Transmission downshift forces - discussion..... Jim, Greg? comments?
07-29-2014, 12:18 PM
#16
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So think of the synchronizers as little bronze clutches, with guide teeth. Every time you do a partial-de-clutch shift (what you describe as a "simultaneus shift"), you force the synchros to work even harder unless your revs are matched perfectly. Yes, your method makes for faster shifting. And in real racing, all the wear items take a back seat to performance so long as you can make it to the finish line first. Gearbox rebuilds are a normal part of racing. Good thing the 928 box is so cheap to rebuild!
Audi discovered at the Le Mans races that the firsty TDI race motors had so much torque that they would chew through gearboxes before the race was over. They couldn't make a box strong enough to hold the torque, so they figured out a way to change gearboxes quickly. They did this for a couple years before rules were changed to force them to rebuild the boxes rather than just swap in a new one. Shortly after that the intake restrictions were tightened and manifold pressure limited so the problem was lessened anyway. I'm sure they still get a fresh gearbox for every race.
Carl's six-speed conversion is going to start looking real cheap to racers who aren't willing to sacrifice performance for gearbox life. The Corvette boxes are relatively cheap to buy, cheap to refresh, and have good upgrade pieces available. So long as your race class rules don't limit you to the Porsche gearbox, this is an obvious path for your program. That opens the door to Sharktuning and better intakes and heads too, without as much fear of gearbox failure.
Audi discovered at the Le Mans races that the firsty TDI race motors had so much torque that they would chew through gearboxes before the race was over. They couldn't make a box strong enough to hold the torque, so they figured out a way to change gearboxes quickly. They did this for a couple years before rules were changed to force them to rebuild the boxes rather than just swap in a new one. Shortly after that the intake restrictions were tightened and manifold pressure limited so the problem was lessened anyway. I'm sure they still get a fresh gearbox for every race.
Carl's six-speed conversion is going to start looking real cheap to racers who aren't willing to sacrifice performance for gearbox life. The Corvette boxes are relatively cheap to buy, cheap to refresh, and have good upgrade pieces available. So long as your race class rules don't limit you to the Porsche gearbox, this is an obvious path for your program. That opens the door to Sharktuning and better intakes and heads too, without as much fear of gearbox failure.
07-29-2014, 07:14 PM
#17
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So think of the synchronizers as little bronze clutches, with guide teeth. Every time you do a partial-de-clutch shift (what you describe as a "simultaneus shift"), you force the synchros to work even harder unless your revs are matched perfectly. Yes, your method makes for faster shifting. And in real racing, all the wear items take a back seat to performance so long as you can make it to the finish line first. Gearbox rebuilds are a normal part of racing. Good thing the 928 box is so cheap to rebuild!
Audi discovered at the Le Mans races that the firsty TDI race motors had so much torque that they would chew through gearboxes before the race was over. They couldn't make a box strong enough to hold the torque, so they figured out a way to change gearboxes quickly. They did this for a couple years before rules were changed to force them to rebuild the boxes rather than just swap in a new one. Shortly after that the intake restrictions were tightened and manifold pressure limited so the problem was lessened anyway. I'm sure they still get a fresh gearbox for every race.
Carl's six-speed conversion is going to start looking real cheap to racers who aren't willing to sacrifice performance for gearbox life. The Corvette boxes are relatively cheap to buy, cheap to refresh, and have good upgrade pieces available. So long as your race class rules don't limit you to the Porsche gearbox, this is an obvious path for your program. That opens the door to Sharktuning and better intakes and heads too, without as much fear of gearbox failure.
Audi discovered at the Le Mans races that the firsty TDI race motors had so much torque that they would chew through gearboxes before the race was over. They couldn't make a box strong enough to hold the torque, so they figured out a way to change gearboxes quickly. They did this for a couple years before rules were changed to force them to rebuild the boxes rather than just swap in a new one. Shortly after that the intake restrictions were tightened and manifold pressure limited so the problem was lessened anyway. I'm sure they still get a fresh gearbox for every race.
