why does retarded timing increase MPG?

[DanHoug]

so i'm pondering this alot.... the IDI fuel quantity is a direct function of engine RPM and the position of the fuel metering valve throttle, correct?

so why, when advanced timing results in more power does it not give better MPG than a retarded timing setting? seems for a given RPM and throttle position, the timing setting that gives more power would result in better MPG because you would have to use less throttle to maintain forward speed against frictional resistance.

i'm missing something here...

-dan

[bbob203]

Dan,
How have you arrived at this conclusion? My timing being slightly more advanced always gave me a gain in mpg's

[DanHoug]

this might be the crux of my issue.... i was under the belief that you timed for either power or economy.. advanced for power or retarded for economy. if this construct is wrong, then my question is answered!!

-dan

[libbybapa]

The timing that gives the most power gives the best fuel economy also.

[DanHoug]

well, what the heck?!! now that makes sense.

where did i get the idea you timed for either power or economy? sheesh.

-dan

[Quantum-man]

What happens to fuel economy with alteration of timing depends on whether you are near the optimum position in the first place, or near the extremes of operation.
Retarding can give more power, because more heat going out of the exhaust [with a TD] results in faster spoolng of turbo, which can ask for more fuel from the pump, and more power quicker... but not so efficiently.

Advancing the pump 'too far' can mean a more efficient combustion, but will be fighting the still ascending piston and crank, thus less efficient conversin to torque and also with less heat 'wasted' to drive the turbo, slightly less acceleration. Adding more fuel in this case can hide the lack of heat in the exhaust, giving more power but drop economy.

All very complicated, but wrt economy there is a place where most energy goes into torque, and less into coolant, and just enough into exhaust that turbo runs at 1 or 2 psi on cruise mode.