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Double clutching is a method of matching RPMs during shifting.
Some claim that it is useful for up-shifting and down-shifting, but logically, it only makes sense when downshifting.
There are three advantages to double clutching when down-shifting. First, no sudden jerk as you downshift, second, less wear and tear on your transmission, and third, (and most importantly!), you can really maximize the power of the turbo on windy roads where speed is changing a bunch or when passing on a two lane highway.
Here's how it works:
You want to downshift. Normally you would push the clutch in, shift, release the clutch. Usually there is a minor jerk as the rpm's surge in the new gear.
When you double clutch, the process is as follows: You push in the clutch and shift into neutral. Release the clutch. Tap the gas to bring the RPM's up slightly past where you want them to be, then you rapidly push the clutch in again and complete the shift by engaging the new gear.
Basically, by tapping the gas, you are doing the work of the synchros for them by bringing your RPM's up to where they need to be. The reason you want to go slightly past the optimum RPM because in the split second it takes to push the clutch in the second time and shift into gear, your RPM's will start dropping again, so that hopefully when you engage the gear, you're right on track.
So what is the "optimum RPM" for downshifting? It just takes getting a feel for it. Go out on an empty road and go a fixed speed in 4th, and try shifting into 3rd normally with your eyes on the tach. Where do the RPM's jump to? That number is where you want to be when you're double clutching. Do another pass at the exact same speed, only this time double clutch, and tap the gas just enough to bring the RPM's up to that number before shifting down to 3rd. The less jerk you have, the closer you are.
Obviously this all becomes second nature and very fast. I double clutch so quickly and habitually that it feels weird to me to NOT double clutch. Until you get to this point, however, one thing to keep in mind is that when you're tapping the gas, that's all you're doing - not holding the pedal down - it's very easy to bring your car up to redline when you're in neutral if you're not paying attention. Once you do get it, your passengers will think you're driving an automatic your down shifts will be so smooth.
So, how does it help on the windy roads and when passing? It all comes down to the turbo. This is the fun part. Since the turbo kicks in at higher RPM's (peaking around 3500-4000), when you need a sudden burst of acceleration, you can downshift (double clutching of course) and bring the RPM's way up into the turbo's useful range. For example: If you're driving 55 in 5th gear behind a tractor trailer and you want to pass, you will have to hit the gas hard and then wait as the RPM's increase to get the turbo boost you need. Alternatively, the double clutch: down shift to 4th, still at 55, now the RPM's are much higher and you can harness the power of the turbo to zip on by. On the windy roads it works best for accellerating hard out of corners into straightaways.
Good luck!
posted by 64.235.16...
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