Carl's six-speed conversion is going to start looking real cheap to racers who aren't willing to sacrifice performance for gearbox life. The Corvette boxes are relatively cheap to buy, cheap to refresh, and have good upgrade pieces available. So long as your race class rules don't limit you to the Porsche gearbox, this is an obvious path for your program. That opens the door to Sharktuning and better intakes and heads too, without as much fear of gearbox failure.
Stuff a cheap, higher output Chevy engine up front, to go with that Chevy transmission!
While I'm certain that used gearbox pieces are getting difficult to find and that this trend will continue, I've got to say that once I can no longer run a Porsche transmission in my Porsche and have to swap in a Corvette transmission, I'm going to have to think long and hard about keeping a 928.
As far as competition vehicles are concerned, once a major component is no longer Porsche, Porsche (and the rest of the racing world) no longer recognizes that car as being a Porsche.
It's some sort of a hybrid, but not a Porsche.
Here's the easy way to figure this stuff out, for the people that didn't "grow up" Porsche.
If PCA won't allow you to run in one of their classes, it's no longer a Porsche.
Pretty simple stuff.
__________________
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Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
07-30-2014, 08:29 PM
#18
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So, greg... that OP'er about synchros..... he isnt completely wrong then?
Im still thinking in the pass through to the downshift gear, the residual friction of a partially pushed down clutch pedal and a synchronized release, spins up the driveline. as we know, just a bad adjusted INT plate, can do this, so much so , that the synchros cant let you even put the car in first or second. But, shifts find on the open road. this to me means , that if this is happening, then during a shift, and a pre rev before a shift, the driveline is spun up. not everyone has a INT problem, and when i fixed it in my car and Scots, downshifts were uneffected...... this means that most peoples timed downshifts are not fulll pedal pushed, so that the driveline can spin up while you go through neutral. should we tell the guy that he wasnt "completley wrong" just not looking at all the other components????
want empirical test proof....... take your car and do a 4th at 80mph to 3rd downshift. no throttle blip vs throttle blip. if you have a weak transmission, it will grind like the dickens with no blip..... doesnt this prove that if a blip helps, that the driveline is spun up as it passes through neutral?
Im still thinking in the pass through to the downshift gear, the residual friction of a partially pushed down clutch pedal and a synchronized release, spins up the driveline. as we know, just a bad adjusted INT plate, can do this, so much so , that the synchros cant let you even put the car in first or second. But, shifts find on the open road. this to me means , that if this is happening, then during a shift, and a pre rev before a shift, the driveline is spun up. not everyone has a INT problem, and when i fixed it in my car and Scots, downshifts were uneffected...... this means that most peoples timed downshifts are not fulll pedal pushed, so that the driveline can spin up while you go through neutral. should we tell the guy that he wasnt "completley wrong" just not looking at all the other components????
want empirical test proof....... take your car and do a 4th at 80mph to 3rd downshift. no throttle blip vs throttle blip. if you have a weak transmission, it will grind like the dickens with no blip..... doesnt this prove that if a blip helps, that the driveline is spun up as it passes through neutral?
07-30-2014, 09:57 PM
#19
Drifting
A blip helps my '83 trans but double clutching does not at all, actually
ensures a nasty grind on 3rd to 2nd downshifts.
To be honest, i only posted a reply to this thread as an excuse to use
this vintage Getrag pic. Just look at this guy.
ensures a nasty grind on 3rd to 2nd downshifts.
To be honest, i only posted a reply to this thread as an excuse to use
this vintage Getrag pic. Just look at this guy.
07-31-2014, 03:04 AM
#20
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scots was pretty bad.... i race it for 3 races at laguna, and a big blip was needed to make the shift from 3 to 2nd. without the blip or a right timed blip, it was a huge grind, once it wouldnt go in. I think 3 shifts over the 3 race weekend, were grinders. that transmission is gone and the 85 is in now..... like Buudddddder!!!!
