me.

 

Will take care of your light with the tune tomorrow

 

Wanted to update this with a better review...

I've had a pretty annoying exhaust leak since installing these. At first the manifold to flange nuts were a bit loose, but a few weeks later it came back. I avoided breathing in fumes by using the re-circulating air function on the AC/Heat. Coupled with my P420, it was time for a new set.

Stupid, I know, but I stand by my original statement that replacing these things 4 times is still cheaper than buying an OEM set. So lets say I got about 30,000 miles out of them.

When I got them in the mail I took a closer look at the flange. It turns out there is a seam there that runs the length of the flange. Clearly this will cause some fitment issues, but you get what you pay for.

I decided to grind it down with a dremel and its been a good fit so far. No more exhaust leak, and with the Xpipe I installed, it sounds great! Also replaced all the O2 sensors with new ones and now my fuel trims are excellent.