By Christian Millard
cmillard@crutchfield.com
last updated 9.21.2001

Test vehicle: 1995 Chevrolet Camaro Z28
Engine: 5.7L LT1 11.4:1 compression, LT4 HotCam (219/228 .525/.525 112)
Injectors: GM Multec 24 lb @ 43.5psi
Exhaust: JetHot Coated 1 3/4" Shorty headers, y-pipe, Flowmaster American Thunder single pipe
Plugs: AC Delco R42LTS gapped at 0.040" ( 1 step colder, for higer compression use)
Wires: Taylor 8 mm over the valvecover custom wires
Ignition: MSD 6AL ignition box

I started this experiment to determine what effect variations in spark advance had on the LT1 at idle.
I believed this to begin with:
1. Higher vacuum means better gas mileage, GM uses the MAP sensor to read the intake manifold vacuum. MAP form of measurement is KPa.  Vacuum has an inverse relationship with MAP KPa.
Max vacuum = Min Kpa,
Min vacuum = Max Kpa,
2. I believed that more spark lead (spark advance or lead time) consumed more fuel, I found this wasn't the the case at idle (see details below.)

This chart is Spark Advance vs. Map. 
We want the lowest MAP because that represents the highest vacuum, which occurs around 34 Kpa
All modifications were done to the Closed Throttle Spark Advance table.
The lower spark values made the engine feel more tame, smoother, yet surprisingly the higher the timing got (up to 36 degrees) the lower the Kpa, the better the vacuum. 
On a side note: I watched with amazement as the engine temperature dropped when I went to the higher spark advance, I'm used to running 173F, it dropped down to 166F sitting at idle.

This chart below is Injector pulsewidth versus spark advance.
Lower pulsewidth for the same rpm indicates that less fuel is needed to maintain stoich (14.7:1 Air to Fuel ratio)
Again 34 degrees advance is the sweet spot of the injector pulsewidths

This chart below shows MAF (airflow) vs MAP vs Spark Advance
Less airflow at the same rpm  means less need for fuel, better gas mileage.
Notice the Maf dropps down to 9 gm/s at 29 -37 degrees, so again 34 degrees is a sweet spot.

The chart below show Spark Advance versus L BLM and R BLM. I was surprised to see the BLMs balance around 19 degrees spark advance, very interesting. 
The generally higher BLM's indicate the PCM is reading a lower than normal resistance from the O2 sensor, meaning a leaner mixture. 
above 450mv indicates a mixture above (richer) than 14.7:1
below 450mv indicates a mixture below (leaner) then 14.7:1

Lower resistance indicated right off the bat that there isn't a wiring problem, simply because wiring problems give higher resistance, making the computer think it is rich.  Instead we have too little resistance.

A lean condition can be caused by any of the following:

• spark plug misfire/ fouled
• injector clogged
• exhaust leak (doubtful)

In conclusion, my hypothesis that more timing consumes more fuel, was incorrect to a point.
It appears that more timing burns fuel more efficiently, requiring less fuel and less air at idle.