I hope everyone is continuing to have a happy holiday season.
On my recitals towards LBC fuel economy, let me add the following
clarifications.
It is true that non-direct gears can suck more power than direct drive and
I also would not doubt that the Laycock overdrive lacks somethin in
efficiency. Still, for a constant power need, an engine turning at a lower
rpm will usually use less fuel than one at a higher rpm. If you assume
that you are below the peak torque rpm for the engine, then maximum power
output decreases with rpm so, for a given power need, you would be closer
to the maximum power potential at a lower rpm than at a higher rpm. This
means that the manifold pressure would be higher at the low rpm power
setting. So, for a given power need below the peak torque speed, reducing
rpm will increase efficiency in two ways- first by redusing friction and
second by reducing pumping losses with an increased manifold pressure
(lower vacuum). Will that make up for changes in inefficiencies of the
gear train? Who knows.
Someone asked about tuning an engine to maximize the vacuum signal at the
PVC port. If my memory serves me correctly, this port opens between the
throttle butterfly and the dashpot slide. If your carb is working
correctly, this area should always be at the same vacuum reading since the
job of the slide is to maintain a constant depression across the metering
jet. You would see some fluctuation during throttle position changes due
to the dashpot oil but at constant load and speed this signal wouldn't tell
you much.
Regards,
Bill Eastman
61 MGA
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