Grumpy

This oughtta be mandatory reading for subing here, IMWO..
Please note 300dF EGT, just scrambled the results. And that they are backpressure sensitive.
In case you don't know Frank, he does the data gathering on the GM C5 race cars.

This is a guestion, and Franks Response

06 Dec 1999
Frank F Parker
gnttype
> 02's are good until around 19-20 psi. then it starts to drop. o2's end
> up at around 675-750mv. this seems to be lean.
> @ the 18psi program the o2 will hover around 825-925mv
> I would like to see them around 1000mv. will turning up the FP solve
> this????
> How high FP can be run on the stock fuel rail and injectors????
> I do not run this program currently BC I think it needs some more fuel.
> i.e. 1000mv
> I would like to run more boost but not until I get the fuel to go with
> it.

I would run a higher base pressure, maybe as much as 10 psi more.
Higher pressures are ok, especially if you have the hp version of the
255L pump.

I was gonna post this later when it was more complete but it may help you.
I measured the reg O2 sensor voltage 2 ways. First with Diacom like we all
do, then at same time also hooked to 8 chan datalogger which gives very
accurate numbers. Also at same time was measuring wide-band O2 so I know
real a.f numbers. Other channels measured boost, fuel press etc etc.

Chart below shows NTK a/f vs measured O2 vs Diacom O2 As others have
soon before, here and on GN list, there is a slight offset to Diacom
values. Not really important since everything is relative.

NTK O2 O2 VOLTAGE Diacom O2 volt

9.9 to 1 0.895 0.982 data point
11.0 0.847 0.941 calculated
12.16 0.807 0.845 calc.
14.4 0.608 0.655 data point
12.8 0.829 0.916 actual
12.25 0.885 0.982 actual

The reason some of the Diacom points are calculated is that Diacom takes
data much slower than the logger.

Biggest thing to see is that the same Diacom point of 0.982 would seem
to be same a/f if you take O2 volt seriously BUT is really 9.9 rich
spike at tipin and a 12.25 point in 3rd gear. Plus notice the 2 points
show 0.01 volt diff from the logger (0.895 vs 0.885)- that is because
the logger has a 12 bit adc . The logger data does reflect the a/f
difference between the 2 a/f #'s of 9.9 and 12.25 but does not correspond
to the differences that the O2 voltage seems to show earlier. ( eg: .895,
.847, .807 trend) .

The difference I suspect may be in the O2 voltage vs temp effect that
has been noted before. The EGT was 1199 deg F when Diacom reported
a O2 value of 0.982 ( widerange=actual 9.9 a/f) and was 1576 deg F
when later in the run Diacom reported the same O2 volt of 0.982 and
widerange was =12.25 a/f

This is interesting stuff BUT PLEASE take everyones warning and be
caerefull about trusting O2 sensor voltage. It is helpfull but as u can
see, can be misleading

Jon88GTA

Very interesting, thanks for sharing. This may help explain a phenomenon noted a while back on the exhaust board. Several listers installed free-flowing exhaust systems and lost power (especially at low RPM, and at low throttle opening). Installing a restriction into the exhaust rememedied the problem.
Is the wide-band O2 sensor accurate regardless of EGT and backpressure?
While we're on the subject, when does the ECM rely on O2 voltage?

Grumpy

Oh boy, that's a whole nutter bucket of snakes. Backpressure has alot to do with the self EGR'ing of an engine. At low speeds especially with bigger then stock cams, when both valves are open there is alot more backpressure relative to manifold vac., so a far amount of exhuast is drawn back into the cylinder, during overlap at small throttle openings. Adding a little fuel and some timing, can help some.
On some codes the ecm does a backpressure calculation. I forget the math in that one, but we'll just say it's clever.

The WB's *heater*, which is a critical part of the design, operates at higher then EGT, so it's not really effected by it. I think at 2,000dF, there maybe a slewing toward lean of .05% or something like that, so even the error just throws you in the cautious side, and only by a slight amount.

The ecm uses the O2 voltage when in Closed loop.