Here are my experiences with this...
First some technical info:
The sole purpose of the O2 sensor after the cat is to determine the
efficiency of the cat. (Basically, the computer compares the readings
from the downstream sensor to the upstream sensor, and it expects to
see more oxygen by volume at the downstream sensor than what the upstream
sensor is reporting, since the cat is supposed to remove the extra junk.)
(The upstream sensor is also used to determine the fuel/air ratio which
the PCM (computer) uses to adjust the mixture at power settings less
than WOT.)
Each sensor has 4 wires. Two are for the heating element (to make
the sensor readings more accurate I'm guessing), one is a ground
provided by the PCM, and the other is a voltage to the PCM (0 to 1volts)
Its been a while since I looked at this stuff but if I remember correctly,
0 volts would mean 100% oxygen, and 1 volt would mean 0% oxygen.
I've been thinking about removing my cat for some time, and
a little while ago, I did some testing with a multimeter on
the downstream sensor just to get a better handle on it. Fortunately,
the O2 sensors just plug right into a wiring harness, so its very
easy to see what would happen if you removed it; just unplug it
and go driving. I did exactly that, and I drove for about 45 miles
with no CEL (check engine light). However, as I was going up a
grade, I was behind a slow car which I was about to pass. As I
was waiting for an oncoming car to go by, I was in 3rd gear at
about 3,000rpm for several seconds. That's when the CEL came on.
I pulled off the road and checked the codes in the PCM, and there
was a code stored about some sort of malfunction with the downstream
sensor (I forget which one it was now). I drove home without resetting
the computer, and didn't notice any difference in the truck. (I didn't
really expect any since this sensor isn't used to adjust any performance
parameters.) The reason I drove around with the sensor unplugged
was to try to determine wether or not I'd get a CEL. (There are
a few Mustangs without cats that I have heard of who are NOT getting
CELs.) So, there is at least one documented case of a CEL as a
result of no O2 sensor. (I have no idea what would happen if
you just removed the cat and left the downstream sensor in place;
that might work as well.)
If a CEL doesn't bother you, you can just reset the computer whenever
you get one, but I'd rather not see it at all since then I've got
to pull the codes and make sure the CEL is because of the O2 sensor
and not a more serious problem. So, I'm working on a module
to simulate a downstream O2 sensor behind a functioning cat.
(Based in part on Sean Meldrum's CKC module. (Curiosity Killed
the Cat)) This module will plug into the wiring harness using
the same connector as an O2 sensor, and will also have to read
the voltages being sent from the upstream sensor to the PCM. I
have most of the parts I need already (stuff anyone can buy at
Radio Shack) I just need some time to put it together.
If anyone knows of a place to get the type of snap-together
connectors used in the wiring harness and O2 sensors, that would
be helpful, since this would allow someone to simply plug it in
without having to do any cutting or wiring.
When I installed my ATR Y pipe (2x2.5" -> 3"), I also removed
the cat and the pipe with the O2 sensor in it. However, I do
still have some bottlenecks. There's a large restriction back
near my muffler where I didn't feel like cutting off my 2.5" ->
3" adaptor so I just slipped a 3" pipe over it, and also I'm
using a 3" flexible exhaust pipe to go over a crossmember after
the Y pipe; I had to hammer a big dent in it to get it to clear.
I also have a couple of leaks, that I didn't really care about
since this is just temporary
After I get that stuff taken care of (I'm going to have a custom
pipe bent up), I'll have a better idea of what the performance gains
and a high 14 every now and again. My best was 14.85 on BFG drag
radials. My times corrected to standard as 14.80s and 14.90s. After
the Y pipe install and cat removal, I ran a 14.80 on street tires;
that time corrected to standard as a 14.66. So, it appears to
have given me 2 to 3 tenths. (and also 2 to 3 mph). Unfortunately,
my rearend self-destructed and I didn't get to make any more
passes. That was the first pass after resetting the PCM too,
so my next runs would've been even faster. By the way, after
removing the cat this time, I drove the truck about 100 miles,
including a couple of runs down the strip, and I didn't get a CEL.
(But I was also careful not to duplicate sitting at a high rpm for
an extended period of time.)
As far as killing low end, I don't have enough info to be
sure. When I took the truck for a spin after removing the cat
and installing the Y pipe, it *did* appear to feel "less
powerful" but that could've been my immagination, since
I hadn't driven the truck for a couple of days. The sound
definitely was different. It was louder, and had more of a
"tuned" or "race" note. (Listening between the exhaust leaks.) ;-)
Once I get my truck back, I'll install my module to do some
additional testing. Once I'm satisfied with it, I'll post
the parts and info required to make one yourself on the DML page.
(If someone is unwilling or unable to roll their own, I might be
willing to sell them for my cost plus maybe a donation to the DML.)
My eventual plan is to have a "quick connect" type of arrangement
which I can use to swap the cat in and out with a minimum of
hassle. (I'm planning to use stainless band clamps.)
-Jon-
.--- stei0302@cs.fredonia.edu ----------------------------------------.
| Jon Steiger * AOPA, DoD, EAA, MP Race Team, NMA, SPA, USUA * RP-SEL |
| '96 Dodge Dakota v8 SLT CC (14.80@92.97), '96 Kolb FireFly 447 |
`--------------------------- http://www.cs.fredonia.edu/~stei0302/ ---'
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