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Drew1209
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Joined: 31 Aug 2016
Posts: 65



PostPosted: Sat Feb 25, 2017 4:57 pm    Post subject: Cat Bypass Reply with quote

I'm just in the process of sticking the exhaust back onto my car, I got thinking about the cats, is there much to gain (noise & power wise) if I remove them and make up a bypass, I'm on standard boxes if that makes any odds
 
  
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Armagreggon85
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Joined: 04 Jan 2017
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PostPosted: Sat Feb 25, 2017 7:37 pm    Post subject: Reply with quote

I seem to remember reading a post from someone who tried it and measured on dyno. If i recall correctly there was a bit available with cat bypass, maybe up to 20-30bhp at the top of the rev range but it caused significant torque and bhp losses low down.

The compromise seemed to be 200 cell cats which gave 10-15bhp peak gains and no loss down low.

Having said that it's not something I've done myself so maybe someone on here with more experuence can comment
 
  
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Demort
Sepang


Joined: 21 Mar 2015
Posts: 2932
Location: Sussex


PostPosted: Sat Feb 25, 2017 8:10 pm    Post subject: Reply with quote

If you remove them then it will fail an mot , you will also get eml light on for post cat lambda sensor faults .

Not sure on a 996 but on a 997 then 200 cell replacement seems to be causing hesitation faults in the low rev range , a remap seems to help .
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Drew1209
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Joined: 31 Aug 2016
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PostPosted: Sat Feb 25, 2017 9:10 pm    Post subject: Reply with quote

Magic cheers chaps.

I'm not fussed about MOT as I'd stick the original cats back on, I did think about welding a boss in for the sensors, I've only got the 2 pre cat lamda's on mine.

If a remap is required though, I think buying uprated cats would make sense, dare say I could get it mapped with no cats but when it comes MOT time this could be problematic when sticking the original cats back on
 
  
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911tom
Barcelona


Joined: 22 May 2012
Posts: 1499
Location: Buckinghamshire


PostPosted: Sat Feb 25, 2017 9:16 pm    Post subject: Reply with quote

As Matt at Fearnsport said to me the other week, 'no one ever fits a cat to make a car faster'.

As far as mot. Just get friendly with your local garage.
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Norfolk & Idea
Imola


Joined: 15 Jan 2016
Posts: 762
Location: South Yorkshire


PostPosted: Sat Feb 25, 2017 9:35 pm    Post subject: Reply with quote

Drew1209 wrote:
Magic cheers chaps.

I'm not fussed about MOT as I'd stick the original cats back on, I did think about welding a boss in for the sensors, I've only got the 2 pre cat lamda's on mine.

If a remap is required though, I think buying uprated cats would make sense, dare say I could get it mapped with no cats but when it comes MOT time this could be problematic when sticking the original cats back on


I'm running de cat. But my system has been tuned accordingly and I'm reliably informed it should pass its MOT as it is. I'll let y'all know how I get on Dont know
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911tom
Barcelona


Joined: 22 May 2012
Posts: 1499
Location: Buckinghamshire


PostPosted: Sat Feb 25, 2017 9:37 pm    Post subject: Reply with quote

Norfolk & Idea wrote:
Drew1209 wrote:
Magic cheers chaps.

I'm not fussed about MOT as I'd stick the original cats back on, I did think about welding a boss in for the sensors, I've only got the 2 pre cat lamda's on mine.

If a remap is required though, I think buying uprated cats would make sense, dare say I could get it mapped with no cats but when it comes MOT time this could be problematic when sticking the original cats back on


I'm running de cat. But my system has been tuned accordingly and I'm reliably informed it should pass its MOT as it is. I'll let y'all know how I get on Dont know


If it's a decat it will fail unless it is the same garage doing the mot as fitted the exhaust for you Very Happy
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Armagreggon85
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Joined: 04 Jan 2017
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PostPosted: Sat Feb 25, 2017 10:58 pm    Post subject: Reply with quote

Norfolk & Idea wrote:


I'm running de cat. But my system has been tuned accordingly and I'm reliably informed it should pass its MOT as it is. I'll let y'all know how I get on Dont know


I'd be interested to know how you get on. How does your car perform without the cats? What did you do to tune it, just a remap?
 
