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Cat Bypass

diverzeusy said:
996ttalot said:
diverzeusy said:
Of interest to me too, any more info on this one anyone may have, has I'm embarking on a Kline exhaust, it came with 200 cell and bypass pipes but do not know which to fit?
My two major concerns being noise and MOT passing 😁
All this to install after the Dinslaken jaunt :thumb:

What model car?
Hi My car is 996 turbo 2000
The problem with the decat is not passing on lamba etc, but on CO normally. I would make the assumption it will fail the MOT although we have got them through with tuning.

If you want the best performance then decat is the solution, then 100 cell and then 200 cell. I am sure Felix will swop out the 200 cell cats for you.

As to CEL, decat or 100 cell will needing coding in the DME.

As to noise, the best sound is from decat, then 100 cell, then 200 cell etc. The noise level will rise with decat being the loudest, but to be clear, the noise level with the Kline is very consistent until you are on boost regardless of decat, 100 or 200 cell.

Ken
 
Martin996RSR said:
I thought I might chime in to give a little more info for early 3.4 NA owners. As has already been mentioned, there is no such thing as a requirement for backpressure. If you look at motorsports where there is no noise restriction, such as drag racing, frequently open stubs are used because these offer the least resistance (the least backpressure if you will). The thing with exhausts on four stroke engines is that they need to be designed to give the greatest exhaust velocity. The faster you can get the exhaust gasses out of the engine, the more power you will make.

In order to achieve this an exhaust needs to be designed with a diameter that is sympathetic to the volume of gas produced. If the bore is too big then you will actually slow down the gasses in much the same way that a river slows down when it widens. Obviously, at different revs then engine produces different volumes of gas, so the ideal exhaust would vary in diameter in line with revs. Until this fantasy pipe is invented we need to settle on a compromise diameter of pipe.

If you are going catless, you will see bigger gains than going with 200 cell cats all across the rev range. This is not just because the cat matrix isn't in the way, but also because the bulb in the exhaust that holds the cat takes energy (velocity) out of the gas by slowing it down where it widens at the start of the cat and then another restriction is faced at the other side of the cat where it tapers back to the normal exhaust diameter.

To get maximum power out of a 996 NA exhaust, the mufflers would also be changed to something that used a straight through design, such as the After Hours one:
https://www.6speedonline.com/forums...ted/232933-996-after-hours-sport-exhaust.html

The only problem with this is the very tight radius of the bend between the mufflers. Although the design is much less restrictive than the normal 996 muffler, more power could be released by increasing that radius.

I'm currently toying with getting something made in the after hours style, but replacing the first muffler with a 200 cell cat, and running the manifold direct to the cat & muffler on it's own side. If I can get the radius of the bends in the exhaust large enough (basically 6" or greater) then I could lose a collossal amount of weight from the rear of the car, and gain somewhere in the region of 15-20 hp.

As regards lamda sensors, I strongly advise you run them. Without the fine tuning of the mixture that they provide you may have trouble with rough running. I'm speaking from recent personal experience with my early 3.4 car here. Even with a decat pipe, I would still keep the lamda sensors. With one of my lamda sensors out of action recently, the bank it was on failed the emissions test for the MOT, and the car only passed because the tester then tested the bank with the working lamda sensor.

Nice post :thumb:
 
996ttalot said:
diverzeusy said:
996ttalot said:
diverzeusy said:
Of interest to me too, any more info on this one anyone may have, has I'm embarking on a Kline exhaust, it came with 200 cell and bypass pipes but do not know which to fit?
My two major concerns being noise and MOT passing 😁
All this to install after the Dinslaken jaunt :thumb:

What model car?
Hi My car is 996 turbo 2000
The problem with the decat is not passing on lamba etc, but on CO normally. I would make the assumption it will fail the MOT although we have got them through with tuning.

If you want the best performance then decat is the solution, then 100 cell and then 200 cell. I am sure Felix will swop out the 200 cell cats for you.

As to CEL, decat or 100 cell will needing coding in the DME.

As to noise, the best sound is from decat, then 100 cell, then 200 cell etc. The noise level will rise with decat being the loudest, but to be clear, the noise level with the Kline is very consistent until you are on boost regardless of decat, 100 or 200 cell.

Ken

Ken great input thank you :thumb: I often wondered about this and what would happen if I changed out my 200cell big bore for the same but 100cell and what extra BHP etc .mine is a 997.1
 
Drew, did you ever get round to this ?

I'm on standard boxes also and was wondering if there is any gain in fitting 200cell cats and keeping standard boxes ?
 
Just to bring this back to life, I fitted my decat pipes today but couldn't get lambda sensors out the old cats, I have ordered new ones from ECP but reading over PPBB post it would seem the lambda sensors are not vital on the early cars.

Mine is DBW but has only the pre cat sensors, could I get away with just blanking the bosses on the decat pipes and let the car work solely off the maf sensor as going by previous posts all the lambda does is trim fuelling at part throttle.

MOT is of no concern at all so not bothered about cel etc
 
Just seen the comments about after hours exhaust, if you go to a fabricator, it should cost you around £600 to make a system similar to it.

I think Stuart has or had one and ive got a copy based on it from the local fabricator. My silencers arent long and slender like the AH but more short and stubby
 

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