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Rev Matching VS. Double Clutching
Hey gang. Found this on another forum. Enjoy.
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Rev-matching and double clutching are two different things that accomplish two different purposes:
Rev-match: When you down shift, put the clutch in and move the gear lever to the next lower gear. This takes time, and the engine is not connected to the transmission because the clutch is in, so the engine speed starts to fall back to idle. Worse yet, when you down shift, the engine will end up going faster than when you started. At 35 mph in 3rd, your engine might be going 2500 rpm, but at 35 mph in 2nd, the engine speed would be 3500 rpm as an example. What you do when you rev-match is use the gas pedal to get the engine from 2500 rpm to 3500 rpm. gear ratios and engine speeds were not really calculated, so the numbers above are simply an example, not actual engine speeds at 35 mph.
Instead of slowly letting the clutch out to spin the motor up match to the speed the wheels are going (3500 rpm in 2nd in my example), you leave the clutch in and tap the gas pedal to "blip" the throttle. This spins the engine up to 3500 rpm using gas in the engine, not the clutch. When the engine is at the proper speed (3500 rpm in this example) let the clutch out. If you do it correctly, there will be no jerk in the car when you let the clutch out. This is the smoothest way to downshift. Once you get the hang of it, it becomes a simple fluid motion.
Double-clutching: is an extra step in a rev-match down shift. The input shaft of the transmission is connected to the clutch and the clutch is connected to the engine. The output shaft of the transmission is always connected to the wheels. So when the clutch is in, the input shaft of the transmission is not connected to anything. The output shaft is always connected to the wheels so if the car is moving, the gears are turning. Putting the clutch in disconnects the input shaft, and it tends to slow down when it isn't connected to anything. Without synchronizers, you would hear a crunch in the gear when you move the gear lever to the next lower. This is because one of the gears is moving (output side) but the input side is not moving. So you are engaging a stationary gear with a moving gear- CRUNCH!
The double-clutch uses engine speed to spin the input shaft of the tranmsission so that the input side and the output side of the gear you are going into are matched, eliminating the crunch.
Here's how you do it: Put the clutch in. Move the gear lever to neutral. Let the clutch OUT momentarily. This connects the input shaft of the transmission (in neutral) to the engine. (Remember that the output shaft of the transmission is always connected to the wheels, so it is moving at a speed proportional to the wheel speed.) Blip the throttle to match the engine speed to the wheel speed. Doing this with the clutch OUT spins the input shaft of the transmission at the proper speed to match the gear you are trying to engage. Now put the clutch back in and move the lever into the next lower gear. Let the clutch out. Like the rev match, this should be one fluid motion, and it takes practice.
Keep in mind the synchronizer also performs the function of spinning the input shaft to the speed of the gear you are trying to engage. So with a synchronized transmission, double-clutching is redundant. The synchronizers already match the speed of the gears so they engage. You don't have to do this yourself with the double-clutch. If you have good synchronizers, double-clutching is a waste of time.
However, keep in mind that double-clutching is from back in the era when cars didn't have synchromesh or the synchromesh was fragile. Newer cars have synchros to match gears between shifts and thus negates the need to double-clutch.
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