  
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Norfolk & Idea
Imola


Joined: 15 Jan 2016
Posts: 762
Location: South Yorkshire


PostPosted: Sun Feb 26, 2017 11:25 am    Post subject: Reply with quote

Armagreggon85 wrote:
Norfolk & Idea wrote:


I'm running de cat. But my system has been tuned accordingly and I'm reliably informed it should pass its MOT as it is. I'll let y'all know how I get on Dont know


I'd be interested to know how you get on. How does your car perform without the cats? What did you do to tune it, just a remap?


It's a massive step change in performance. Up around 120bhp from an already cheeky 500, so circa 620bhp now. It's epic.

Mods comprise of stainless Kline full system (decat) including Kline equal length headers, enlarged IPD intake plenum, new and re-gapped plugs and custom tune to bring it all together. All the work was carried out by 9E.
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Norfolk & Idea
Imola


Joined: 15 Jan 2016
Posts: 762
Location: South Yorkshire


PostPosted: Sun Feb 26, 2017 11:31 am    Post subject: Reply with quote

911tom wrote:
Norfolk & Idea wrote:
Drew1209 wrote:
Magic cheers chaps.

I'm not fussed about MOT as I'd stick the original cats back on, I did think about welding a boss in for the sensors, I've only got the 2 pre cat lamda's on mine.

If a remap is required though, I think buying uprated cats would make sense, dare say I could get it mapped with no cats but when it comes MOT time this could be problematic when sticking the original cats back on


I'm running de cat. But my system has been tuned accordingly and I'm reliably informed it should pass its MOT as it is. I'll let y'all know how I get on Dont know


If it's a decat it will fail unless it is the same garage doing the mot as fitted the exhaust for you Very Happy


Uh oh, I'll call Ken at 9E for the best way forward. I have a pair of 200 cell cats to slot in if needs be.... I'm just a bit concerned doing that may throw up lights. Or I'll give my local test centre lads a heads up. If we have to fit the cats it shouldn't be a biggie.
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Drew1209
Trainee


Joined: 31 Aug 2016
Posts: 65



PostPosted: Mon Apr 17, 2017 6:51 pm    Post subject: Reply with quote

Is there much to be had on NA 3.4 removing the cats, I know a few responses above are for the turbo, if I decide to do the bypass what would I do with the lambda sensors, I've only got the pre cat sensors or do the bypass pipes have a boss to screw the sensors in.

Exhaust wise just now I'm running the standard Porsche silencers and I'm not really wanting to go much louder
 
  
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poppopbangbang
Nürburgring


Joined: 25 May 2015
Posts: 442



PostPosted: Mon Apr 17, 2017 7:44 pm    Post subject: Reply with quote

Drew1209 wrote:
Is there much to be had on NA 3.4 removing the cats, I know a few responses above are for the turbo, if I decide to do the bypass what would I do with the lambda sensors, I've only got the pre cat sensors or do the bypass pipes have a boss to screw the sensors in.

Exhaust wise just now I'm running the standard Porsche silencers and I'm not really wanting to go much louder


Short answer yes, mine gained 21bhp from binning them along with fitting FVD manifolds. No check engine lights or similar...... but to be fair mine didn't even show a check engine light when it had no lambda sensors fitted at all - it did log it as a fault but no CEL. It seems the early e-gas 3.4L cars simply don't care what's happening emissions wise, which would make sense as these were prior to it being a requirement for OBD2.

