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      12-08-2011, 06:23 PM   #23
steve515
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This method certainly works to smooth out the low-rpm shifts, but I was under the impression that rolling onto the throttle while releasing the clutch was bad for the clutch? Please correct me if I am wrong.

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Originally Posted by jphughan View Post
The RPMs at which you shift determine how much (if any) throttle you should be adding before the clutch engages. The reason for this is that the rate at which RPMs fall while the clutch is in is pretty constant through the rev range, but the difference between RPMs between gears at a given speed increases as you go higher in the rev band.

So for example, at 40 MPH in 1st gear you're probably turning 8400 RPM, and at the same speed in 2nd gear you're turning maybe 6400 (not sure about that, but it'll work for the example). That means that there's a 2000 RPM difference. So as long as you can complete your shift in the amount of time it takes for the engine to lose 2000 RPM, you're good without applying any throttle during the shift.

But let's say you shift at 15 MPH. In 1st gear, that might mean you're turning 2500 RPM, and in 2nd gear that might be 2000 RPM. That's only a 500 RPM difference, but given that the shift itself takes about the same amount of time, the engine will fall below the optimal engagement RPM in that amount of time, so you need to add some throttle prior to clutch engagement there to slow down their fall or even bring them back up to where they need to be for a smooth shift. Think of it as rev-matching on an upshift.

Generally speaking, the higher up in the rev range you shift, the less throttle you need because the more time you have to shift due to the higher differences between RPMs in the gears.
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      12-08-2011, 06:29 PM   #24
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Quote:
Originally Posted by steve515 View Post
This method certainly works to smooth out the low-rpm shifts, but I was under the impression that rolling onto the throttle while releasing the clutch was bad for the clutch? Please correct me if I am wrong.
Mismatched revs are the main source of wear with a clutch (ignoring abuse such as riding the clutch). Technically if you could hold the throttle so the engine was at the perfect RPM to match the gearbox speed when you re-engaged the clutch, that would be less wear than staying off the throttle completely while re-engaging the clutch with an RPM discrepancy. The reason people say not to engage under power is because the gearbox input shaft is already under a load (imposed by the turning of the wheels), so it's better for only one moving component to have a load than two when they're reconnecting. But if you can set it up such that both components are under the same load, then you're golden -- and if you can set it up so they're at least really close, then you're silver, so to speak.

The fact is that this engine drops revs so fast that waiting until the clutch is fully engaged before touching the throttle at all just isn't realistic except for 1>2 and 2>3 shifts performed at redline. Re-engaging the clutch while under a little power isn't bad, and I think you'll find it makes for the smoothest solution, which in turn means the least wear on your clutch and other driveline components. Most "How to drive stick" guides actually say to gradually apply throttle as you let out the clutch too.

That's what I do on upshifts into 2nd, 3rd, and 4th. For upshifting into 5th and 6th, I actually do a traditional throttle blip just as I would a rev-matched downshift and then stay off the throttle until the clutch is fully engaged again because I've found that's actually smoother for me than re-engaging under power. Although when I go straight from 4>6, I just come in under a little power without an intermediate blip.

I guess if I did a throttle blip upshift for ALL gear changes that would be less wear than getting on throttle a little early as long as I did the blip and re-engagement properly, but that would be tricky to learn and cause shifts to take a lot longer. And honestly it'd be a lot harder to get that method right consistently, which means more wear and less smoothness when you get it wrong.
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Last edited by jphughan; 12-08-2011 at 06:49 PM..
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      12-08-2011, 06:33 PM   #25
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Some excellent considered responses in this thread - just to lower the tone though.......DCT FTW
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      12-08-2011, 09:19 PM   #26
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I find it not to be a problem at the start of the drive (engine still cool/cold ... oil still cool. Shifting from 1st to 2nd seams to be quite normal. However, after the engine oil is near or at normal operation temperature I find it may exhibit clutch noise as described by the original poster.

When my engine is at normal temperature I seam to control the problem (issue described by the OP) by simply gently releasing the clutch and controlling the RPM at the time of the shift.

Depending on the situation (car slightly in motion - just before a traffic light change etc.) I may continue in second gear. I would say 90% of the time I start in first gear. I find smooth gear changes from 1st to 2nd in my case is a direct result of a) engine oil temperature and good control of the clutch (i.e. letting the clutch engage in a smooth manner by RPM control and slow release of the clutch).

Works fine for me.
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      12-08-2011, 11:13 PM   #27
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M3 bullitt, we need a modern interpretation

Quote:
Originally Posted by jphughan View Post
The RPMs at which you shift determine how much (if any) throttle you should be adding before the clutch engages. The reason for this is that the rate at which RPMs fall while the clutch is in is pretty constant through the rev range, but the difference between RPMs between gears at a given speed increases as you go higher in the rev band.

So for example, at 40 MPH in 1st gear you're probably turning 8400 RPM, and at the same speed in 2nd gear you're turning maybe 6400 (not sure about that, but it'll work for the example). That means that there's a 2000 RPM difference. So as long as you can complete your shift in the amount of time it takes for the engine to lose 2000 RPM, you're good without applying any throttle during the shift.

But let's say you shift at 15 MPH. In 1st gear, that might mean you're turning 2500 RPM, and in 2nd gear that might be 2000 RPM. That's only a 500 RPM difference, but given that the shift itself takes about the same amount of time, the engine will fall below the optimal engagement RPM in that amount of time, so you need to add some throttle prior to clutch engagement there to slow down their fall or even bring them back up to where they need to be for a smooth shift. Think of it as rev-matching on an upshift.

Generally speaking, the higher up in the rev range you shift, the less throttle you need because the more time you have to shift due to the higher differences between RPMs in the gears.

Thanks for sharing man!!! Great stuff!!! Reminds me of all the cool upshifting sounds from Steve McQueen's mustang in Bullitt (from reading the zillions posts about the carchase, the sounds were actually dubbed in from a GT40). That car chase will live for eternity.

Here's one of the many youtube links for you guys who have not seen this (shame on you):
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