Double Clutching?
05-21-2012, 12:17 PM
#1
Race Car
Thread Starter
Double Clutching?
I got into the habit of double-clutching on downshifts in my 3.2 (915), partially because I found it smoother and partially because the double clutch action forces me to take the time to think more about each downshift (perhaps that's why they're smoother haha).
My question, is there any reason to double-clutch? I clutch in, shift to neutral, clutch out, blip the throttle with the clutch pedal "out", then clutch back in to shift into gear. Does clutching in for the entire shift and blipping with the clutch pedal "in" achieve the same affect? Is double-clutching redundant?
What does your shifting look like?
My question, is there any reason to double-clutch? I clutch in, shift to neutral, clutch out, blip the throttle with the clutch pedal "out", then clutch back in to shift into gear. Does clutching in for the entire shift and blipping with the clutch pedal "in" achieve the same affect? Is double-clutching redundant?
What does your shifting look like?
05-21-2012, 12:38 PM
#2
Rennlist Member
Two part shift. Clutch in, remove from engaged gear. Move lever partially through neutral to the pressure point for the next desired gear. Maintain light pressure until you "feel" that the desired gear will engage smoothly. Complete shift. Release clutch.
Clutch in...
1. Shift out of engaged gear, moving the shift lever to the pressure point for the next gear.
2. When the next gear is "ready", complete the shift.
Clutch out.
It's a feel thing. When it's habit, some shifts will be noticeable to a passenger, most will not. Double clutching is not necessary, but you don't mention what engine speed you do most of your shifts at. 3,500 rpm on a cold engine, and about 4,000 revs on a warm/hot engine works best, and after your shift the engine will be in the power band enough to accelerate smoothly.
One additional note about shifting. NEVER leave the trans in 5th gear below about 55 mph! Always try to do your 5 to 4 downshift at about 60 mph.
Clutch in...
1. Shift out of engaged gear, moving the shift lever to the pressure point for the next gear.
2. When the next gear is "ready", complete the shift.
Clutch out.
It's a feel thing. When it's habit, some shifts will be noticeable to a passenger, most will not. Double clutching is not necessary, but you don't mention what engine speed you do most of your shifts at. 3,500 rpm on a cold engine, and about 4,000 revs on a warm/hot engine works best, and after your shift the engine will be in the power band enough to accelerate smoothly.
One additional note about shifting. NEVER leave the trans in 5th gear below about 55 mph! Always try to do your 5 to 4 downshift at about 60 mph.
05-21-2012, 01:12 PM
#3
Skunk Whisperer
Rennlist Member
Rennlist Member
Join Date: Nov 2011
Location: Winston Salem, NC/Ararat, VA
Posts: 5,236
Likes: 0
Received 4 Likes
on
2 Posts
Peter, read your book and thoroughly enjoyed it.
It is my understanding and observation, that a synchronized transmission doesn't require rev matching or double clutching to engage the desired gear position. Rev matching on downshifts, just makes the gear change smoother, like for corner entry, in my experience. This sound correct?
Non-synchronized manual transmissions, like the one in my class eight road tractor, require double clutching or floating(moving in and out of gears without the clutch while selecting the desired gear at the correct engine speed). I have been able to float my manual cars at lower engine speeds but once the rpm's get above 3k, I use the clutch to eliminate errors. The low-end torque of the 911 SC motor makes floating much easier.
It is my understanding and observation, that a synchronized transmission doesn't require rev matching or double clutching to engage the desired gear position. Rev matching on downshifts, just makes the gear change smoother, like for corner entry, in my experience. This sound correct?
Non-synchronized manual transmissions, like the one in my class eight road tractor, require double clutching or floating(moving in and out of gears without the clutch while selecting the desired gear at the correct engine speed). I have been able to float my manual cars at lower engine speeds but once the rpm's get above 3k, I use the clutch to eliminate errors. The low-end torque of the 911 SC motor makes floating much easier.
05-21-2012, 01:50 PM
#4
Race Car
Thread Starter
Two part shift. Clutch in, remove from engaged gear. Move lever partially through neutral to the pressure point for the next desired gear. Maintain light pressure until you "feel" that the desired gear will engage smoothly. Complete shift. Release clutch.
