Tate,
>Would a single high flow 3" in & out cat help? That's the route I
took.
Yes, high flow cats can be almost as good as none at all, according
to some dyno tests I've seen.
Pat,
<< The cat delete is normally worth 10 HP or more on most vehicles. I
agree that with headers AND a low restriction DUAL exhaust,
subsequently deleting the cat may produce so little restriction as to
somewhat compromise low end torque. >>
>Have you done any dyno testing to prove this?
I have seen dyno tests from several Impala owners in our club who
have documented this via dyno testing, plus I have witnessed back to
back testing at the track where it was worth a tenth and 1 mph or so.
Also several of the GM vehicles exported to Saudi and which come from
the factory with no cats, are factory rated by GM as having about 10
HP more. The Impala has dual cats which flow better to start with,
the Dak and GC have a seemingly more restrictive single cat (my GC
cat has a 2.5" inlet and 2.25" outlet) which would probably benefit
even more.
>90% of the people out there that I've talked to will agree it's not
worth it on a near stock truck.
I'd say just the opposite. The more stock it is (no headers, stock
exhaust) the bigger the gain would be. For example you could keep the
factory manifolds and exhaust (which isn't bad on the Dak R/T or the
GC 5.9 for example) and delete the cat for a nice gain in power and
sound. The effect would be similar to putting on headers, or a
free-flowing aftermarket exhaust. I agree that if you did all three,
headers, free flow exhaust, AND cat delete, there would be a definite
loss of low end and bias towards more power on the high end.
Jon,
>The cat removes oxygen? I figured that oxygen was OK, so a cat
would let it by, while trapping the hydrocarbons (or whatever)
Whoops! I am the one who got that BACKWARDS! (I think I must've been
up too late <g>). You are correct here. The effect of the cat is to
burn any leftover unburned HCs in the exhaust stream. These unburned
HCs would normally DISPLACE oxygen. Therefore the less unburned HCs
there are, the LEANER the mixture, and the more oxygen will
proportionally be present.
Here is a table which helps describe it:
Unburned Oxygen
Mixture Hydrocarbons Content Voltage
---------------------------------------------
Rich More HCs Less O2 High (>450mv)
Lean Less HCs More O2 Low (<450mv)
>>Exactly the opposite is true. More voltage indicates MORE oxygen.
>Hmmm... You sure about that? My FSM says otherwise:...
>Its possible that the FSM is wrong I suppose; stranger things have
happened...
Maybe, but not this time. As you can see from the table above, both
you and the FSM were correct on this! Less voltage indicates more
oxygen, and vice versa. The "less is more" deal can be hard to keep
straight sometimes. <g>
The basic function is that the oxygen sensor compares the oxygen
content of the exhaust gas mixture with the oxygen content of the
outside air, and generates a voltage that is proportional to the
amount of oxygen it detects. When the exhaust gas mixture contains a
HIGH amount of oxygen (LEAN mixture), the sensor outputs a LOW
voltage that is below approximately 450 millivolts. When the exhaust
gas mixture contains LOW oxygen (RICH mixture), the oxygen sensor
outputs a HIGH voltage that is above approximately 450 millivolts.
In my experiences, the output voltage from the upstream oxygen sensor
varies (transitions) back and forth with wide swings between
approximately 100 and 900 millivolts, while the downstream sensor
transitions are much narrower swings at a lower voltage overall, and
which vary between about 350 and 450mv.
>BTW: Where'd you find that info? Is there a good place to look this
stuff up?)
Yes there is. There is a FANTASTIC book on OBDII produced by Chrysler
for technician training. I don't have it here with me (which might be
why I misstated the voltage oxygen relationship), but tomorrow I will
be visiting the location where it is stored and I'll dig out the
info. It only costs a few dollars, and is an EXCELLENT tutorial on
OBDII. It has helped me on several occasions, with both Chrysler and
non-Chrysler vehicles since OBDII is an industry standard that is
non-marque specific.
>I already have most of the stuff I need to do it. :-) I might give
it a shot anyway, just out of curiosity.
Please do! There are several ways to skin a cat as they say, so your
approach may work and give us an alternative method.
>I was just planning on spitting back the voltage from the front O2
sensor, divided by ten. (simple voltage divider circuit)
An interesting approach, although a little more complicated
connection wise that what I have been using so far.
