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The part of this that does not make sense is turning back the timing.
I am sure that anything you will gain in fuel mileage by going lean will be lost by retarded timing.
For fuel economy you normally want the timing to be advanced as much as possible.
This would probably be in the 35 - 40 range at part throttle cruise at high vacuum.
The best way to achieve this without getting into detonation under high load is with a vacuum advance.
The simplest method of tuning for fuel economy is usually done with a vacuum guage.
The settings that will give you the best fuel economy will usually result in the highest vacuum readings at a given steady state speed. Do your testing on the same road under the same conditions.
There are also fuel flow guages available at fairly reasonable prices, normally used in aircraft and boats, that will give you instantaneous fuel use figures in gallons or liters per hour, as well as total gallons or liters used over a period of time. The ones for fuel injection are a little more complicated as they need two senders, one for the fuel delivery and one for the fuel return line - mini computer in the gauge subtracts one from the other. But this is the way to go if you really want to play with tuning for fuel economy.
You should find that with both going lean and advancing the ignition timing
you will get an improvement up to a point at which further changes in the same direction produce no additional improvement and then the economy will get worse.
John
V-Performance.com
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