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Thanks Greg. I will follow up with you directly to see if you can take this on over the next few months.I'd love to have the car up and running in the spring but understand if your backlog goes beyond that.
Luckily, transmissions are not anywhere near as time consuming as doing an engine...so I can generally "fill" one in, while waiting for the machine shop to finish their work.
Or do one on a weekend.
Also nice is that Kyle has become a "certified disassembler, cleaner, and inspector" (certified by me...pretty tough standards.) And because he has 30+ year old eyes and not 70 year old eyes, he catches details that I might miss, these days.
The cleaning and inspection process takes up the major portion of the time required, so this really helps, if he gets a break to help me.
The 928 PSD was a fantastic development!
Really great for a GT car when/if someone encounters ice, snow, rain, or anything slippery.
For ultra high performance use (like on a race course, a parking lot slalom, or even very aggressive canyon driving) they prove to be inadequate (for above average drivers.)
Why?
The delay time in actuation becomes an issue. And because there is a delay, the faster the corner, the tighter the corner, the more of an issue this becomes.
For the PSD to function, it first has to detect a difference between the two rear wheel speeds. This signal is processed by a fairly slow (very slow, by modern standards) computer, which then sends a signal to the PSD hydraulic pump assembly, which sends hydraulic fluid to a slave cylinder, which then pushes on a lever to increase the amount of "locking" of the rear wheels. At this point in time, the amount of "locking" required to force the rear wheels to travel at the same speed is an unknown amount....so the system then begins changing the amount of "locking" until this occurs.
So, by definition, the PSD will only work once the car is in a corner.
And since the time for engagement is a "fixed" period of time, the faster the corner, the farther into the corner the vehicle will be.
And the more aggressive the corner (tighter), the more time it will take for the system to figure out how much locking is required.
What this "delayed locking" does to the car depends on a bunch of things....which I'll discuss, next.
The 928 PSD was a fantastic development!
Really great for a GT car when/if someone encounters ice, snow, rain, or anything slippery.
For ultra high performance use (like on a race course, a parking lot slalom, or even very aggressive canyon driving) they prove to be inadequate (for above average drivers.)
Why?
The delay time in actuation becomes an issue. And because there is a delay, the faster the corner, the tighter the corner, the more of an issue this becomes.
For the PSD to function, it first has to detect a difference between the two rear wheel speeds. This signal is processed by a fairly slow (very slow, by modern standards) computer, which then sends a signal to the PSD hydraulic pump assembly, which sends hydraulic fluid to a slave cylinder, which then pushes on a lever to increase the amount of "locking" of the rear wheels. At this point in time, the amount of "locking" required to force the rear wheels to travel at the same speed is an unknown amount....so the system then begins changing the amount of "locking" until this occurs.
So, by definition, the PSD will only work once the car is in a corner.
And since the time for engagement is a "fixed" period of time, the faster the corner, the farther into the corner the vehicle will be.
And the more aggressive the corner (tighter), the more time it will take for the system to figure out how much locking is required.
What this "delayed locking" does to the car depends on a bunch of things....which I'll discuss, next.
Greg, given what you've explained so far, is a plain old LSD better than a PSD (especially for "above average drivers")? Is "variable locking" the only advantage a PSD has over a LSD?
Greg, given what you've explained so far, is a plain old LSD better than a PSD (especially for "above average drivers")? Is "variable locking" the only advantage a PSD has over a LSD?
For performance, the "late" 2 friction disc limited slips, are virtually useless.
They have so little locking effect (7 to 15 ftlbs), that they are pretty much only good for slippery road conditions. (Although Porsche called these 40% limited slips, they are actually an optimistic 10% unit.)
The early 4 friction disc units have the ability for the two friction discs (which touch each other, stock) to be separated, creating an active 4 friction disc limited slip.
]These can be used for performance purposes, when modified, like this. [40 ftlbs. (a "true" 40% limited slip) is simple to achieve. Although it would be awful to drive, even 80% locking is possible...but needs to be avoided, at all costs.}
The current trend for high performance use is to have a variable locking limited slip...say 45/60 (45% under acceleration and 60% under braking.)