>How are you simulating the exhaust stream without knowing what the
front sensor is reading? From what you've said, I'm gussing you're
spitting back a voltage that varies occasionally?
Exactly. You just simulate an O2 sensor. At first it would be reading
a constant voltage, and then within a minute or so (after it heats
up), it should be transitioning much like the upstream sensor,
although it should be doing so more slowly, at a lower voltage, and
with MUCH narrower swings between min and max. Basically we have a
very low frequency, low amplitude oscillator, with a time delay
before starting. It is interesting that many of the PCMs now look for
the sensor to be non-functional at first, and then start functioning
as they warm up.
>I was planning to cannibalize the connector from the downstream
sensor for the module, but I also wanted a male and female which I
could use to make a nice neat plug-in up at the front sensor. Also,
in the future, I plan to make a mixture gauge which will read the
voltage from the front O2 sensor, so I'm going to want some of 'em
for that project too.
I know where one can get Packard connectors, used primarily by GM
(and somewhat by Ford and Chrysler), but I don't know about some of
the Chrysler stuff. I'll give you his number and you can ask about
the Chrysler stuff.
>Currently, I've got a set of JBA headers with 1.5" primaries which
feeds an ATR Y pipe with 2.5" arms and a 3" "collector", then a 3"
pipe to the back, where everything exits through a Gibson 3" single
exhaust.
Now that is what I'd call a "free flowing" system. <g>
>I haven't actually tested it yet (I haven't put it together yet),
but here's my basic idea so far, maybe with your prior O2 sensor
experience, you'll notice a problem I haven't:...
It actually looks interesting to me, I'd love to see it tested. Your
approach has the benefit of functioning exactly like a true O2 sensor
would, including the non-functionality during warmup which some of
the PCMs look for.
The thing is to keep the voltage transitions at a much lower
amplitude than the front sensor, and not just at a lower voltage
overall. I think you may need to modify your circuit for that
purpose, but then again I don't know what the threshold is for the
PCM to set a code, so your circuit may work as is.
>Speaking of scanners, do you know where I could find the protocol
for the OBD-II connector under the dash? I'd like to try building my
own scan tool, since I can't afford a commercial one.
The protocols are an industry standard which anybody can obtain I
believe. One problem is there are several OBDII standard protocols,
including ISO 9141-2 (the one Chrysler and virtually all Asian and
European mfrs. use), SAE(J1850) (used by GM I believe), and a couple
more. You should be able to get copies of these from places like
Global Engineering Documents:
Global Engineering Documents
15 Inverness Way East
Englewood, CO 80112-5704
(303)792-2181
(800)854-7179
(303)792-2192 FAX
They are a source for copies of virtually all industry standard and
engineering documents and SHOULD be able to get them, although I have
never tried to get these. Let me know if indeed they have the OBDII
standard documents and how much they cost.
>I don't care for exhast clamps because they crimp the pipe and make
it impossible to take apart... :-( That's the reason I have my 2.5"
-> 3" adaptor still in my system. I tried to take the crimps out with
a pipe expander, but no dice. I'm going to have to cut it off. :-(
I like the band clamps because they make a nice seal and they don't
crimp the pipe. They do allow it to rotate though.
The standard clamps will only crimp the pipe if you overtighten them,
like the factory ones were on my GC. With 13 miles on the truck I
couldn't separate the pipes even though I heated them orange with a
torch, they were crimped together so firmly. I had to cut them apart.
When refitting a new pipe, I used a pipe expander to remove the crimp
from the factory exhaust. I got an industrial strength one from Lisle
tools which works with an impact gun, the manual ones I had were
useless on the heavy gauge factory 409 stainless tubing. To insure a
crimp-free seal which will not rotate even though I have anti-seize
on the joints, I used two clamps per side and tightened them just
tight enough to prevent rotation. I have no crimps and can easily
separate the pipes in the future.
A friend of mine who owns Casper's Electronics has been manufacturing
O2 sims that work well for a couple years now. Call John Spina at
(847-247-0484) or email him at <mailto:caspers@wwa.com>, tell him I
sent you. He could comment further on your simulator design, or sell
you a functioning sim if you just want to get on with it. <g> He also
sells the Packard weatherpack connectors, and may be able to help you
with Chrysler ones, although when we made up my sim, we cannibalized
my downstream sensor for the plug. Scott.
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