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Discussion Starter #1
Question for those of you who have removed the catalytic converter on your Elise/Exige. I have a 2005, NA (non-SC) and I would like to de-cat it. This decreases the back pressure quite a bit so is an ECU tune absolutely required to not burn up the exhaust valves? Or does a cat-delete not cause the car to run that much leaner? Just trying to play it safe.

If you say "probably ok" then I'll go ahead and do it, weld in an extra O2 sensor bung and just log lambda for a bit to make sure I'm ok. If you say probably not, then I won't even go that far and I'll start poking around for software to tune my ECU. I have access to a nice dyno for a day and experience with ECU calibrating, but I don't have software for the ECU.

Thanks a lot,

Keith
 

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I would definately get one of the tunes friendly to de-cat, and purchase a CEL eliminator. My car is De-Catted at this time, but it is supercharged too. Reasons you will want a de-cat friendly tune:
1. Engine will likely run leaner.
2. No effective HP gain without a tune.
 

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I would definately get one of the tunes friendly to de-cat, and purchase a CEL eliminator. My car is De-Catted at this time, but it is supercharged too. Reasons you will want a de-cat friendly tune:
1. Engine will likely run leaner.
2. No effective HP gain without a tune.

I know I have commented on similar questions in the past but ill chime in a bit here to clear up some misconceptions about this topic…

1. Backpressure is a myth when related to exhaust flow/performance… the actual term you are referring to is the effect of scavenging which is a much more accurate way in explaining performance directly related to exhuast flow
2. There are no performance disadvantages of running no cat… however not having one will enhance emissions (which will pose an issue if you reside in a state/county that tests this)
3. A tune may help, but not getting one won’t necessarily hurt… many other vehicles ECU’s tend to go into ‘safe mode’ when an 02 sensor doesn’t get correct readings if a cat is bypassed (which will normally cause an engine to run rich, not lean)… running rich is common in ‘safe/limp mode’ so you are not jeopardizing burning rings, which will happen if an engine runs too lean/hot, however running too rich can also cause a decrease in power since it drastically changes the air/fuel ratio (this will also hinder your fuel economy)… but from what I have read on the lotus specifically, the ECU doesn’t trigger any ‘safe mode’ when the 2nd o2 sensor throws a code, & only triggers a check engine light if you are unable to trick the sensor
4. Sound levels will naturally increase due to the lack of exhaust flow restriction & you may smell more unburnt fuel in the cabin when driving around (you may also see a slight decrease in fuel economy)…

In the end, running a de-cat/test pipe is not bad for your engine or performance as long as you are aware of the changes that come with this type of modification
 

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Driftwood,
I agree on scavenging versus back pressure (I just thought it best to leave that sleeping dog alone). I will not speak to reversion issues either, but the cat does act as a significant "reflection" point, dyno charts show it, top tuners work around it.
On supercharged engines there is a benefit in de-cat, NA engines very little gain.
Header selection makes a big difference in scavenging advantages with a de-cat, I can't speak to the stock headers characteristics with a de-cat. Also agree on second oxygen sensor after Cat, it is there to confirm the presence of a cat, but has no input on the actual lean/rich tuning characteristics.
 

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I just installed a Larini de-cat on my 2006 Elise with Larini Sport Exhaust, and I am very impressed. The car finally sounds as aggressive as it looks, throttle response is greatly increased, the engine runs cooler, it spits fire occasionally, and it even smells better with an amazing aphrodisiac-like exhaust fume. The difference for me was like night and day and I haven't even got the Vision Function reflash yet. Unfortunately after the de-cat installation I got the CEL warning and my shift indicator light has stopped working, but hopefully a CEL Eliminator will correct this. But otherwise the de-cat is a big bang for your buck and I would highly recommend this upgrade to any Elise/Exige owner.

Note- Slight modification to the Larini de-cat was necessary in order to achieve proper fitment to the Larini Exhaust with a stock header configuration.
 

