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Cat Bypass

Drew1209

Well-known member
Joined
31 Aug 2016
Messages
261
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
 
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
 
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 .
 
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
 
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.
 
Drew1209 said:
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:
 
Norfolk & Idea said:
Drew1209 said:
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 :D
 
Norfolk & Idea said:
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?
 
Armagreggon85 said:
Norfolk & Idea said:
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.
 
911tom said:
Norfolk & Idea said:
Drew1209 said:
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 :D

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.
 
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
 
Drew1209 said:
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 :D
 
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:
 
Phil 997 said:
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.
 
poppopbangbang said:
Phil 997 said:
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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