Fuel Econ and Hiway Mode

[Bob Valentine]

With the seemingly renewed interest in fuel economy lately around here,
I've spent some time messing with Hiway Mode in the '730 / 8D / AUJP
calibration.   Seen mention of it in the archives, but no details of the
settings.   With 93 Octane pushing $1.90 around here, want to get as much
as I can for what I pay.  

Here is what I found in the ANHT_HAC and what I've done.  None of it would
have been possible without the ANHT_HAC,  thanks (for the 10^6 time) to
whomever wrote it.

$81A7  Min coolant for Hiway Mode	moved FFh to 76h  (70deg. C)
$81A8  Max kpa for Hiway Mode	moved 2Bh to 32h (50kpa)
$81A9  Min RPM for Hiway Mode	moved FEh to 40h (1200 rpm)

$8482 	Lockout time for Hiway Mode	moved 0Ah to 01h (1 second)
$8484	Min speed for Hiway Mode	moved FFh to 01h (1 mph)

Table at $8485 for A/F vs. Kpa
Table at $81AB for S/A vs Kpa

The results are pretty cool.    As soon as all conditions are met, 02
sensor voltage drops to around 75-85 mv, and stays there until
acceleration.   Integrator locks at 128, and no error codes set.     Having
a WB02 would be nice here, since I already know my 02 doesn't read 14.7 A/F
correctly.    Plugs do not show any detonation or burn signs.   Transitions
to power are just fine, butt-o-meter indicates a slight improvement.  

Keep in mind I run no cats; I'm not sure what extended lean A/F ratios
would do to a cat but I doubt it would be good.    I haven't done enough
highway driving to make any definitive statement as to improved mileage,
but mixed driving is up a good amount, 150 miles and still 1/4 tank left,
usually refill time by now.

Initially I was planning on trying to add EGR back in, but I thought about
it and couldn't see how diluting the A/F mixture would help economy.
Also, turning on Hiway didn't require adding any wires and hoses - just
changing bits. 

-> Bob Valentine
-> bob at tecmark.com
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