Clutch in...
1. Shift out of engaged gear, moving the shift lever to the pressure point for the next gear.
2. When the next gear is "ready", complete the shift.
Clutch out.
It's a feel thing. When it's habit, some shifts will be noticeable to a passenger, most will not. Double clutching is not necessary, but you don't mention what engine speed you do most of your shifts at. 3,500 rpm on a cold engine, and about 4,000 revs on a warm/hot engine works best, and after your shift the engine will be in the power band enough to accelerate smoothly.
One additional note about shifting. NEVER leave the trans in 5th gear below about 55 mph! Always try to do your 5 to 4 downshift at about 60 mph.
Clutch in...
1. Shift out of engaged gear, moving the shift lever to the pressure point for the next gear.
2. When the next gear is "ready", complete the shift.
Clutch out.
It's a feel thing. When it's habit, some shifts will be noticeable to a passenger, most will not. Double clutching is not necessary, but you don't mention what engine speed you do most of your shifts at. 3,500 rpm on a cold engine, and about 4,000 revs on a warm/hot engine works best, and after your shift the engine will be in the power band enough to accelerate smoothly.
One additional note about shifting. NEVER leave the trans in 5th gear below about 55 mph! Always try to do your 5 to 4 downshift at about 60 mph.
The rpms you mention shifting at above, are these downshift rpms?
05-21-2012, 02:08 PM
#5
Nordschleife Master
My question, is there any reason to double-clutch? I clutch in, shift to neutral, clutch out, blip the throttle with the clutch pedal "out", then clutch back in to shift into gear. Does clutching in for the entire shift and blipping with the clutch pedal "in" achieve the same affect? Is double-clutching redundant?
When you are in neutral, with the clutch out, and rev, what you are doing is speeding up the gear you are about to downshift into. You are rev-matching it to the same speed so that the synchros don't do any (or at least do a minimum) work in accelerating the loose gear as you pull the slider onto it to engage it. On a 915 you can make quicker downshifts by doing this and at the same time are reducing the wear on the synchros because you aren't making them work.
If you rev the engine while the clutch pedal is pushed to the floor, the revs of the engine have zero impact on the internal components of the gearbox. The usual purpose of this kind of rev is to speed up the engine to closer to the speed that the mainshaft is moving so that you minimize the shockload and/or slippage of the clutch when you release it on the downshift. But it has no effect on the synchros. The synchros will still have to speed up the loose gear for the downshift when you pull the gearshift lever into gear.
05-21-2012, 02:23 PM
#6
Rennlist Member
Thanks Peter! What about downshifting? Do you blip the throttle as you perform the second part of your shift to rev match?
During downshifts a gentle throttle blip can help a gear change, and the timing of the rpm increase should coincide with engagement of the desired gear. This actually reduces some of the working load on the synchro parts. A properly functioning 915 doesn't necessarily need rev matching, but doing it can't hurt.
The rpms you mention shifting at above, are these downshift rpms?
No, those are for up shifts.
During downshifts a gentle throttle blip can help a gear change, and the timing of the rpm increase should coincide with engagement of the desired gear. This actually reduces some of the working load on the synchro parts. A properly functioning 915 doesn't necessarily need rev matching, but doing it can't hurt.
The rpms you mention shifting at above, are these downshift rpms?
No, those are for up shifts.
http://porsche.wikidot.com/
Scroll the left side of page, click on "915" under page tags, and you'll find my 9-part repair Tutorial!
05-21-2012, 03:03 PM
#7
Nordschleife Master
Join Date: Feb 2011
Location: Deep Downtown Carrier, OK
Posts: 5,297
Likes: 0
Received 6 Likes
on
6 Posts
If you rev the engine while the clutch pedal is pushed to the floor, the revs of the engine have zero impact on the internal components of the gearbox. The usual purpose of this kind of rev is to speed up the engine to closer to the speed that the mainshaft is moving so that you minimize the shockload and/or slippage of the clutch when you release it on the downshift.
Matt is on the money here. The throttle blip (when one is not double clutching) has no other effect than to reduce (best case - negate) the shock on the clutch and thus the car...as the clutch engages.