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Driftwood,
I agree on scavenging versus back pressure (I just thought it best to leave that sleeping dog alone). I will not speak to reversion issues either, but the cat does act as a significant "reflection" point, dyno charts show it, top tuners work around it.
On supercharged engines there is a benefit in de-cat, NA engines very little gain.
Header selection makes a big difference in scavenging advantages with a de-cat, I can't speak to the stock headers characteristics with a de-cat. Also agree on second oxygen sensor after Cat, it is there to confirm the presence of a cat, but has no input on the actual lean/rich tuning characteristics.
oh I gotcha, my response wasn’t completely directed toward yours... more or less a feed off of what you had already mentioned

& I totally get the back pressure debate which is a whole other topic by itself :crazyeyes

I’m certainly a big advocate of not running a cat on performance oriented cars... the advantages simply outweigh the disadvantages (regardless of N/A or forced reduction. less restriction & wieght savings is always a good thing for performance)…

& to the hippies… for the thousands of people not running cats on cars probably causes less than .000000000001% of our environments issues. Just sayin
 

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Discussion Starter #7
I was under the impression talking about back pressure instead of scavenging is the same thing just backwards because they are inversely related. Back pressure is just a more common term to throw around. Scavenging comes from the time both sets of valves open and the exhaust pulls the fresh charge into the chamber. How could you maximize scavenging without reducing back pressure? Our cars aren't cammed that hot, even on the high end, so I assume the power gain from reducing the back pressure (increasing scavenging) is minimal.

As for the O2 sensor not working. I have worked with cars in the past where this throws the ECU into open loop (no feed back from the O2 sensor) which causes it to run rich to keep things cool. That doesn't hurt the engine and generally it isn't rich enough to cause power loss.

At any rate, I thoroughly agree without an ECU tune it's just a waste of time other than making a racket. I'm just trying to hold off an an ECU tune until I put together an intake as well (just breaking up the cost).

Thanks for your input though, keep the discussion going :)

Keith

p.s. I've done engine building and ECU calibrating before, I just don't know the capability of the stock ECU.
 

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I cannot recommend highly enough against removing the catcon. First, unless you get a proper reflash, it will throw a check engine light and in many jurisdictions you won't be able to get it inspected. From a sound and performance perspective, just use a good metal matrix cat - I use these and the technology has advanced to such a point that there really is no meaningful performance loss with this kind of cat. I run JUST a metal matrix cat on my turbo car and the sound and performance are excellent. Finally, and I know it sounds preachy, but for a street car what makes your desires for sound and maybe some tiny bit of power more important than everyone else's (who has catcons) and the environment?

Now if it is a track only car, then I agree, the cat should go in part because the constant non-stoichiometric A/F ratio I think is hard on a cat, but on the street you are running stoichiometric the vast majority of the time and should use a cat.

Finally, if you do it, do it right. A constant CEL may well cause the computer to actually reduce performance. I don't know all the internal algorithms, but I do know in many cases an illumitaed check engine light causes changes in fuel and spark timing designed to protect the engine by reducing power, and a failed cat can cause a fire, so I woudl not be surprised to see that the ECU makes calibration changes when it detects the secondary O2 sensor not giving a proper signal.
 

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Discussion Starter #10
I drive the car on the road a slight majority of the time. Most of the time this summer will be driving the car to track days and autocrosses. I understand cars without cats are very loud and smelly. This wont be my first one. I like the smell so that's fine. The sound is going to take some getting used to I'm sure, considering it is already kind of a loud car and it's going to get worse.

Thanks for the help guys, I really appreciate it. I'm still so new to the Lotus community and Lotus cars that this forum with all the expertise flying around, makes life MUCH easier.

