At 12:35 PM 4/13/99 , you wrote:
>OK, lets say we have an R/T with a ported throttle body (52 millimeter or B&F
>Stage 2 equivalent), Mike Leach headers, low restriction cat back exhaust,
>and a high flow air filter. This will increase the airflow into the motor
>substantially. At most RPMs the oxygen sensor will sense the additional air
>and widen the pulsewidth of the factory fuel injectors and match the
>additional airflow with the correct amount of fuel. Does any one know if the
>O2 sensor works at Wide Open Throttle? I don't think it does... it goes to
>open loop and the injector's pulsewidth is changed yet again to work at the
>factory set "fuel tables" which may at times be less fuel that needed as the
>factory settings will not know that you added these additional airflow
>devices.
Yep. Well, technically, the O2 sensor does work at WOT, but it isn't
used by the computer to determine the amount of fuel to spray into the
engine. The PCM does indeed go into open loop at WOT. (I have found that
it also goes into open loop during decelleration, such as when engine braking.)
> There is the possibility of adding too much air and the factory
>computers "fuel tables" will not provide enough fuel... leading to a possible
>lean condition that can break a motor. Does anyone know if this is possible?
Its possible, but I doubt that you'd come close to this on a naturally
aspirated motor. I have OBD-II diagnostic software which allows me to plug
my laptop into my truck and monitor various parameters in the PCM. I did
some research for Frank and Bruce specifically to determine wether or not
leaning out with a larger TB was going to be an issue. Basically, I did
this by running at WOT with various combinations of computers and throttle
bodies, while graphing the upstream O2 sensor's output.
An O2 sensor ranges from 0 to 1 volt, and is basically inversely
proportional to the amount of oxygen in the exhaust. 0v would mean 100%
oxygen (full lean), and 1v would mean no oxygen (full rich). As most folks
know, you want to be at stoich (14.7 parts air to 1 part fuel.) That's
about .45v. Unfortunately, there's no way to measure the exact A/F ratio
with a standard oxygen sensor. Sensors are available which are accurate
enough to do this, but they can cost several hundred dollars. Basically,
the only thing an automotive sensor is good for is to tell you wether you
are lean or rich. In closed loop, the computer will spray extra fuel until
the O2 sensor voltage crosses a certain threshold somewhere between .5v and
1v, at which point the engine is considered rich, and the PCM cuts back on
the fuel until it is lean (sensor crosses a threshold between 0v and .5v),
then back rich again, etc... (This threshold varies from engine to
engine.) However, during open loop (such as at WOT), the computer uses
lookup tables to determine the proper amount of fuel. Its actually pretty
interesting to watch... The graph of the O2 sensor is all over the place,
but when the pedal goes down, it pegs at one voltage and stays there,
varying only by maybe .01-.02 volts.
Unfortunately, I didn't have a stock TB to test with (I had already
ported mine), but the "leanest" my truck went was .88v, certainly well
above the .45v stoich, even accounting for some error. I tried all
possible combinations of my ported TB, the Stage II, my stock PCM, and the
MP PCM. The .88v was with my stock PCM. The MP PCM usually had the O2
sensor pegged at .92v. Obviously MP has built a bit more leeway into the
MP PCM. Perhaps trying to compensate for bigger throttle bodies, blowers,
and the like?
I have recently purchased an Intellitronix digital air/fuel guage. I
haven't had a chance to install it yet, but it should be interesting to see
what results it will give me, and to see how well it corresponds with the
signal that the PCM is getting. (I just haven't had the time to install it
yet.) Hopefully it will be fairly accurate, so I can monitor the A/F ratio
under hard accelleration. (The connection to the data link connector is
severed by the PCM under hard acceleration, so I can't use my laptop to
monitor anything under those conditions.)
Some data on the intake/exhaust on my truck: 1996 318ci V8 with stock
internals, stock fuel injectors, 14x3" open element air can with a K&N
filter, JBA "ram horn" headers with 1.5" primaries, ATR mandrel bent Y pipe
with 2.5" arms that join to form a 3" collector, then its 3" pipe back to a
3" Gibson muffler (straight through, with baffles), and 3" mandrel bent
pipe out the back. (no cat) Both of the throttle bodies that I used (my
home ported & polished version and the Stage II) probably flowed somewhere
between 700-750cfm. So, its a fairly well breathing low restriction
system, but it was still fairly rich at WOT.
-Jon-
.--- stei0302@cs.fredonia.edu ------------------------------------.
| Affiliations: DoD, EAA, MP Race Team, NMA, SPA, USUA. RP-SEL |
| '96 Dodge Dakota v8 SLT CC (14.58@93.55), '96 Kolb FireFly 447 |
`----------------------- http://www.cs.fredonia.edu/~stei0302/ ---'
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