07-31-2014, 10:00 AM
#21
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I've seen some cockpit videos and checked some data and yes, many people time the downshift so the chutch pedal passes the engagement point long after the gear is disconnected and the rpms are going up. It happens naturally when you start the three moves (both feet and right hand) at the same time.
That is "double clutch" skipping the first clutch kick, which is not needed to disengage the gear anyway, specially if you blip.
So I tend to think OP is right in theory: if you completely disengage a healthy clutch before you touch the gear lever and you engage it after you have your hand back in the steering wheel, what you do with the throttle doesn't make any difference. But that's not the way we downshift, so he's wrong in practice: blipping does make a difference even if you don't attempt to double clutch.
IMHO.
Ps: Throttle blip is made in racing for various reasons, but the one I find most important is to avoid the torque spike to the wheels that in turns causes locks.
Ps2: Except in dog boxes, in which it is also made to help the gear disengage and avoid blisters in the palm of the hand.
That is "double clutch" skipping the first clutch kick, which is not needed to disengage the gear anyway, specially if you blip.
So I tend to think OP is right in theory: if you completely disengage a healthy clutch before you touch the gear lever and you engage it after you have your hand back in the steering wheel, what you do with the throttle doesn't make any difference. But that's not the way we downshift, so he's wrong in practice: blipping does make a difference even if you don't attempt to double clutch.
IMHO.
Ps: Throttle blip is made in racing for various reasons, but the one I find most important is to avoid the torque spike to the wheels that in turns causes locks.
Ps2: Except in dog boxes, in which it is also made to help the gear disengage and avoid blisters in the palm of the hand.
07-31-2014, 10:31 AM
#22
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It's a Tremec, used by Dodge, Aston Martin, Ford....
Originally designed by Borg-Warner which, as you already know, makes the synchros for the 85+ transmission in the 928.
I understand where you are coming from so I don't disagree with the premise, but if PCA will not allow a 928 to run with a Borg-Warner / Tremec transmission, it's complete and utter BS.
There are cars running with PCA that don't even have an original Porsche chassis, much less an "original" transmission.
Unless you can show me a production model of 911 made by FABCAR.
What about billet lower control arms? Moton shocks? MOTEC ECU's? Non-OEM installed carbs?
How far do we push this? Only 78-84 transmissions allowed since they are 100% Porsche? Then I guess the automatics are banned.
If we can agree the transmission is generic, then we are left with the differential from the Vette which is specific to those cars. So what's "now allowed"..the housing? The internals? Most racing Vette's don't even use the original differential, they install a Quaife or something else, so the internals are not an issue since one could also install a 3rd party diff in a 928 and nobody would be the wiser.
Bottom line, trying to state originality based on what a car club will allow is nonsense, way too many politics involved. Maybe Mark can share his story about the Cadillac's with the central seating position in the Speed GT series.
On that note, last time I checked PCA will allow you to run with a RUF, newer ones don't even have a Porsche serial number.
Bottom line, I don't disagree with you, but the reality is there is no consistency with racing bodies so it really shouldn't matter, but I know it would.
Originally designed by Borg-Warner which, as you already know, makes the synchros for the 85+ transmission in the 928.
As far as competition vehicles are concerned, once a major component is no longer Porsche, Porsche (and the rest of the racing world) no longer recognizes that car as being a Porsche.
It's some sort of a hybrid, but not a Porsche.
Here's the easy way to figure this stuff out, for the people that didn't "grow up" Porsche.
If PCA won't allow you to run in one of their classes, it's no longer a Porsche.
It's some sort of a hybrid, but not a Porsche.
Here's the easy way to figure this stuff out, for the people that didn't "grow up" Porsche.
If PCA won't allow you to run in one of their classes, it's no longer a Porsche.
There are cars running with PCA that don't even have an original Porsche chassis, much less an "original" transmission.
