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      09-30-2011, 05:31 PM   #1
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6MT question

When I accelerate under full throttle and switch gears from 1 to 2 or 2 to 3, I always either jerk hard or skid the tires. What\'s the proper way to shift quickly near redline? When I shift at ~6800, second gear would land around say 6000... but when I shift fast, I release the clutch around 6500, so naturally the car jerks. How do I fix this? Does a modified CDV help? Thanks.
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      10-03-2011, 05:27 PM   #2
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It's all in the timing of the throttle and clutch and yes a CDV delete helps a lot. Really, they only way to get it right is to keep practicing. The goal is to have minimum clutch slip while getting back to full throttle as quickly as possible.
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      10-07-2011, 10:31 AM   #3
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is it possible to redline and shift smoothly if you dump the clutch?
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      10-07-2011, 11:09 AM   #4
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is it possible to redline and shift smoothly if you dump the clutch?
Only if you time it right. The reason why the car "jerks" is because the speed in which the input shaft on the transmission isn't spinning at the same speed as the engine side.

Let's say you're in 2nd gear at 6,000 RPM. In order to have a smooth shift to 3rd, your engine's RPM needs to be at exactly 3,950 RPM to engage 3rd. The formula is simple:

((next gear's ratio)/(current gear's ratio)) * RPM

It's simply timing and practice. You can either time your clutch release at exactly 3,950 RPM for the shift between 2nd and 3rd, or you can release your clutch smoothly to let some slippage smooth out the transition.
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      10-08-2011, 11:58 PM   #5
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that's a long wait before you're on full throttle again, though... there's no way to speed shift smoothly then? no trick?
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      10-09-2011, 01:19 AM   #6
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Quote:
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It's all in the timing of the throttle and clutch and yes a CDV delete helps a lot. Really, they only way to get it right is to keep practicing. The goal is to have minimum clutch slip while getting back to full throttle as quickly as possible.
+1..CDV and SSK is a very good mod..I had noticeable improvement w the CDV..
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      10-09-2011, 02:58 AM   #7
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Quote:
Originally Posted by jetblackcoupe View Post
that's a long wait before you're on full throttle again, though... there's no way to speed shift smoothly then? no trick?
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It's simply timing and practice. You can either time your clutch release at exactly 3,950 RPM for the shift between 2nd and 3rd, or you can release your clutch smoothly to let some slippage smooth out the transition.
I thought I was pretty clear. You can either time your clutch dump at exactly 3,950 rpm or allow the clutch to slip slightly. So if you release the clutch at say, 4.,500 rpm but do it slower rather than a dump, the shift should be as smooth as waiting for the rev to drop and then sidestepping the clutch.

It takes either practice or patience, and a combination of both.
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      10-11-2011, 09:01 PM   #8
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thanks, I understand what you're saying. But what I'm asking is ... let's say you're trying to get from 0-60 as fast as possible... disengage clutch at let's say 6800... by the time i have shifted and want to hit the gas again for optimal acceleration, the RPM has only dropped to ~6500... now by your logic I would have to really slowly let go of the clutch for slippage. All this "patience" will slow my acceleration... So how can I even manage to keep up with automatic cars? Or do I just have to jerk/skid for quick shifting?
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      10-11-2011, 09:14 PM   #9
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Your acceleration will not necessarily slow down. As long as the clutch is even partially engaged and your are at or near full throttle, you'll put power down. Like Hack said, you'll almost like DCT if you do it right. Automated manuals (I should say good automated manuals) will have perfect timing every shift. You're "cushioning" the falling revs so that the energy from the revs being higher than it would in gear goes to the ground more than it goes to wheel spin or the engine mounts.
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      10-12-2011, 11:18 AM   #10
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Quote:
Originally Posted by jetblackcoupe View Post
thanks, I understand what you're saying. But what I'm asking is ... let's say you're trying to get from 0-60 as fast as possible... disengage clutch at let's say 6800... by the time i have shifted and want to hit the gas again for optimal acceleration, the RPM has only dropped to ~6500... now by your logic I would have to really slowly let go of the clutch for slippage. All this "patience" will slow my acceleration... So how can I even manage to keep up with automatic cars? Or do I just have to jerk/skid for quick shifting?
You have to keep in mind, there's no way around the physics of how cars work. Even automatics will take some time between gears for the torque converter to spool up. So while it'll change gears almost instantaneously, the torque converter does not lock-up fully until the transmission side has caught up with the engine side, and until then you can put your foot to the floor and all the automatic will do is slowly release that power to the driveline.

