Quote:
Originally Posted by wheelhaus
Rev matching is for the engine to re-engage the clutch at match speed, it does nothing for the synchros (because they're on the other side of the clutch) unless you're double clutching a downshift.
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I am trying to visualize what you are saying there and it makes sense.
On an upshift -
When you press the pedal and release the clutch (engage the throwout bearing), you decouple the engine from the trans input shaft.
Then when you pull the trans out of gear the output shaft and the input shaft are now free to spin independently.
The output shaft will keep spinning at the previous rate because it is connected to the rear wheels.
However the input shaft is not hooked to the engine, or the output shaft.
It is spinning under it's own momentum, and slowing (quite rapidly BTW).
Now you attempt to press into the next gear.
The input shaft needs to be spinning slower now to engage with the next gear up and the synchros will provide the friction to match the speed of the input shaft to the output shaft (now through a different gear ratio).
If you shift slowly then the input shaft may have slowed too far and the synchros have to speed UP the input shaft.
If you shifted really quickly, then the synchos will have to slow down the input shaft.
IF you wait exactly the right amount of time, the two shafts will be spinning at the right rate to engage with NO PRESSURE on the synchros.
SO long story short, there still is a rev matching going on with the clutch disengaged, it just has nothing to do with engine revs.
This is also why you can shift with no clutch.
When you upshift, it is easy to push the trans out of gear if you are not applying any load (not accelerating).
Then as you ease it into the next gear (clutch still engaged - attached to engine, engine revs falling) you put pressure on the synchros, but it wont let it in gear until they are matched. As the revs drop naturally, the gears will match up and it will slip right into gear with no grinding.
I dont do that regularly but I demonstrated it to my teenager while teaching him to drive stick, and he gets it perfectly.