Unless you can show me a production model of 911 made by FABCAR.
What about billet lower control arms? Moton shocks? MOTEC ECU's? Non-OEM installed carbs?
How far do we push this? Only 78-84 transmissions allowed since they are 100% Porsche? Then I guess the automatics are banned.
If we can agree the transmission is generic, then we are left with the differential from the Vette which is specific to those cars. So what's "now allowed"..the housing? The internals? Most racing Vette's don't even use the original differential, they install a Quaife or something else, so the internals are not an issue since one could also install a 3rd party diff in a 928 and nobody would be the wiser.
Bottom line, trying to state originality based on what a car club will allow is nonsense, way too many politics involved. Maybe Mark can share his story about the Cadillac's with the central seating position in the Speed GT series.
On that note, last time I checked PCA will allow you to run with a RUF, newer ones don't even have a Porsche serial number.
Bottom line, I don't disagree with you, but the reality is there is no consistency with racing bodies so it really shouldn't matter, but I know it would.
07-31-2014, 11:00 AM
#23
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When doing rev match on downshifts, I try to go into gear just as the clutch is releasing for smoother engagement.
There are other times I want the revs to be higher or lower to help loosen the rear on entry, then I hesitate releasing the clutch just a fraction to insure the transmission is fully in gear. For example setting up to enter a tight chicane.
There are other times I want the revs to be higher or lower to help loosen the rear on entry, then I hesitate releasing the clutch just a fraction to insure the transmission is fully in gear. For example setting up to enter a tight chicane.
07-31-2014, 01:34 PM
#24
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Probably the guiding factors have to do with how much mismatch is present when the 'next' gear is engaged, and how much force is appled during that mismatch. The tiny central driveshaft adds a small amount of inertia, and with zero coupling between clutch pieces there's a very predictable amount of force that the synchros need to transfer. Sliding out of a gear without the clutch is fine so long as there's no speed dfference between the pieces as the gear (and the synchonizer) uncouple. So with clutch still engaged, neutral throttle and slide out of gear to neutral, blip the throttle in neutral to match revs, then de-clutch before engaging the next gear should be OK. The "partial de-clutch" as you go from neutral to the next gear is a crapshoot, with synchronizers working to match the speeds of the gears. How much coupling with the engine, and more importantly how big is the RPM mismatch? At that point you are effectively balancing the duty between the engine-end clutch and the synchronizers (clutches) as they work to match the speeds of the engine and all the connected gears.
PDKwith the dual clutches makes all this moot. The computer manages the sequential engagement of the gearsts, while the clutches at either end are selectively engaged for rev matching. Sweet!
PDKwith the dual clutches makes all this moot. The computer manages the sequential engagement of the gearsts, while the clutches at either end are selectively engaged for rev matching. Sweet!
07-31-2014, 02:28 PM
#25
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Thread Starter
When doing rev match on downshifts, I try to go into gear just as the clutch is releasing for smoother engagement.
There are other times I want the revs to be higher or lower to help loosen the rear on entry, then I hesitate releasing the clutch just a fraction to insure the transmission is fully in gear. For example setting up to enter a tight chicane.
There are other times I want the revs to be higher or lower to help loosen the rear on entry, then I hesitate releasing the clutch just a fraction to insure the transmission is fully in gear. For example setting up to enter a tight chicane.
some instant rear brake bias.
Probably the guiding factors have to do with how much mismatch is present when the 'next' gear is engaged, and how much force is appled during that mismatch. The tiny central driveshaft adds a small amount of inertia, and with zero coupling between clutch pieces there's a very predictable amount of force that the synchros need to transfer. Sliding out of a gear without the clutch is fine so long as there's no speed dfference between the pieces as the gear (and the synchonizer) uncouple. So with clutch still engaged, neutral throttle and slide out of gear to neutral, blip the throttle in neutral to match revs, then de-clutch before engaging the next gear should be OK. The "partial de-clutch" as you go from neutral to the next gear is a crapshoot, with synchronizers working to match the speeds of the gears. How much coupling with the engine, and more importantly how big is the RPM mismatch? At that point you are effectively balancing the duty between the engine-end clutch and the synchronizers (clutches) as they work to match the speeds of the engine and all the connected gears.
