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Discussion Starter #1
As title says really. Car is going into work for a week or so to have some bits done and the wheels refurbed and a new set of tyres. I have a supersprint x-pipe and Eisenmann race exhaust, but i want it a bit louder.

one of the guys suggested removing the cats and putting a straight pipe in it's place. a quick call to roy at CA reveals that his supplier has stopped making decat pipes, so we are thinking of cutting them off and making my own decat pipe.

Had a quick search but couldn't find many people who have done it. So, how many people have done it? What pipe did you use? Did you have any ill effects from this? Any warning lights? I am going to leave the sensors in the exhaust, emissions will never be a problem for the MOT :biggrinbounce:

Thanks in advance
 

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Discussion Starter #3
no cats, and supersprint headers would be about the loudest i think it will go! Just got to save up for the headers now, after the rear BBK!

Good thing about being in the motor trade is that if i decide it's too loud or i don't like it i can whip the car back into work and refit the cats in my lunch hour for example!

really thinking about doing it though, as you know the e39's are very well sound proofed, i have to have my windows open to enjoy my exhaust to the full!

cheers
 

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Going from stock cats to supersprint cats, my car got significantly louder. So yes, I can assume your car will get fairly loud with no cats/x-pipe/E Race. Al Gore recommends to leave cats on.:)
 

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That sounds like a really bad idea idea.

Do you know that your O2 sensors reley on DC voltage signals to adjust fuel trim?
Isn't that done by the pre-cat O2 sensors which he's leaving in place?
 

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That sounds like a really bad idea idea.

Do you know that your O2 sensors reley on DC voltage signals to adjust fuel trim?
as was mentioned above, it's the precat o2 sensors that are used for mixture control while in closed loop mode. In the UK, i'm not sure if missing cat's would throw a cat efficiency code and trigger an ses light. In the US it would, i bet it won't on a euro car.
 

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Discussion Starter #9
hmmm...can't decide. The guys at work think that by leaving then pre-sensors in place, there shounld not necessarily be a warning light.

Roy said that by fitting a decat pipe, emissions aside that there shouldn't be any problems with warning lights per say.

100cel racing cats are about £1300, whereas this is a much cheaper option, but obviously i don;t want to cause any harm to the car, so if it turns out it is going to be a bad idea, then i will save up for some 100cel cats instead.

just thinking...
 

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In Califonia it's 25 years-to-life for screwing around with emission controls. In europe I don't know.

I am sure you already know the aft O2S will be sending funtionality and rationality information to the PCM. If those inputs are out of range, you will get a MIL.

I bet it will sound just like Dale Earnhardt Jr.'s Chevy.
 

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ow.

I am sure you already know the aft O2S will be sending funtionality and rationality information to the PCM. If those inputs are out of range, you will get a MIL.
In the US, yes you will get a ses light, in the UK, not necessarily. They're obd is not as intrusive as ours. Same reason they don't get an ses light for carbon blockage.
 

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Your low and mid range performace will suffer even with o2 eliminators. Why not install a high flow cat in place of the stockers? I installed the CA OBDII Compliant units from Magnaflow on my 540i with good results (no CEL). You will gain some volume but retain performance.
 

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Your low and mid range performace will suffer even with o2 eliminators. Why not install a high flow cat in place of the stockers? I installed the CA OBDII Compliant units from Magnaflow on my 540i with good results (no CEL). You will gain some volume but retain performance.
why even bother with 02 eliminators? the post cat o2 sensors do nothing for performance/drivability, and the cat's offer little restriction, so performance will not suffer.

Keep in mind, the service engine soon light does not work the same way in europe as it does here.

the op is doing it for more noise, and any cat is going to have some noise reduction.
 

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Discussion Starter #14
in all honesty, i have made a decision not to decat the car at all. I guess, that if there had been a decat pipe available from the likes of Supersprint for example, then it would be a proven method like the e34 M5 where they do make a decat pipe.

have priced up headers and hi-flow cats and it comes to nearly £3,000. I thought a quicker way around it would be to take the cats off, and couldn;t remember whether the M5 had sensors after the cats or not - which they do!

It has been interesting to see the different responses, and i am glad that i have read them all and taken all of the information in!

I am going to save for either the headers and hi-flow cats or a Kelleners shorter ratio diff and chip with delimit, but that will be for next year i think now!

many thanks to everyone for the info!

cheers and have a good weekend!
 

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why even bother with 02 eliminators? the post cat o2 sensors do nothing for performance/drivability, and the cat's offer little restriction, so performance will not suffer.

Keep in mind, the service engine soon light does not work the same way in europe as it does here.

the op is doing it for more noise, and any cat is going to have some noise reduction.
The post O2 sensors are responsible for monitoring the performance of the CATs. If in europe the CATs performance is not monitored, why include the post O2 sensors at all? The CEL I reference has nothing to do with engine performance, simply the computer telling you there is something wrong, in this case, it would be inoperable CATs. Personally, I hate looking at the CEL but if you don't mind that glowing lamp, have at it.

If you remove the CATs and do not change the fuel mapping you will lean the motor out at lower RPMs. Leaning the motor out results in a decrease in power (torque). Granted this is not a large change but there is a change none the less.

If you are a non believer, I suggest you try dynoing a vehicle pre and post cat removal.
 

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Do a search on decating the E39 on this board. One member did extensive dyno comparisons pre and post decating. His conclusion was that a few hp was gained in the process.

My car is decatted. I bought it like this so I dont know what was involved (car has no piggy back chip and shows no errors on the BMW diagnostic equipment). When I installed new MAFs 6 months ago, I was flowing 150L/h at WOT, car sounds good too
 

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Discussion Starter #17
hmmm...interesting to know. Pretty much not going to do it now, going to go down another road. Too much risk as here doesn't seem to be a definitive answer for what will happen and the best way to do it etc...

cheers
 

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Removing the cats completely will make any car sound like crap (IMO). I just installed some high flow cats (Evosport) and did some custom stainless piping in the process.

The car sounds AMAZING and I can definitely feel some extra umph up top in the higher RPM range; no difference below 4K, but @ WOT, she definitely pulls better with the high flow cats. And the tone of the car is a lot more aggressive, with a loud and throaty growl on downshifts and throttle blips.


Don't remove the cats, just get some high flow ones. My entire high flow cat project cost roughly $400, which includes parts and labor. Compare that to the $3K supersprint race cats.

I am sure I will some more gains once I get a custom software upgrade as well.
 

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I'm seriously considering this. The exhaust shop that did my Magnaflow install was pointing out the bad design in the inlet/outlet of the stock cat

I'm having them build me an X-pipe (for$250!) to replace the stock resonators, and was thinking of removing the cats as well. If it's too loud, or I have issues, then I can just install some high-flow cats later, or maybe add a resonator.
 
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