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Old 06-22-2009, 09:32 AM   #12 (permalink)
Mechanical Engineer
Join Date: Jun 2008
Location: Richmond, VA
Posts: 190

The Truck - '02 Dodge Ram 1500 SLT Sport
90 day: 15.15 mpg (US)

The Van 2 - '06 Honda Odyssey EX
90 day: 22.63 mpg (US)

GoKart - '14 Hyundai Elantra GT base 6MT
90 day: 30.43 mpg (US)
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I would lean toward the valve adjustment, providing that there was too much valve lash and they made the necessary adjustments to reduce lash.

I know from testing with other Honda engines (20hp air-cooled v-twins) that they run very differently in response to valve lash. An engine with more lash reduces effective valve lift and valve overlap. Engines with lower lift and overlap operate more efficiently at low engine speed and will idle at a higher speed for a given throttle opening. Reducing lash will reduce idle without adjusting the throttle opening and will move the more efficient power band up the RPM range. For someone who drives for fuel efficiency this is counterproductive.
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