PDKwith the dual clutches makes all this moot. The computer manages the sequential engagement of the gearsts, while the clutches at either end are selectively engaged for rev matching. Sweet!
PDKwith the dual clutches makes all this moot. The computer manages the sequential engagement of the gearsts, while the clutches at either end are selectively engaged for rev matching. Sweet!
07-31-2014, 05:00 PM
#26
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By the way, before any crazy mind tries it, DON'T downshift to 1st at 130 kph. Even if you plan to keep your left foot down all the time. The clutch disc centrifuges the pads, believe it or not. 
And that is in "normal" car. Who knows what could happen to a over reving torque tube.
And that is in "normal" car. Who knows what could happen to a over reving torque tube.
07-31-2014, 05:06 PM
#27
Drifting
gear oil:
Using gear oils for limited slip diffs (with FMs) are usually fine to use in non limited slip trans's. I have a hunch (FMs) may actually slow shifting in older
boxes like in my '83.
I have heard the Subaru guys say using Mobil LS in their non LS manuals caused slower shifts.
I don't have a LS trans in my 1983 S and I'm thinking of switching to
Redline 75W90 NS.
911 and 944 owners with manuals and slow syncros swear by it, saying it really helps shifts.
Ultimately I'd like to install a later 5 speed.
Interesting to me, 928s received their slicker shifting box for MY '85 while the 911 did not until MY 1987.
When the 911 did finally get a better shifting trans (G50) for '87 it was a signal to all who could read the tea leaves
that the 911 was no longer being phased out of production (rather the rumor).
My 1986 Carrera shifted lousy (type 915 trans) and was a much more vague shift experience due to it's linkage design i guess.
By comparison the '83 928S is MUCH better even with slightly tired 2nd and 3rd gear syncros.
Using gear oils for limited slip diffs (with FMs) are usually fine to use in non limited slip trans's. I have a hunch (FMs) may actually slow shifting in older
boxes like in my '83.
I have heard the Subaru guys say using Mobil LS in their non LS manuals caused slower shifts.
I don't have a LS trans in my 1983 S and I'm thinking of switching to
Redline 75W90 NS.
911 and 944 owners with manuals and slow syncros swear by it, saying it really helps shifts.
Ultimately I'd like to install a later 5 speed.
Interesting to me, 928s received their slicker shifting box for MY '85 while the 911 did not until MY 1987.
When the 911 did finally get a better shifting trans (G50) for '87 it was a signal to all who could read the tea leaves
that the 911 was no longer being phased out of production (rather the rumor).
My 1986 Carrera shifted lousy (type 915 trans) and was a much more vague shift experience due to it's linkage design i guess.
By comparison the '83 928S is MUCH better even with slightly tired 2nd and 3rd gear syncros.
Last edited by The Fixer; 07-31-2014 at 10:03 PM.
07-31-2014, 07:20 PM
#29
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Thread Starter
before i had my 84 box worked on, the 2nd gear synchro was a mess. I used swepco. it actually made a pretty large difference. then, when i blew up that gear box 5th gear on the dyno, and went to the 85 borg warner, life was really great at the track, shifting wise!
07-31-2014, 07:24 PM
#30
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Thread Starter
no one every responded to my comments about not matching revs, how that can blow up the short shaft, or 5th gear. chirping the tires has way more force in 3rd coming from down from 4th than the engine can ever produce on acceleration..... one way uses the total kinetic energy of the engine at High rpm to driven RPM, and the other uses the KE of the car at speed backward from the tires, through the gear box to the driveline.
moral of the story, match those downshift RPMs!
moral of the story, match those downshift RPMs!
