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).

[632 Regal]

sounds like a doctor here.