gut the cat/replace the cat

[Unregistered]

Lots of folks, esp. Porsche people, gut their catalytic converter, or simply substitute a straight through "test" pipe to let the engine breathe better.

Anybody with a 535i do this? Results?

Anybody making "test" pipes out there?

[winfred]

the modern honeycomb catylist is just not as restrictive as people think, the old thermal reactors sucked so did the ones with the little coated bb's. my 535 had this abortion on it when i got it, no cat a flowmaster then a glass pack in the back, it sounded like ****, after i installed a full exhaust with cat intact it runs better, a certain amount of backpressure helps lowend torque a little anyway

[Unregistered]

Well, as a matter of fluid dynamics, a straight pipe still has to be a lot less restrictive than any honeycomb or other internal obstruction. And, it seems likely that any desirable back pressure would still result from the resonator and muffler further downstream.

So, has anybody tried a straight "test" pipe in place of the cat. converter. Results?

[Tiger]

In any engine, you need some restriction for smooth constant exhaust pressure for better torque and smooth engine response. Engine are not designed for zero backpressure... which result in peaky and rough engine responses. Like Winifred said, it runs like ****.

[George M]

Anonomous one....you are mixing your metaphors a bit. Gutting a BMW CAT will actually increase backpressure, not decrease it. It will create more of a chambered affect in a cannister ill devises for sound containment without the pellets...i.e. will sound like sh!t as Winfred said. Adding a straight pipe is a better solution but again the stock US exhaust system is not designed accoustically to run with a replacement pipe so the exhaust note will not be as clean. I believe certain early E-34's in Europe ran without a CAT. Further you will lose an increment of low end torque...where you need it most on the street in favor of an imperceptible top end gain if at all.
George

[632 Regal]

would it even run with the O2 sensors and a gutted cat? I think error code is in order.

[George M]

OBDI cars will run fine with a gutted CAT or replacement pipe with a bung mounted O2 sensor. OBDII cars with O2 sensor in front and behind the CAT will only run OK if you run a surrogate O2 sensor behind where the CAT used to be to dupe the DME.
George

[winfred]

obd1 would be ok, the sensor is upstream of cat

[Dave M]

would it even run with the O2 sensors and a gutted cat? I think error code is in order.

The exhaust below is from the 91 525im and has run 'errorlessly' with the O2 sensor placed in the crotch behind the straight pipe. CAr idles fine and sounds relatively quiet (wouldn't remember what it sounded like before anyhow).

http://volcano.photobucket.com/albums/v11/dave_macisaac/Bimmer/Exhaust/Exhaust.jpg

[632 Regal]

sounds like a doctor here.

[Tiger]

In any case, you will not pass Smog test.

[winfred]

1 sensor per pipe cars will work fine, 2 sensor per pipe (obdII 96+) cars need a dummy post cat sensor signal to avoid computer problems, if you fart around with the cat


sounds like a doctor here.

[Unregistered]

Since I posted the catgut/testpipe question earlier, most folks on this list have said the car would run poorly without the cat in stock configuration, mainly due to backpressure concerns. Then, it turns out that one 525i guy has been using a pair of straight pipes instead of the cat, without problem.

Today I also asked essentially the same question of Porsche 944 Rennlist members. Almost all respopndents who gutted cats or installed "test" pipes said their car ran better and with more power, but with changed or somewhat louder exhaust sound. Several had passed emmissions tests in their respective states with a "test" pipe installed!

Granted, this is an apples vs. comquats comparison, because Porsches and BMWs have obvious differences. But, do the respective exhaust gasses know that?

Now, having originally posed this question, I'm still wondering if a gutted cat or "test" pipe replacement might not work pretty well on a BMW 535i, provided the O2 sensor is properly placed in front of the cat location. I'm also wondering if a "test" pipe with insulating exhaust wrap tape on it (to keep the heat and noise insulated inside the pipe) might actually be cleaner and more efficient than an old cat, because the retained heat would help to neutralize unburned exhaust gasses, i.e., perform some of the cat function. For that matter, why not go ahead and wrap as much as possible of the exhaust system, from manifold to tailpipe?

Your thoughts?

[George M]

all your questions have been answered prior to your last post excepting the wrapping of the exhaust. Heat abatement isn't an issue outside of the engine compartment... CAT's are about as hot as it gets and noise containment by using a wrap is irrelevant as well as noise emission is mostly out of the exhaust....you don't want to wrap a hollowed CAT or test pipe...no value added. As I stated earlier, a test pipe is a better solution...I have run them before on other cars. Both gutting a CAT and/or adding replacement pipes with be louder. As to driveablity...again no real advantage performance wise either way as production CAT's on these cars flow well.

Finally, Jeff you asked for a bit better explanation on OBDII cars. Here is a pretty good summary of why OBDII cars need a MIL eliminator attached to the rearward O2 sensor to dupe the DME into not spitting a code when running a replacement pipe or gutting the CAT:

http://corner-carvers.com/wiki/index.php?OBDII%20and%20O2%20Sensors%20Explained

HTH,
George