Keith
 

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Let me be emphatic, the second O2 sensor is there only to verify you have a functioning cat on the elise/exige. It has zero value to your air to fuel ratio. It will not throw you into limp mode on an elise/exige. Most people add a CEL eliminator so they can see their shift light and to know if something is really wrong with their engine (when the CEL comes on for a critical reason). Racing or hi-flow cats sometimes throw a CEL with some cars and tunes. For those who can get away with a racing cat, I say more power to them for trying to do the right thing. I also agree the removal of a cat may make it difficult to impossible to pass emissions tests (unless you have a buddy at the emissions station). I am kinda old school, my first several cars came from the factory without a cat, I like the smell (but that is a personal preference). I am enough of a kid (inside) that I kinda dig the flame which sometimes shoots out the exhaust during a high-rev shift.
05 Elise Freak, the removal of the cat also helps with a standing-wave issue called reversion. The issue includes the dynamics of the thermal pulse as well as the actual flow of exhaust gasses, but I don't feel like typing up the 6 page treatise I did on this topic a few years ago on another performance interest group. The intake equivalent is called stand-off (but lacks the thermal pulse which complicates things), and is clearly visible with older carburated cars as a fine mist seen floating around the venturi at certain rpm ranges (depending on the Q factor of the intake track).
 

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Let me be emphatic, the second O2 sensor is there only to verify you have a functioning cat on the elise/exige. It has zero value to your air to fuel ratio. It will not throw you into limp mode on an elise/exige. Most people add a CEL eliminator so they can see their shift light and to know if something is really wrong with their engine (when the CEL comes on for a critical reason). Racing or hi-flow cats sometimes throw a CEL with some cars and tunes. For those who can get away with a racing cat, I say more power to them for trying to do the right thing. I also agree the removal of a cat may make it difficult to impossible to pass emissions tests (unless you have a buddy at the emissions station). I am kinda old school, my first several cars came from the factory without a cat, I like the smell (but that is a personal preference). I am enough of a kid (inside) that I kinda dig the flame which sometimes shoots out the exhaust during a high-rev shift.
05 Elise Freak, the removal of the cat also helps with a standing-wave issue called reversion. The issue includes the dynamics of the thermal pulse as well as the actual flow of exhaust gasses, but I don't feel like typing up the 6 page treatise I did on this topic a few years ago on another performance interest group. The intake equivalent is called stand-off (but lacks the thermal pulse which complicates things), and is clearly visible with older carburated cars as a fine mist seen floating around the venturi at certain rpm ranges (depending on the Q factor of the intake track).
Agree 100%.

I ran the car stock for a year, then it has been running decat+arqray. There is no change in power at all, and there is no "safe mode" it runs in, when the second 02 sensor detects no cat.
 

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Discussion Starter #13
Addertooth,

Thanks for the input. Is reversion an issue worth considering with our cars? I have yet to pull apart my head for any reason (luckily) so I can't tell if there's any stains in the intake ports from reversion. I was under the impression that Lotus would tune the camshafts and exhaust to minimize it but I know it's always there, no matter what. Correct me if I'm wrong but reversion is when exhaust gas is pushed back into the intake path because when the intake valve opens the chamber still has exhaust in it at a higher absolute pressure than the intake port. Right? And by either making the intake valves open a bit later, or by getting the exhaust out of the chamber faster (freer flowing exhaust system or more exhaust cam duration) you reduce reversion?

Either way, I'm pulling the cat on this car just like I did on my other cars :) I also already got an ECU tuned for a de-catted car. Now a back-to-back dyno comparison may be in the mix to verify what is happening and double check things are kosher.

I can't wait to instal this stuff! Too bad we just got 8 inches of snow today :(
 

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Discussion Starter #14
Also, regardless if the second O2 sensor is responsible for AFR (lambda) correction, if there is more air moving through the engine in general, the first O2 sensor will see this and hopefully throw in a little more fuel to compensate, no? Again, this all stems from the fact that I have no idea what the capabilities of our cars are. All I know is that the temperature and humidity variations of upstate NY seem to confuse it quite regularly so it doesn't like to idle very smooth some days. So I assume the capabilities are limited.
 
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