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Discussion Starter #1
This is a follow-up to the question I posted last year: https://www.lotustalk.com/forums/f154/power-loss-after-de-cat-460722/
Since then I have done some work and clarified a few unceranties.
Now looking for advice on how to move forward.

Last Summer my mostly stock 08 Elise SC had a minor fire in the trunk due to exhaust heat.
To prevent this from happening again I gutted the cat.

That caused CEL throwing P0420, which always came back shortly after I clear it.
But more importantly, I was down on power.
The car feels fine but I am losing 8-10 mph on the straight.

I had the engine inspected, and replaced belts, pooleys, spark plugs.
Replaced post-cat O2 sensor
Checked for loose pieces that could be left over in the cat
Nothing helped.

Question: will P0420 cause the ECU to adjust in a way that would explain power loss?
If so - perhaps all I need is to figure out how to suppress it? What would be a good way to do this? Or is it rather that both are caused by the same signal but are independent of each other?
BTW, the ECU is also throwing P0630 (missing VIN) but I don't expect that to matter. Right?

One of the things I am considering is replacing the cored cat with a straight pipe.
I would imagine that may help only as long as the problem is specifically with my gutted cat, rather than "something other than stock".
Another option is a "sport cat". Will I still have the issue with heat? I sometimes run 75 min sessions.
Thought about a tune but didn't find anything for MP45

Thoughts?
Suggestions?
 

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Cored cat is the issue, but you should also retune.

I had this same issue, besides the fire. mr2 turbo with a turbo upgrade. gutted the cat... went slower at the 1/4 mile track. 104mph (cat) vs 101mph (gutted).
Then I had a 2.5" straight down pipe made. 109mph! Open downpipe!

The gutted cats create a huge restriction (turbulence).
 

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Yes, the straight pipe should fix your issue. The CEL does not affect the tune... does not go into lower power mode.

I just ordered this one, looks like a nice quality unit.

https://www.monkeywrenchracing.com/product/mwr-cat-delete-pipe-05-lotus-elise-exige-2-eleven-2zz/

They also claim it prevents CEL. Unique downstream O2 sensor fitting means no annoying check engine light even without a catalytic converter.

Somehow I doubt it. You can get a fouler and add it to the downstream sensor.
 

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Discussion Starter #6
I am also curious - what's the benefit of suppressing CEL?
For inspection purposes you can always just clear the code....
 

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Thanks.
Why did you pick it over BWR's?
My car is mostly track but I would like to keep it street derivable. In light of that what would you recommend?
Any thoughts on 2bular "Silenced De-Cat Pipe"?
https://www.2bular.co.uk/elise-sc-2zz-ge
I'm hoping their claim of no CEL will work, which is why I chose it. I also like the long initial pipe, which acts like a long tube header, and then increases velocity just before the second sensor.

I like the 2bular, just didn't want to wait 6 months.
 

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Not sure how much louder the cored cat would be, however I did get the pipe installed tonight.

Very nice quality, and it came with a 90 degree fouler for the second sensor.

I did have a small issue, the end that slips into the muffler was a bit too tight. I tried reducing the outer diameter by grinding it, but got no where fast. I just ended up cutting about an inch off the pipe, and then grinding down the end to make it slip in easier.

I still couldn't get the hagar111 exhaust centered... the pipe still would not move in enough. I just put the Arqray back on.

It did sound much louder when doing a cold start, but driving around town was only slightly louder. I did however scare my GF getting on a freeway on ramp. She said it felt WAY faster than before. Car revs so much easier. Very happy with this mod! CEL free, but we only drove about 15 miles.
 

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You can take your decat or any other exhaust tubing to any muffler shop and they can reduce or expand the end so it slips on properly.
 

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A tip: make sure there are NO leaks in your exhaust system when you put it back together. I was once really shocked at how a super small leak caused loss of power and CELs. My others cars would not have cared. ;) That being said removing the back pressure of the cat will cause you to lose some low end power. Some can tell (I could) some can not... its not massive.
 

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Discussion Starter #13
I have an update.
Got the de-cat pipe from BWR and O2 sensor emulator. Have not installed it yet.
I also got the OBDII tool to clear codes.

The consensus opinion seems to be that P0240 from the post-cat sensor should not matter at all. Sould not affect the maps and performance.
I have some evidence to the contrary from the last track day.
I found that if I clear the code before each session (it doesn't take long before it comes back), I have higher top speed on the straight, close to what it was before I cored the cat.

Based on that it looks like the fault code from the post-cat O2 sensor does matter.
 

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Maybe someone like Turbophil can chime in.

On a side note, i have a few hundred miles on my MWR Decat. No CELs so far.
 

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A tip: make sure there are NO leaks in your exhaust system when you put it back together. I was once really shocked at how a super small leak caused loss of power and CELs. My others cars would not have cared. ;) That being said removing the back pressure of the cat will cause you to lose some low end power. Some can tell (I could) some can not... its not massive.
This! If you want driveability problems, have any exhaust leak between the exhaust ports and either of the oxygen sensors. Further, you want to make sure there's no leak downstream of an oxygen sensor for a foot or so. I don't want to go into historical detail, but trust me that there's personal pain associated with this lesson.

Do whatever you have to do to make those leaks go away or you will be miserable.
 
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