Disclaimer: I like to bring threads back from the dead.
P.S. Drag racing information follows...
hock:
While the phrases "speed shifting" and "power shifting" are used interchangeably most of the time, some people still differentiate between the two. Both phrases have been bastardized and most people define them very vaguely or entirely incorrectly. Neither of the two techniques refer to completely clutchless shifting (at least not today). I'll talk about powershifting, since that seems to be the direction this thread has taken.
The trick is to keep the gas pedal held to the floor while stabbing the clutch pedal and simultaneously shifting into the next gear. Obviously the RPMs will be rising rapidly while the clutch is disengaged, so the stab and shift has to be FAST. If executed improperly, you'll roast your clutch. If you have no power, you'll bog your motor. If you're traction limited, then you'll burn some rubber. If executed properly with a setup that can benefit from a powershift, the clutch will engage and the revs will drop while the motor pulls the car up to speed. If you have the power and traction, and your driveline can handle the stresses of this technique, then you can save a couple tenths per shift. This is especially true for cars with a large turbocharger, as this keeps it spooled up between shifts. You can tell if someone is powershifting at the track when you hear the revs rise during a shift, instead of hearing them drop or even stay constant.
Some people only depress the clutch pedal halfway while popping the shifter into the next gear. This is probably because they're not fast enough to engage the clutch again before the revs shoot through the roof. Failure to fully disengage the clutch puts a ton of heat into the clutch disc and will glaze the pressure plate very quickly. The trick is a quick stab of the clutch. Clutchless shifting in a synchronized transmission is really not a bright idea.
