What is double clutching? Technique for rev-matching the transmission’s intermediate shaft to the output gear that is to be selected.
How do I double clutch?
1. Power is removed and clutch is disengaged (clutch pedal down).
2. Transmission is shifted from original gear to neutral.
3. Clutch is re-engaged (clutch pedal up). (Driver now has control of intermediate shaft speed by controlling engine speed).
4. Driver blips the throttle to match intermediate shaft speed to speed of new gear. (This takes practice to get the right match).
5. Clutch is disengaged (pedal down).
6. Transmission is shifted from neutral into new gear.
7. Clutch is re-engaged (pedal up) and power is applied.
Steps 1- 3 can be done casually or quickly. Steps 4 – 6 must be done quickly so that the intermediate shaft doesn’t slow down again before it’s engaged. If step 7 is also done quickly, the engine will also be ‘rev-matched’ to the rest of the driveline so that engagement will be smoother.
When should I double clutch? The only instance one may need to double clutch would be during the 2-1 downshift on a 02-04 WRX. Many find this a useful technique for a smooth 2-1 downshift due to the WRX’s transmission design flaw, or in any gear where there appears to be problems with the synchro. The 2004+ STi and the 2005 Legacy/Outback have different first gear synchro system and do not experience the same problems when performing the 2-1 downshift. Otherwise, this is really an advanced shifting technique and is not required for the average or above average driver in most cases. There are those of the opinion that double clutching will save wear and tear on the transmission synchros though.
What is flat shifting? In a nutshell, flatshifting is where your right foot never leaves the gas pedal. You quickly engage the clutch at the shift point, shift gears, and side step the clutch. While terrible on an OEM gearbox, this can safely and effectively be performed on some aftermarket gearboxes for improved racing times. Some dogbox transmissions also take well to another method. This method requires a slight lifting of the gas to unload the dogs, a lightning fast shift, then back fully on the gas. Though you technically can perform either of these two shifting actions in an OEM synchro equipped gearbox, it falls along the same lines as parachute packing. Yes, you can do it yourself, but it’s best left to professionals.