The point is to NOT allow it to drop back to idle. You should be revving
the engine to or slightly above the point the lower gear "expects". The
idea is that it's less wear for the gearbox to slow the engine than it is
to speed it up while matching speeds. It's better to be a bit high than
a bit low. When you're downshifting, the revs will always be higher for
the new gear (unless you coast quite a ways and let everything slow down).
Ideally:
clutch in (revs begin to fall)
gear lever to neutral (revs continue to fall)
clutch out (revs still falling)
apply throttle (revs now above speed required for new gear)
since the clutch is out, the input shaft is now spinning at just
above the "right" speed for the new gear as well
clutch in (revs falling again, but from a higher point)
gear lever to lower gear (revs fall to the "right" point)
synchro doing little to no work since input shaft already
spinning at matching speed
clutch out
no slip, since engine spinning at speed matching input shaft
In practice, you usually end up doing something like this:
clutch in, blip throttle, gear lever to neutral (one motion)
clutch out (with revs way above what's required)
clutch in, gear lever to new gear
clutch out
OR
clutch partway in, neutral, blip throttle, new gear
All timed so the blip happens just as the lever passes through neutral.
This trades some clutch wear for less synchro wear, and a smoother shift
(less wear on donuts and CV joints). Done right, there's not much clutch
wear, either.
james montebello
On Tue, 6 Aug 2002, Joe Elliott wrote:
With all this talk of double-clutching, maybe now is a good time to
ask (again) if anyone can venture a guess as to why my GTV6 won't
drop back to idle fast enough to allow for effective
double-clutching. No, my throttle body does not stick, and yes, I
know my car has two flywheels.
Thanks,
Joe
'82 GTV6
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