VE stability

["John S. Gwynne" <jsg@jsgpc.mrcday.com>]

I've been collecting tuning data through the ALDL now for 3 1/2
months and correcting my VE table with BLM and O2 data ('730 5L sbc,
sc, cam, headers, axxc bin).

using data only during close loop mode and testing to ensure a
more-or-less steady state with rpm and map corresponding to the
correct blm cell (aldl frame are not a snapshot of a given instant of
time), I do a long term average (i.e., more than 10-20 hours of engine
run time) to correct the VE table (letting transient errors average
out over time). There's a little more in terms of weighting and
smoothing, but that's the jest of it. As a result, IMHO the car runs
better (drive-ability) and BLM cell counts are always less then +/-6 overall
and under +/-3 for some conditions. Now the real question....

I looks to me as if something is varying with operating conditions to
prevent me from narrowing this last 4% of error (it's not that much,
but it has my curiosity). The two conditions I see this best are 1-2 hours of
around-town driving versus 4 hour of highway mixed with 1-2 hours
of around-town. It looks to me like something in the system is changing
characteristics after a couple of hours of drive time.

Has anyone else seen this?

A few of my thought of what may be going on.... appreciate comment and 
suggestions....

-- baro measurement screwed up because of sc 
I'm collecting data on this now. The condition does seem more
pronounce at partial throttle conditions.

-- EGR
EGR partial pressure correction needs updated for exhaust system

-- inlet air temperature reading inaccuracy
MAT sensor may be influenced by block/manifold temp

-- engine/cylinder conditions
carbon in the cylinders changing the burn after highway driving

-- sensor temperature sensitivity
MAP sensor may be somewhat temperature sensitive (1-2%)

-- limitation of the tuning procedure
maybe, but it does look like it in the data.

-- O2 sensor
temperature/accuracy sensitivity of the O2 sensor.






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