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Old 08-12-2009, 10:46 AM   #82 (permalink)
Bearleener
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Join Date: Mar 2008
Location: Berlin, Germany
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Frugolf - '96 VW Golf wagon
90 day: 40.36 mpg (US)
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Engine-off coasting with a manual transmission:
Shouldn't it be better to leave the transmission in gear and hold the clutch until the bump start?
Seems as if synchronizer wear would be more of a problem than clutch throwout bearing / pressure plate wear. Of course, by now my left leg is a bit shorter than the right one from all the clutch holding.

For synchronizer see PaleMelanesian's comments above. When the car is rolling with the engine off and you put it into gear, the tranny has to synchronize the engine's 0 RPM with the tranny's whatever RPM; not easy. If instead you simply disengage the clutch and turn the engine off without going into neutral, then when you want to restart the gears are already meshed and the synchronizer isn't needed.

For the clutch have a look at the illustration on HowStuffWorks "Fly Wheels, Clutch Plates and Friction"
When the engine is off, everything in the clutch is stationary except the clutch plate and output shaft to the tranny. That is, the throwout bearing and pressure plate are not grinding against each other (no wear), in contrast to when the engine is on.

Furthermore, if you leave the car in gear during EOC that's one less clutch press-and-release cycle (unless you pop it out of gear without the clutch), i.e. less usage of the throwout bearing.

Then of course there's the safety issue of being able to quickly restart the engine even when both hands are full (e.g. swerving).
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