87+ manual transmission destruction, rebuilding, discussion etc....
08-15-2018, 05:26 PM
#32
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Tuomo, what clutch do you have in the car that transmitted enough torque to do _that_ to whatever gear that was? Spec?
08-15-2018, 05:31 PM
#33
Nordschleife Master
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08-16-2018, 05:42 AM
#34
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What is this?
I’ve got a fairly securely strapped and crated five speed in my garage. Can you help me on figureing out what it is? I’d have to tear down the crate to get access to every surface, but just opening it a little gives me this view point above.
I can guesstimate from those numbers that it’s a “GT” transmission. The question is does it have an LSD, like I’d expect the GT’s to have? Or is the 31 really 3L which might make it 1990 GT with PSD (or should I call it PTSD since my car is ‘87).
08-16-2018, 08:32 AM
#35
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I’ve got a fairly securely strapped and crated five speed in my garage. Can you help me on figureing out what it is? I’d have to tear down the crate to get access to every surface, but just opening it a little gives me this view point above.
I can guesstimate from those numbers that it’s a “GT” transmission. The question is does it have an LSD, like I’d expect the GT’s to have? Or is the 31 really 3L which might make it 1990 GT with PSD (or should I call it PTSD since my car is ‘87).
Åke
08-16-2018, 08:46 AM
#36
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08-16-2018, 09:01 AM
#38
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08-16-2018, 09:11 AM
#39
Drifting
I’ve got a fairly securely strapped and crated five speed in my garage. Can you help me on figureing out what it is? I’d have to tear down the crate to get access to every surface, but just opening it a little gives me this view point above.
I can guesstimate from those numbers that it’s a “GT” transmission. The question is does it have an LSD, like I’d expect the GT’s to have? Or is the 31 really 3L which might make it 1990 GT with PSD (or should I call it PTSD since my car is ‘87).
I've got one of those too in mine with 18k miles.
08-18-2018, 12:44 PM
#41
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I'm still on the part of the learning curve of these manual transmissions in which every increment of new information makes me realize that I know even less than I though I would. It's kind of embarrassing how little I know about these things, for someone who likes to think about engines for fun.
For example, there's the gear oil. It's my understanding that the fluid in the differential is the same as in the main gear box. It's also my understanding that ideally the synchronization rings (or whatever) would want different kind of oil than the gears, which would want a different kind of oil than the limited slip differential. Is the sum total of it that the API GL-5 fluid should be used? The GL-5 oil is going to have more anti-wear and extreme-pressure additives, so that's good for gears. But it's my (uncertain, emerging) understanding that some of those EP additives chemically corrode some kinds of synchronizers and also make the oil a little too slippery for them. The hypoid gear in the rear end, on the other hand, slides a lot more than regular gears. Those hypoid gears sliding creates both extreme contact pressure and heat, and by my logic appears to wipe off the oil from the gears, making the elastohydrodynamic oil film thickness thinner. So that would require a lot of anti-wear and extreme-pressure additives, which in turn hurt the synchro performance. And what does the LSD want, the same stuff as synchros?
Would higher oil viscosity help making the synchros stick better and also protect the gears better, especially the hypoid gear in the rear end? And then there's the consideration that the higher viscosity gear oil is going to produce a lot more heat, which if left unchecked will expand the case and kill the transmission. Also, shifting cold might not be fun with very high viscosity gear oil. So thermostatically controlled external oil cooler would be necessary with higher oil viscosity. What would be the next step up in terms of viscosity from 75W90? Has anyone tried something like that?
For example, there's the gear oil. It's my understanding that the fluid in the differential is the same as in the main gear box. It's also my understanding that ideally the synchronization rings (or whatever) would want different kind of oil than the gears, which would want a different kind of oil than the limited slip differential. Is the sum total of it that the API GL-5 fluid should be used? The GL-5 oil is going to have more anti-wear and extreme-pressure additives, so that's good for gears. But it's my (uncertain, emerging) understanding that some of those EP additives chemically corrode some kinds of synchronizers and also make the oil a little too slippery for them. The hypoid gear in the rear end, on the other hand, slides a lot more than regular gears. Those hypoid gears sliding creates both extreme contact pressure and heat, and by my logic appears to wipe off the oil from the gears, making the elastohydrodynamic oil film thickness thinner. So that would require a lot of anti-wear and extreme-pressure additives, which in turn hurt the synchro performance. And what does the LSD want, the same stuff as synchros?