The "smoother" the engagement and transition the better they work....the friction discs must be very "tame".
Combined with modern ABS, this style limited slip allow very deep trail braking, which keeps the front end "pointing" into the corner (60% locking under hard braking.). (Without ABS but with a sequential transmission, this also works very well.)
After the trail braking event, getting gently back into the throttle will allow the rear of the car to rotate some. (Transition period, as the limited slip "cams over" and changes from 60^ to 45%.)
After the rear rotation, heavy throttle can be applied and the car will be neutral or gently point, as the limited slip regains its 45% lock.(The steering wheel can be unwound during this time.)
(Think of this as a mild understeer, to a mild oversteer, back to a neutral event.
Of course, the car must have the suspension very well balanced, in order for this all to occur.
And when it all works together, there's only braking and acceleration involved...zero "coasting".
This is a very fast way through corners....and takes a high amount of skill....something that must be gradually learned.
Walt Moss showed me this technique (quite a few years back).
Very frightening, the first time, as a passenger!
A quick update, I decided to send the transmission off for some refurbishment. As Greg pointed out above the synchros were fairly worn and now was a good time replace them and go through the rest of the internals.
Per a few recommendations, I am using an 120qt ice-chest for transport. I have verified that a 110qt cooler would also work but this is the one I found used on craiglist. It's all packed up, insured and sent off to the shipper today.
Last edited by Michael Benno; 01-13-2022 at 05:54 PM.
I'm interested to hear about your flywheel/clutch plan. It sounds like you are going dual disc? I have the impression that the way to do that is to use an 85/86 flywheel, but somehow change the timing sensor ring the later style. I am thinking of putting a '91 motor into my '79 cab. Lots of ways to go on the transmission, but my first thought to retain the 5 speed and dual disc clutch from the '79 and somehow come up with a flywheel that will accept that clutch and have the correct timing ring.
I'm interested to hear about your flywheel/clutch plan. It sounds like you are going dual disc? I have the impression that the way to do that is to use an 85/86 flywheel, but somehow change the timing sensor ring the later style. I am thinking of putting a '91 motor into my '79 cab. Lots of ways to go on the transmission, but my first thought to retain the 5 speed and dual disc clutch from the '79 and somehow come up with a flywheel that will accept that clutch and have the correct timing ring.
If you are going to mate a twin disc clutch to a S4/GT/GTS motor then you will need to have hybrid flywheel made that uses the timing gear from a S4/GT/GTS matted to the flywheel for the respective twin disc clutch. There are a few vendors that will make this hybrid flywheel assembly for you and there are a few rennlist posts of people doing it themselves, Since you already have the 79 clutch assembly All you need is the motor and the hybrid flywheel.
For me, I need to purchase all the clutch parts. I am waiting on the pre-assembled clutch assembly and hybrid flywheel to be shipped to me.
I have a fixture to locate the later model timing ring onto the '85/'86 flywheel, indicate it in, and then tack weld it in place.
Karl:
Your '79 intermediate plate/pressure plate is located on the flywheel with a 90 degree "step" on the flywheel. This will not work on the '85/'86 style flywheel. You will need a later model intermediate plate and pressure plate, with the dowels.
may i ask , what's wrong with the later style single disc ? Why do you need the twins ??
While I dont have any direct experience with a dual disc clutch. This route was recommended to by several trusted individuals. My understanding in speaking to others more experienced that me, is the single disc clutch does not offer as much clamping force as the dual disc. This becomes a problem for durability on higher HP/Torque motors like GT/GTS and boosted/stroker motors. In reviewing the threads about swapping to the dual disc clutch there are other benefits such as a much lighter rotating assembly and better modulation because the engagement is much lower in the stroke and the engagement takes place over longer travel. As I remember the single disc clutch in my S4. The engagement was at the top of the stroke in a very narrow range.
Hopefully others with direct experience will chime in on the benefits of the dual disc over the single.
12-13-2021, 05:55 PM
