<VV> Gas Mileage

[Brent Covey]

Frank wrote;

 >   Since our gasoline is changing to 10% ethanol, you're correct
> that there would be a drop in fuel mileage from the lower energy
> value of ethanol compared to straight gasoline.  However, ethanol
> has about 70% of the heating value of gasoline so with 10% ethanol
> in the fuel, the OVERALL heating value would be expected to drop
> only about 3%.

On the face of it, this should be the case, but on cars with fixed metering
systems (carbs that have no feedback provisions for mixture calibrations)
you generally discover a somewhat exagerated effect when fuel energy drops
for a given volume- the weaker fuel tends to knock you out of a zone where
cruising can be maintained at small throttle openings and maximum ignition
advance, in essence, you have to put your foot in it farther, more often,
and make more adjustments to maintain your originally desired
speed/performance. These exaggerated motions reduce efficiency if they
happen much.

Forcing open the enrichment circuits in the carbs, killing the vacuum
advance off and sometimes requiring small changes in calibration (jetting,
initial timing advance etc) which can impact mileage in a cascading fashion;
when you need a small increase in output, say for a little headwind or
slight grade, you have to really lean on the pedal compared to the more
robust fuels, to get the original power that was availible with a smaller
motion, and can at once lose much or all of the vacuum advance, open the
enrichment ports if they arent already, squirt fuel from the accelerator
pumps, and may be doing this with the original timing slightly reduced from
'optimal' because the slightly leaner fuel mixture pings a little easier
despite the enhanced octane in the ethanol.

You end up running with a pingy mixture, heavier throttle, less advance and
in a badly metered zone of carburetion more frequently, in short. How big an
impact this has will vary somewhat car to car of course, and with the
specific circumstances of your driving style, speed and conditions.

Experience driving in California a few years back with the ~14% MTBE fuels
in cars with known calibrations suggested the power loss was comparable to
reducing main jet size about 3-4 sizes. Cars with feedback systems like
oxygen/knock sensors can make minute adjustments to ensure mixtures stay in
the ballpark and timing etc is less affected and will generally see a
smaller impact from fuel energy variations.

Small losses of fuel energy have a somewhat disproportionate negative impact
on cars that were originally calibrated for a higher energy fuel. The most
practical solution is re-jet a little richer (about 3 greater than original)
to restore the original power and this usually has the least impact on
overall economy, rather than leaving things too-lean for the newer fuels.

Different areas have different blends, so a little experimentation is
usually needed to find optimum calibrations for a specific car in the area
its normally operating in.

Brent Covey
Vancouver BC