Would higher oil viscosity help making the synchros stick better and also protect the gears better, especially the hypoid gear in the rear end? And then there's the consideration that the higher viscosity gear oil is going to produce a lot more heat, which if left unchecked will expand the case and kill the transmission. Also, shifting cold might not be fun with very high viscosity gear oil. So thermostatically controlled external oil cooler would be necessary with higher oil viscosity. What would be the next step up in terms of viscosity from 75W90? Has anyone tried something like that?
08-18-2018, 04:57 PM
#42
That’s a great looking custom exhaust, I won’t comment on your other issue regarding oil except to say when I looked the exact issues you are facing to improve the operation of the transmission I concluded the way to go was the Corvette transmission as it separates the differential from the transmission. Then the compromises are more limited. Hey we could work on that together, I have too many projects.....
08-18-2018, 11:20 PM
#43
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I'm still on the part of the learning curve of these manual transmissions in which every increment of new information makes me realize that I know even less than I though I would. It's kind of embarrassing how little I know about these things, for someone who likes to think about engines for fun.
For example, there's the gear oil. It's my understanding that the fluid in the differential is the same as in the main gear box. It's also my understanding that ideally the synchronization rings (or whatever) would want different kind of oil than the gears, which would want a different kind of oil than the limited slip differential. Is the sum total of it that the API GL-5 fluid should be used? The GL-5 oil is going to have more anti-wear and extreme-pressure additives, so that's good for gears. But it's my (uncertain, emerging) understanding that some of those EP additives chemically corrode some kinds of synchronizers and also make the oil a little too slippery for them. The hypoid gear in the rear end, on the other hand, slides a lot more than regular gears. Those hypoid gears sliding creates both extreme contact pressure and heat, and by my logic appears to wipe off the oil from the gears, making the elastohydrodynamic oil film thickness thinner. So that would require a lot of anti-wear and extreme-pressure additives, which in turn hurt the synchro performance. And what does the LSD want, the same stuff as synchros?
Would higher oil viscosity help making the synchros stick better and also protect the gears better, especially the hypoid gear in the rear end? And then there's the consideration that the higher viscosity gear oil is going to produce a lot more heat, which if left unchecked will expand the case and kill the transmission. Also, shifting cold might not be fun with very high viscosity gear oil. So thermostatically controlled external oil cooler would be necessary with higher oil viscosity. What would be the next step up in terms of viscosity from 75W90? Has anyone tried something like that?
For example, there's the gear oil. It's my understanding that the fluid in the differential is the same as in the main gear box. It's also my understanding that ideally the synchronization rings (or whatever) would want different kind of oil than the gears, which would want a different kind of oil than the limited slip differential. Is the sum total of it that the API GL-5 fluid should be used? The GL-5 oil is going to have more anti-wear and extreme-pressure additives, so that's good for gears. But it's my (uncertain, emerging) understanding that some of those EP additives chemically corrode some kinds of synchronizers and also make the oil a little too slippery for them. The hypoid gear in the rear end, on the other hand, slides a lot more than regular gears. Those hypoid gears sliding creates both extreme contact pressure and heat, and by my logic appears to wipe off the oil from the gears, making the elastohydrodynamic oil film thickness thinner. So that would require a lot of anti-wear and extreme-pressure additives, which in turn hurt the synchro performance. And what does the LSD want, the same stuff as synchros?
Would higher oil viscosity help making the synchros stick better and also protect the gears better, especially the hypoid gear in the rear end? And then there's the consideration that the higher viscosity gear oil is going to produce a lot more heat, which if left unchecked will expand the case and kill the transmission. Also, shifting cold might not be fun with very high viscosity gear oil. So thermostatically controlled external oil cooler would be necessary with higher oil viscosity. What would be the next step up in terms of viscosity from 75W90? Has anyone tried something like that?
They did not want GL-6 or GL-7 used, because of the potential to damage the bronze syncros.
