Cat - Lambda prope - what happens?
 

The left side of my new SSI's has a socket in the tube for some kind of sensor I guess. Probably for cars that used to have a cat? This made me think.

What do you guys with cat equipped 911s do when you switch to SSI's?

All cat cars have a lambda prope that controls the cat by sending signals to the engine control box, right? I wonder if it's placed before or after the cat? If it's placed after the cat what will happen as you remove the cat? It must be rather sensitive if it sits after the cat? Will the DME get confused signals or what?

Could someone please explain this to me?

Mikkel:

Not all models with catalytic converters have O2 sensors (lamda probes), my '78 SC, for instance had a cat. but no lamda (the lamda, for SC's, came along in '80, I believe). The O2 sensor is mounted before the cat., senses the amount of O2 in the exhaust and adjusts the fuel mix to achieve as close to stoichiometric (approx. 14.7:1) as possible (depending greatly on the sophistication of the system, earlier less sphisticated, later very sophisticated). Some very recent models (all cars, not just Porsche) also use a variety of O2 sensor after the cat. to ensure the least amount of emissions possible. There's, obviously, much more to the differing systems, when they were implemented, etc. but that's cat.'s and lamda's in a "nutshell".

Jerry M
'78 SC

Mikkel- here's some of what I learned in school:

1. The O2 sensor is a probe that is put in the exhaust air stream before the cat. If I remember correctly, the O2 sensor has 2 platinum surfaces sandwitched with some material inbetween. The difference of oxygen in the exhaust vs. the outside air creates a small fluctuating dc voltage (usually between .25 to .75V) that is sent to the computer and tells it to slightly ajust the average mixture level to get stochiometry.

2. The O2 sensor was made so the cat would work better.
The reason why the mixture fluctuates is because the cat works best when the mixture switches from a little lean to a little rich(many times a second).
This ensures the cat stays clean and has enough O2 stored for the times of heavy acceleration when there is little O2. The cat has platinum and/or paladium honeycomb/balls inside which converts any unburned gasses to CO2 and water.
The closer the O2 sensor and cat are to the engine, the faster they warm up and work efficiently.

3. The O2 sensor voltage shifts from high to low too rapidly to be read with even some cheaper scanners: the sample refresh rate is just not fast enough to pick up the number of cross-counts(how many times the high/low voltage changes per/sec).
Even though the high and low voltages may still be good, the O2 may not shift fast enough and can be called a "lazy O2 sensor".

4. The O2 sensor after the cat is manatory for 1996 OB2 and later. This one basically tells the computer how well the cat is working. If the voltage is the same as the one before the cat the computer will set a code.
OBD2 also does self-checks("monitors") with the sensors by richening the mixture/opening the EGR etc.. and seeing if the O2/ MAP voltage follows.

It gets more scientific than that, but hopefully this gave some basic working information.

Thanks guys. Good explanations that confirmed my basic ideas.

So in theory even an OBD car (like a 993 Turbo) will run properly in spite of having its cat removed?

I've heard people talk about how cat cars will run badly if you remove the cat. I guess this is mostly down to rumors and less facts.

Hurry up and get those bits fitted - I have simular waiting to get fitted and want to know the results!!!

I've heard they're (SSI's) rubbish on Carreras. You may aswell bring them down the road to me Rob!;)

OBD1 will run fine w/o a cat. OBD2 will not. OBD2 measures pre and post cat gas w up to 4 <FONT FACE="SYMBOL">l</FONT> sensors. It is causing no end of trouble to owners of '96 on US 993 cars.


ROW cars thru at least 98 are OBD1. US cars form 96 on are all OBD2

More than you ever wanted to know about Lambda