gain in power by removing catalytic converter

[E61Silver]

Originally Posted by Gary92

im pretty sure the sound would be completely different without a cat

So that's a reason to do it

[Wagner]

Originally Posted by Gary92

im pretty sure the sound would be completely different without a cat

Pretty sure you're wrong unless by completely different you mean, crap. I've modified many cars [BMW M3s, X5s, 545, 645, 350Z, Mustangs, etc], unless the mufflers (which actually MUFFLE the sound) and pipes are changed the only difference will be noise, not a great sound

[Wagner]

Originally Posted by N S

In the mustang world we have MIL eliminators for off-road x pipes that eliminate the cats. Essentially they just jump out the connection so the computer thinks everything is working as designed. Not sure if BMWs have something similar.

As for removing cats for performance gains - yes, there are gains to be had. Cats are extremely restrictive. A more environmentally friendly mod may be to find a set of "high-flow" cats that flow better.

IMO the gains don't justify the hassle though. Maybe on a street/strip car, but not on an X5.

Agree, with added modification. Cats alone are not worth it, no matter what you have.

[codered]

Originally Posted by lo_jack

Do you really think the cats are choking down 20 horse? Move up to a bigger cat. Keep in mind, the cat is matched to the exhaust system, so if you are holding back any significant power, it is in the design and diameter of the entire exhaust system, which cannot be blamed on the cat. One good 3" in/out ceramic metal matrix is good for way beyond the horse these motors put out NA. But the pipe on these cars is no where near 3" ID, now is it.

You could probably do a cam and some kind of homemade longtubes, true duals all the way down with an h pipe, and still not outflow the capability of one good quality 3" converter.

If your cat is clogged, then yes it could be dragging some power, but not because it was working properly and holding you back. Cats fail when you have to much raw fuel hitting them, or you do not get them up to operating temp, or both. Has this been happening to you?

It will run with out them, but whats the point? Unless you have a blower or a turbo, you don't need to take the converter off. But if you want to make some carcinogens, go for it.

The main reason I want to completely remove the both cats is that for a year or more there is a problem with the engine no one has been able to fix.

Once you push the engine really hard, meaning between 4000-5000rpm or more , for longer than couple of seconds the engine starts to shake/vibrate really bad and loses power. I have to stop the car, switch the engine off and then on again for the problem to disappear. It is worse if the engine is not at optimal temperature.

I have been to different dealers here in Romania but the truth is no one gave me an exact answer to what the problem is...on different tester equipment the error seems to change...mainly is "misfire on cylinders 4,6,8".

I have changed the spark plugs, lambda sensors, I have checked the head gaskets, change the acceleration sensor (hopefully that's how in translates in english) . No change.
I read somewhere that if you have engine misfires the cats will go bad and I suppose they are after misfires for such a long time..
There is not much I can change apart from what I already had , I guess, to fix this. And it's not going to cost much to put straight metal pipes there..

This is actually my last option, long time avoided (I know it's bad for the environment and for the people).

I'm thinking that if I put any cat instead , they will still break sooner or later because of the engine misfires, that's why I want to completely remove them to identify the problem.

There is a guy here in Romania who removed them and after a dyno test, the gain was 17hp. However, I cannot see his dyno results on paper.

Anyway, thank you all for replying back to me, you are really very helpful, much appreciated!


Kaz

[codered]

Originally Posted by Wagner

Pretty sure you're wrong unless by completely different you mean, crap. I've modified many cars [BMW M3s, X5s, 545, 645, 350Z, Mustangs, etc], unless the mufflers (which actually MUFFLE the sound) and pipes are changed the only difference will be noise, not a great sound

By changing the "mufflers" you mean changing the entire exhaust system or just parts of it?
I'm guessing the mufflers are just the 4 metal pipes at the end of the exhaust? Will larger one make any difference?
Sorry about my poor understanding of english, I'm not a native speaker.In romanian dictionary "muffler" refers to the whole exhaust system which I suppose is wrong/incomplete.

[meandmyX]

i skpped over most, but taking the cat out sounds like something ricers and trucks (ford/chevy) do.........could be wrong

[X5girl]

Removing the cats will release upwards of 5 bhp and much more if the cats are faulty. If running for any length of time on a missfire, the unburnt fuel enters the cats and burns in there, seriously overheating them. The matrix can melt, break-up and block and in this case 20 bhp is feasable.

HOWEVER

Tour car is OBD2 and so removing the cats will tell the ecu the cats it thinks you have are ineffective as the post cat O2 sensors will see the same emissions as the pre cat sensors and throw a check engine light/fault code.

It could be done by using some trick post cat sensors giving false readings but imho its not worth it. Fit sports hi-flow cats and sports rear silencers, no fault codes, more power and nice sound. Delete that heavy middle resonator too and save weight and add sound.

X5Girl, xxx

BTW... removing cats NOTICEABLY changes exhaust note and sound level.

[Wagner]

Originally Posted by X5girl

Removing the cats will release upwards of 5 bhp and much more if the cats are faulty. If running for any length of time on a missfire, the unburnt fuel enters the cats and burns in there, seriously overheating them. The matrix can melt, break-up and block and in this case 20 bhp is feasable.

HOWEVER

Tour car is OBD2 and so removing the cats will tell the ecu the cats it thinks you have are ineffective as the post cat O2 sensors will see the same emissions as the pre cat sensors and throw a check engine light/fault code.

It could be done by using some trick post cat sensors giving false readings but imho its not worth it. Fit sports hi-flow cats and sports rear silencers, no fault codes, more power and nice sound. Delete that heavy middle resonator too and save weight and add sound.

X5Girl, xxx

BTW... removing cats NOTICEABLY changes exhaust note and sound level.

Sorry have to disagree with most of that, starting with 20bhp. I'd love to see one, just one, dyno chart with a X5 with cats removed posting a delta of 20bhp after modification. Just is not possible.

As for the O2 sensors, correct.

OP: Mufflers are simply the two items at the end of the exhaust, box like in size with the exhaust tips coming out of them.

[X5Kiwi]

Once you push the engine really hard, meaning between 4000-5000rpm or more , for longer than couple of seconds the engine starts to shake/vibrate really bad and loses power. I have to stop the car, switch the engine off and then on again for the problem to disappear. It is worse if the engine is not at optimal temperature.

I have been to different dealers here in Romania but the truth is no one gave me an exact answer to what the problem is...on different tester equipment the error seems to change...mainly is "misfire on cylinders 4,6,8".

Sounds to me like you have a fuel supply problem, perhaps a faulty fuel pump? Those symptoms sound like when at high revs, it is slowly loosing fuel pressure until it can no longer supply the injectors, so the engine starts missing.

When my X5 ran out of fuel (fuel sender stopped working, causing fuel gauge to be inaccurate), the error codes stored were all misfire codes.

[codered]

Originally Posted by X5Kiwi

Sounds to me like you have a fuel supply problem, perhaps a faulty fuel pump? Those symptoms sound like when at high revs, it is slowly loosing fuel pressure until it can no longer supply the injectors, so the engine starts missing.

When my X5 ran out of fuel (fuel sender stopped working, causing fuel gauge to be inaccurate), the error codes stored were all misfire codes.

I have changed the fuel pump and the fuel filter a while ago,was on the list to fix the engine shake. No luck.