You don't want/need anything thicker than 75/90....especially if you control the transmission temperature at 210 degrees. I don't think you want the temperature to be lower than that. What most people don't understand is that the Borg Warner style synchro has the friction surface "hidden" from the gear oil, which makes it difficult to both circulate the gear oil and cool the gear oil at the friction surface of the synchro. I add spray systems to race transmissions to not only keep oil right at the engagement point of the gears, but mostly to allow gear oil to spray directly onto the synchros.
I won't bore everyone with the evolution of the different synchros and what applications they came from....it's very involved and not everything makes sense.
In the '85/'86 transmissions, Porsche used bronze synchros with a sprayed Moly friction surface in all 5 gears (and reverse). In 1987, they switched to a sintered iron synchro in 1st-3rd gears and kept the bronze synchros in 4th, 5th, and reverse. The sintered iron synchros had the same Moly surface....they were just stiffer and didn't distort as much as the bronze pieces. The Moly surface is identical to the surface on the clutches for the limited slips.
Current technology, in most all manual transmissions, replaces the Moly surface for a Carbon Fiber surface. The benefits are huge. Better friction. Larger friction surface because of the way they retain oil (no oil "slots" required, like the Moly synchros have.) The Carbon Fiber won't "smear" like the Moly surface when it gets hot. The Carbon Fiber surface is way more resistant to wear. The list goes on and on.
I have Billet Steel Carbon Fiber lined synchros custom made for the 928 transmissions I do, in house. Fantastic pieces. Half the price of a Porsche steel synchro. Virtually bulletproof.
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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!
08-19-2018, 12:54 PM
#44
Nordschleife Master
Thread Starter
Greg -- Thanks for the info on the carbon-fiber synchros. Do those carbon-fiber friction surfaces put any requirements on the lubricant?
I'm learning more about these transmission issues. Does anyone know or have an educated guess about how much spring twisting did Porsche spec into the torque tube drive shaft? I think all these shafts in the driveline are designed to work as twisting springs to make the gears last as torque fluctuates during the crankshaft revolution. Note that this not really an issue when changing the steel alloy to, say, 300M as all steels have approximately the same Young's modulus of elasticity and thus only the shaft diameter matters there. Going a couple fractions of mm larger on the diameter is not meaningful, but Porsche changing from 25mm to 28mm shaft might have changed the spring properties a lot.
More I think about this, more questions I have. What kind of bearings do these shafts have? Can they be preloaded more than stock for additional shaft stability? How exactly parallel are the shafts from the factory? Can one add steel inserts to the case for bearings to increase the stability of the shaft locations? There are a lot of questions when I start from zero knowledge...
I'm learning more about these transmission issues. Does anyone know or have an educated guess about how much spring twisting did Porsche spec into the torque tube drive shaft? I think all these shafts in the driveline are designed to work as twisting springs to make the gears last as torque fluctuates during the crankshaft revolution. Note that this not really an issue when changing the steel alloy to, say, 300M as all steels have approximately the same Young's modulus of elasticity and thus only the shaft diameter matters there. Going a couple fractions of mm larger on the diameter is not meaningful, but Porsche changing from 25mm to 28mm shaft might have changed the spring properties a lot.
More I think about this, more questions I have. What kind of bearings do these shafts have? Can they be preloaded more than stock for additional shaft stability? How exactly parallel are the shafts from the factory? Can one add steel inserts to the case for bearings to increase the stability of the shaft locations? There are a lot of questions when I start from zero knowledge...
Last edited by ptuomov; 08-19-2018 at 02:41 PM.
08-19-2018, 01:57 PM
#45
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On paper the Autos may be more turbo friendly and some say they can handle more power than a manual. But as always in practical terms I am not so sure it is so cut and dry. What I found out through hundreds of dyno tests is the few autos I used for testing stuff, they can't seem to take the increased load of a forced induction system, repeatedly. It seems at or around dyno run number 20? they start to slip badly and I had at least 2 that wouldn't even move the rollers by this time. You gotta pull them off put a fan on them for a few hours due to the heat-soak and then try again. Those probably needed rebuilding, but there were never any other indicators in road use, and as far as I know they are still going, even to this day 10 years later. Scared the crap out of me the first time, thought I had taken out a tranny completely on someone else's 928! I had similar results on a few others and was wondering if anyone else has experienced this?
Have a great one,
Dave
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