In other words, the blip serves to smooth out the transition throughout the entire drivetrain down to the tire patch at the very end (clutch engages) of the downshift.
We do not want to unsettle the car because of the downshift. It is for this reason that the blip should occur just prior, or just as, the clutch engages - just as the clutch pedal comes up.
Smooth is good - at speed, smooth is a must.
Matt is on the money here. The throttle blip (when one is not double clutching) has no other effect than to reduce (best case - negate) the shock on the clutch and thus the car...as the clutch engages.
In other words, the blip serves to smooth out the transition throughout the entire drivetrain down to the tire patch at the very end (clutch engages) of the downshift.
We do not want to unsettle the car because of the downshift. It is for this reason that the blip should occur just prior, or just as, the clutch engages - just as the clutch pedal comes up.
Smooth is good - at speed, smooth is a must.
Trending Topics
05-21-2012, 04:53 PM
#8
I haddah Google dat
Rennlist Member
Rennlist Member
Yes, there is a reason to double clutch, especially on an old 915 with Porsche's split ring synchros. The two different revs do two different things.
When you are in neutral, with the clutch out, and rev, what you are doing is speeding up the gear you are about to downshift into. You are rev-matching it to the same speed so that the synchros don't do any (or at least do a minimum) work in accelerating the loose gear as you pull the slider onto it to engage it. On a 915 you can make quicker downshifts by doing this and at the same time are reducing the wear on the synchros because you aren't making them work.
If you rev the engine while the clutch pedal is pushed to the floor, the revs of the engine have zero impact on the internal components of the gearbox. The usual purpose of this kind of rev is to speed up the engine to closer to the speed that the mainshaft is moving so that you minimize the shockload and/or slippage of the clutch when you release it on the downshift. But it has no effect on the synchros. The synchros will still have to speed up the loose gear for the downshift when you pull the gearshift lever into gear.
When you are in neutral, with the clutch out, and rev, what you are doing is speeding up the gear you are about to downshift into. You are rev-matching it to the same speed so that the synchros don't do any (or at least do a minimum) work in accelerating the loose gear as you pull the slider onto it to engage it. On a 915 you can make quicker downshifts by doing this and at the same time are reducing the wear on the synchros because you aren't making them work.
If you rev the engine while the clutch pedal is pushed to the floor, the revs of the engine have zero impact on the internal components of the gearbox. The usual purpose of this kind of rev is to speed up the engine to closer to the speed that the mainshaft is moving so that you minimize the shockload and/or slippage of the clutch when you release it on the downshift. But it has no effect on the synchros. The synchros will still have to speed up the loose gear for the downshift when you pull the gearshift lever into gear.
05-21-2012, 05:04 PM
#9
Rennlist Member
When learning to drive standard (in my 62 beetle) I was shown how to double clutch on downshifts. I've been naturally doing it ever since, especially in trucks that refuse to make a downshift unless you do double clutch the damn thing. LOL
The way I think about it, (right or wrong) is that when upshifting your input shaft naturally slows down due to friction in the transmission, so the speed difference between gears becomes less over time. When downshifting, it is the opposite. The speed difference increases over time. The input shaft is still slowing down from the speed it was at when you disengaged that gear, and now has to speed back up to, and beyond that point, to match with the lower gear. Helping out by spinning the input shaft up to more closely match the lower gear before trying to engage it takes a load of the synchros.
The way I think about it, (right or wrong) is that when upshifting your input shaft naturally slows down due to friction in the transmission, so the speed difference between gears becomes less over time. When downshifting, it is the opposite. The speed difference increases over time. The input shaft is still slowing down from the speed it was at when you disengaged that gear, and now has to speed back up to, and beyond that point, to match with the lower gear. Helping out by spinning the input shaft up to more closely match the lower gear before trying to engage it takes a load of the synchros.
05-21-2012, 09:01 PM
#10
Pretty much everything you can do with your feet is in this video, including left foot braking.
IMO, if you like double clutching, and you are good at it you should. Don't see a lot of it in this video however.
Oh yeah. It is pretty much the only way to get first at speeds over 20 MPH. Why would you do that ? Well, if you have to ask...