And on a manual you can do exactly the same. You can clutch in, change gears and almost immediately slowly release the clutch as you apply throttle. Again, it's all in timing and practice. And when I mean slowly release the clutch, I mean instead of sidestepping the pedal to get it top pop immediately, you smoothly release the pedal in about 1/2 second. When you have the timing down right, you can release the clutch nearly as fast as you can move the shifter from one gear to another, and do it smoothly.

Look, I can try and explain this to you and spend the next 5 posts "describing" it to you. My best suggestion to you is, is either keep practicing it until you get it just right, before you blow up your driveline, or have someone who can do this demonstrate it to you so you'll have at least some reference on how to execute it.

On a side note, my car came naturally aspirated with solid lifters and individual throttle butterfly for each cylinder, and the second I let off gas at 7,900 RPM it'll drop immediately to ~5,000 RPM in neutral, almost perfect for the next gear. I can stab the clutch in between gearshifts. Can't do that in a turbocharged engine.
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      10-12-2011, 11:33 AM   #11
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There was a guy on M5 Board who had lots demo videos about shifting that were pretty cool.
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      10-12-2011, 05:20 PM   #12
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thanks for your replies everyone
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      11-09-2011, 01:42 AM   #13
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so it takes longer for turbocharged engines to decrease rpm? making shifting inherently slower?
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      11-09-2011, 11:31 AM   #14
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Quote:
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so it takes longer for turbocharged engines to decrease rpm? making shifting inherently slower?
No, not necessarily. It is, however, more difficult to make fast shifts smooth. There's momentum stored in the drivetrain and it's still making a bit of power so as you release the clutch you're still putting power to the ground even though you're not on the throttle. As the clutch slips an the engine spools down you need to get back on the throttle so there's no interruption in power. This is what we mean by timing. It's all about how fast you pull the clutch out and how soon you get back on the gas. An engine that spins down faster is just easier to shift quickly.

Like Hack said, it's really difficult to describe this on a forum. You need to see it in action.
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      11-09-2011, 05:38 PM   #15
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No, not necessarily. It is, however, more difficult to make fast shifts smooth. There's momentum stored in the drivetrain and it's still making a bit of power so as you release the clutch you're still putting power to the ground even though you're not on the throttle. As the clutch slips an the engine spools down you need to get back on the throttle so there's no interruption in power. This is what we mean by timing. It's all about how fast you pull the clutch out and how soon you get back on the gas. An engine that spins down faster is just easier to shift quickly.

Like Hack said, it's really difficult to describe this on a forum. You need to see it in action.
I understand now, I've tried what you guys said and it works relatively well. It's just that I always thought, for maximum acceleration, you would clutch in, change gears, and dump clutch as quickly as possible. When I tried that, tires screeched, so I knew something was wrong. It's kind of annoying to have to do anything relatively "slowly" (i.e. slipping instead of dumping) while you're trying to move fast.
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      11-22-2011, 09:00 PM   #16
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With the 335 you just have to be patient between shifts. It's not like an S2000 where you can just slam through the gears. It may seem like an eternity to wait for the revs to fall (especially 1 > 2), but that's just because you're driving - if you were a passenger, it wouldn't seem long at all. I did a driving video a while back and realized this.
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      12-03-2011, 11:48 PM   #17
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Quote:
Originally Posted by jetblackcoupe View Post
I understand now, I've tried what you guys said and it works relatively well. It's just that I always thought, for maximum acceleration, you would clutch in, change gears, and dump clutch as quickly as possible. When I tried that, tires screeched, so I knew something was wrong. It's kind of annoying to have to do anything relatively "slowly" (i.e. slipping instead of dumping) while you're trying to move fast.
You drive a turbo-charged car with single throttle body and electronically controlled direct fuel injection and a hydraulic lifter. You want to be able to keep the foot buried and side-step the clutch to keep maximum acceleration? You'll need to have a carburetor equipped engine with individual throttle bodies and a solid lifter. On the N5X engines, the RPM is programmed to stay high on shifts to keep emissions down as well as keep the boost up (in the case of the N54/N55) to prevent too much lag, plus the hydraulic lifters and electronic throttle map will never allow your engine to respond that quickly to throttle changes.

