Gas mileage and headers

[Marteney, Steven J.]

Well, the addition of 1 3/4" SLP headers to my 89 Formula 355 (165 ECM,
MAF), ZZ3 cam, flat-top 10.2:1 piston ride has dropped my hiway mileage a
tad.  I haven't formally checked it yet but today I would usually have 1/4
tank showing on the gauge for my ride home from work (40 miles) and I only
have less than an 1/8.  Mileage at last fill-up was 17.3mpg, but that had
some in town driving in it so I discounted it.  Now with the gauge on empty
when it's normally not, I'm puzzled.  Will be checking mileage in about 2
hours when I leave work but wanted to get this email out for review asap.
Mileage had not varied at all prior to the headers.  Well, it varied, but I
was always able to make it home at this point in the week.  So, I guess my
BROAD question is how can headers adversely effect gas mileage?  I would
think it would be the same or better.

I've been scanning with Diacom on rides to and from work and the results for
75mph, 2300rpm with cruise on are:
1) BLM actually DOWN maybe 1 count to 121-122, was 128 before the headers.
2) BLI about 125-127 and varies +/- 8.
2) Pulse width the same as before, about 3.1-3.2msec
3) TPS voltage the same or less at 1.04V (was typically 1.04V to 1.08V)
4) MAF reading about the same
5) Idel Air Position at 129, up from previous 120 (still don't know how to
read that.)
6) Bumped base advance up to 10, was at 7.  No audible knock, no spark
control counts.
6) A/C now off, whereas before it was on

The last one I think is the biggest clue, while the car was laid up for 2
weeks whilst I put the headers on (don't ask) the weather changed here.
(Central Florida, from 85-90 during the day to high 70's.)  I've never
driven this car in anything but high heat and humidity, but I thought that
would hinder mileage more than this 50% humidity I'm experiencing now.
Also, the engine temps are way down.  It usually ran on the hiway at about
190F and now is around 179F.  Performance is WAY up after the headers.
Butt-o-meter is pegged and one of those home dyno thingies shows 30 ft-lbs
more torque and 10 more peak hp.

I'm just wondering if I'm merely fighting slight weather change or if the
headers have screwed with the tune.  I noticed the oxygen sensor (which was
new 6 months ago) gets located further down the pipeline.  Although, I can't
understand why the all the variables are staying the same above but mileage
decreases.  Do I need to go back to 7deg base advance?  (BTW - no trouble
codes either.)

Thanks for letting me think out loud.  Sorry for the length.  Any insight is
GREATLY, GREATLY appreciated.  Still learning... 

Steve
PS - In addition to the headers, I added a Random Tech converter.  Nice
unit, great sound.  :-)  Very happy, just paying a little more money for gas
than I think I have to.
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