If it doesn't have post cat lambdas then fill your boots Very Happy
 
  
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Phil 997
Yas Marina


Joined: 05 Dec 2015
Posts: 8501
Location: Bournemouth,Dorset

2009 Porsche 997 Carrera 4S

PostPosted: Mon Apr 17, 2017 10:03 pm    Post subject: Reply with quote

Have any of you with decat noticed the supposed low down torque and BHP loss as that was what I was told would happen if I decatted mine and as its a road car it didnt make sense to loss any low down to get top end gain that I would rarely use . So I went 200 cell big bore to get low down gains without any top end loss. but I am interested in what you found , mine is a NA car not a turbo Thumb Thumb
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poppopbangbang
Nürburgring


Joined: 25 May 2015
Posts: 442



PostPosted: Mon Apr 17, 2017 10:13 pm    Post subject: Reply with quote

Phil 997 wrote:
Have any of you with decat noticed the supposed low down torque and BHP loss as that was what I was told would happen if I decatted mine and as its a road car it didnt make sense to loss any low down to get top end gain that I would rarely use . So I went 200 cell big bore to get low down gains without any top end loss. but I am interested in what you found , mine is a NA car not a turbo Thumb Thumb


No, none at all. This is probably the old "you need some back pressure" myth rearing it's head again. Essentially the best possible option regardless of turbo or NA is as little back pressure as possible as this can only help cylinder filling/minimise residuals. In some cases you will need to make changes to the engine calibration to accommodate this but in my experience the single lambda per bank 3.4L cars just work and show gains everywhere.
 
  
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Phil 997
Yas Marina


Joined: 05 Dec 2015
Posts: 8501
Location: Bournemouth,Dorset

2009 Porsche 997 Carrera 4S

PostPosted: Mon Apr 17, 2017 10:17 pm    Post subject: Reply with quote

poppopbangbang wrote:
Phil 997 wrote:
Have any of you with decat noticed the supposed low down torque and BHP loss as that was what I was told would happen if I decatted mine and as its a road car it didnt make sense to loss any low down to get top end gain that I would rarely use . So I went 200 cell big bore to get low down gains without any top end loss. but I am interested in what you found , mine is a NA car not a turbo Thumb Thumb


No, none at all. This is probably the old "you need some back pressure" myth rearing it's head again. Essentially the best possible option regardless of turbo or NA is as little back pressure as possible as this can only help cylinder filling/minimise residuals. In some cases you will need to make changes to the engine calibration to accommodate this but in my experience the single lambda per bank 3.4L cars just work and show gains everywhere.


Lovely thanks PPBB you are spot on the "you need some back pressure" was the reasoning I was told. Thumb Thumb
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Drew1209
Trainee


Joined: 31 Aug 2016
Posts: 65



PostPosted: Mon Apr 17, 2017 10:46 pm    Post subject: Reply with quote

PPBB, excellent mate that was what I was looking for, with the cats removed does it increase sound levels by much. 20bhp is some gain, if anywhere near that it has to be worthwhile.

So basically the cars management can cope with the fact there's no cats, might be a stupid question but with no lambda sensors how can it alter fuelling ?
 
  
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poppopbangbang
Nürburgring


Joined: 25 May 2015
Posts: 442



PostPosted: Mon Apr 17, 2017 11:29 pm    Post subject: Reply with quote

Drew1209 wrote:
PPBB, excellent mate that was what I was looking for, with the cats removed does it increase sound levels by much. 20bhp is some gain, if anywhere near that it has to be worthwhile.

So basically the cars management can cope with the fact there's no cats, might be a stupid question but with no lambda sensors how can it alter fuelling ?


Cats of the late 90's weren't anywhere near as good as modern stuff in terms of flow so whilst on a modern car you won't generally see much of a gain on something like a 3.4 996 the cats are fairly restrictive! There are quite a few dyno plots kicking about showing the effects of a decat. They do get a fair bit louder without them in though so it's worth considering if you have neighbours that aren't car fans. I have a single silencer per bank and that's it so mine is fairly loud, although from inside you can barely hear it it's over the noise limit for the circuit where I work Very Happy

I can't speak for the later 3.4 and 3.6 cars with post cat lambdas as I don't have any experience of them, aside from to say you will get a CEL as that is the purpose of the post cat lambdas - however the early DBW throttle cars really do not care at all. The pre cat lambdas are purely used as fuel trim so without them it just runs from the MAF. The ECU uses a mathematical model of the engine to determine the correct amount of fuel to inject based on feedback from the MAF i.e it knows with good accuracy exactly how many grams of air are flowing into the engine at any moment in time so it knows exactly the volume of fuel to inject to achieve the air/fuel ratio the model tells it to target.