My final advice to you is, if you want to achieve the FASTEST possible acceleration time? It's not going to be possible to avoid some jerkiness in between shifts when you dump the clutch. Even the last generation SMG, in the most aggressive and fastest shift mode, you will feel a hammer on the back of your ss every time you shift because, well, there's a difference between engine speed and road speed. ONLY on DCT type transmission where there's two flywheels and two drive-shafts engaged at all times where it is possible to NOT feel that change in gear, because the next gear is already spinning at the same speed as the engine.

Or ditch the N54/N55 and go with any of the naturally aspirated M engines. The individual throttle bodies makes the engine rev up and down almost as quickly as your foot can move, and those cars are tuned to produce far more smog than your run of the mill BMW engines so they're not programmed to "hang" up high in rev to keep NOx emission down (as much).
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      12-04-2011, 01:21 AM   #18
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Quote:
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You drive a turbo-charged car with single throttle body and electronically controlled direct fuel injection and a hydraulic lifter. You want to be able to keep the foot buried and side-step the clutch to keep maximum acceleration? You'll need to have a carburetor equipped engine with individual throttle bodies and a solid lifter. On the N5X engines, the RPM is programmed to stay high on shifts to keep emissions down as well as keep the boost up (in the case of the N54/N55) to prevent too much lag, plus the hydraulic lifters and electronic throttle map will never allow your engine to respond that quickly to throttle changes.

My final advice to you is, if you want to achieve the FASTEST possible acceleration time? It's not going to be possible to avoid some jerkiness in between shifts when you dump the clutch. Even the last generation SMG, in the most aggressive and fastest shift mode, you will feel a hammer on the back of your ss every time you shift because, well, there's a difference between engine speed and road speed. ONLY on DCT type transmission where there's two flywheels and two drive-shafts engaged at all times where it is possible to NOT feel that change in gear, because the next gear is already spinning at the same speed as the engine.

Or ditch the N54/N55 and go with any of the naturally aspirated M engines. The individual throttle bodies makes the engine rev up and down almost as quickly as your foot can move, and those cars are tuned to produce far more smog than your run of the mill BMW engines so they're not programmed to "hang" up high in rev to keep NOx emission down (as much).
thank you, much appreciated
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      12-04-2011, 09:28 AM   #19
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would a lighter flywheel help at all in terms of rev-matching?
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      12-04-2011, 07:07 PM   #20
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would a lighter flywheel help at all in terms of rev-matching?
I'm not sure about turbo cars, but on an NA car, a lightened flywheel will help your engine spin up and down quicker, so less hang and easier full-throttle shifts. For a DD, an excessively light flywheel will be more annoying than helpful. Launching the car will be tough since the car will be much more prone to stall. And since the revs fall quicker making a nice smooth granny shift will be tougher since the revs will go below your target revs for the next gear.

For turbo cars, a very good tune will help more than a flywheel and is cheaper. Good tunes will be able to get rid of most of the rev hang but this may be at a detriment to your cats since it will blow more unburned fuel through the exhaust. That's one of the reasons race cars burble and pop when they slow down.
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      12-04-2011, 10:12 PM   #21
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wait so a piggyback can solve this issue????
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      12-04-2011, 10:19 PM   #22
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Depends on the piggy back. My guess is no since most tuners are going for power.
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