For example lets say we have 100gs of Air per second entering the engine and for the given torque request (a function of pedal position) and engine speed the ECUs model tells it that it needs to achieve lambda 1 or 14.7:1 AFR (14.7 parts of air to 1 part of fuel). Some quick math in the box then tells it that for the next injection event it needs to inject 6.8(and a bit) grams of fuel, it knows the specific gravity of pump fuel is about 0.76 (or it was in 1999) so it knows that it needs to inject 5.168ml of fuel. As the ECU also knows the injector size and the operating pressure in the fuel rail it can then determine exactly how long to open the injector for to provide this amount of fuel on the next injection event.

The lambda sensors are just used as a trim to this calculated value and then only in engine operating regimes where Lambda 1 is targeted (to keep the cats happy and operating), once you get past 4500RPM and 60% or so torque request then the ECU will operate purely from it's model based on a measure of inducted airmass.

It's because of this system you can bolt a load of mods onto an engine 300CC bigger than that which the ECU was originally calibrated for and find it still achieves 0.91L at WOT and peak power RPM exactly like it did on the standard engine because the MAF is still able to accurately measure the increased air flow and the value is still within the model the ECU uses to calculate fueling.

What's really clever is that if the MAF fails the ECU then reverts to a speed/density operating regime where it uses throttle position as it's primary load sensor and operates from another model that ties throttle position and RPM to the expected amount of air mass, it's still pretty accurate on fueling even at that! Unless you've done the above load of mods and a larger engine thing.... if you have the model is quite a way off and it's properly lean under load Laughing
 
  
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poppopbangbang
Nürburgring


Joined: 25 May 2015
Posts: 442



PostPosted: Mon Apr 17, 2017 11:52 pm    Post subject: Reply with quote

To expand on this slightly here is some data from my car taken during testing today:



The top green line is GPS speed, The second blue line is Lambda on the left bank, the third purple line is MAF (this is an arbitrary reading unfortunately rather than an SI unit), the fourth dark blue line is engine RPM and the lower red line is TPS.

From the graph you can clearly see the relationship between throttle position, MAF reading and lambda. With the throttle wide open and the engine RPM around 5000RPM it is running purely from the model in the ECU to determine how much fuel to inject and we can see it's pretty good at it by how stable the Lambda is at 0.9 +/- 0.1. Note how the MAF reading reflects the shape of the throttle position trace, as throttle position directly relates to the inducted airmass and therefore the MAF sensor output.

Also quite interesting to see it's done just over 100mph to 125mph in top gear with a slow throttle roll on in a little over 5 seconds and I came off the throttle about 3/4 of a second before peak speed was reached! Surprised
 
  
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Dammit
Spa-Francorchamps


Joined: 23 Sep 2016
Posts: 336



PostPosted: Tue Apr 18, 2017 8:55 am    Post subject: Reply with quote

I have a single 100 cell cat in the 850R- which you can see through, so it's not what I'd call hugely restrictive. Of course there will be a defreeenof restriction but I can pass an MOT anytime, anywhere. What's the performance that I am leaving on the table, and does the convenience out weigh it?
 
  
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Phil 997
Yas Marina


Joined: 05 Dec 2015
Posts: 8501
Location: Bournemouth,Dorset

2009 Porsche 997 Carrera 4S

PostPosted: Tue Apr 18, 2017 10:21 am    Post subject: Reply with quote

This is turning into a very interesting thread Thumb thanks for the input gents. Thumb Thumb
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