Petza914

I have one of Carl's 6-speed kits in my supercharged Spyder (convertible). The original tranmission in that car was terrible with 2nd and 3rd gear synchros requiring double-clutching just to be able to shift. Took the supercharged car and made it slow because of the shifting. Rather than putting money into the original 5 speed with a rebuild since the gear ratios didn't really match the extra 125 HP and torque now that the engine was supercharged, I went down the Z06 6-speed path. Sourced the trans, Torque Tube, shifter box etc and had it shipped to Carl in WI. Drove it up there and dropped it off on a Sunday while he was out of town at a race and picked it up the next Thursday and drove it home to SC. It made the car SOOOO much better - being able to shift quickly like a regular manual, much better gear ratios for the supercharged engine, ability to run 90 mph on the highway at 2,000 rpm, etc.

A few things to mention - the special SPEC clutch discs he uses for this kit look like they have sintered metal contact areas and although they're full circumference and not pucked style, they are very grabby, so smooth take offs at anything other than idle speed or aggressive launches are challenging with some shudder. Also, there is noise and vibration transmitted to the car even with the the "street" mount option that I selected. Neither of those things were deal breakers for me and the car was much better to drive. After about a year, I did have an issue where the clutch pedal would get stuck when things would get hot. Took the clutch apart and used the proper grease on the splined shaft (which looked super dry) and that resolved the sticky pedal issue. At that time I also installed a pair of OEM friction discs to make the clutch engagement smoother - that was a mistake, as the first full throttle pull in 3rd gear had the engine spooling up, but the car no longer accelerating. The clutch was slipping. Took the clutch out again and went back to the SPEC friction discs and slippage was gone.

Fast forward a couple years and I was driving the car, applied the brakes hard and heard what sounded like a slide-hammer below me. Yep, I'm assuming due to the improper way the torque tube bearings were installed, they slide back and forth in the torque tube under heavy acceleration and braking so the car has pretty much been parked on the lift for a couple years while I've been working on other ones. I drive it around town when I have to get it off the lift to work on something else because it's a CIS car and they need at least some occasional exercise. Still fires right up and drives well. Occasionally, I hear it catch a piece of whatever is loose in the torque tube and whip it around inside. My plan is to remove it (which means dropping the whole torque tube and rear suspension (as it won't come out like a normal 928 torque tube) and send it off to Dan's guy (Dundertaker) to have it reworked with new bearings that are installed properly with the retainer rings and grooves upgrade he did on Dan's. Just haven't found the time to do that yet, but need to as this car is a lot of fun and has been to SITM a few times and the Atlanta Rendezvous.

Based on the way the car drove with the crunchy 5-speed compared to this Z06 trans, I would definitely do this mod again - just wish it had been done the right way originally so I didn't have to deal with the TT failure a few years after. From what I can see, I don't have any fitment or component alignment issues - maybe that will change when I'm the one taking things down and I'll also be checking the welds on the cradle based on some of Sterling's observations.

If I can get the torque tube sorted out and I paired this with Greg's new Tilton single disc clutch, I'd probably have the ideal driveline package. Who knows, if I delay long enough, maybe Hans' new clutch package will be available and I can find an affordable setup with different friction discs that hold the power and torque, wihtout being grabby like the current SPEC ones are.

hacker-pschorr

Your engine is not boosted either.

GregBBRD

Corvette transmissions are more modern and have carbon fiber synchros. 928 transmissions are of old design have moly coated synchros, which get hot, wear, and gall. They also get crunchy when they get hot, which is caused by the inability to get fresh gear oil into the synchro area. (993 transmissions have hollow shafts and drllings to "solve" this.)

We have modern carbon fiber synchros for 928 transmissions, solving the crunchy issues, the wear issues, and the gear oil supply issue. (They require very little lubricant, while also building far less temperature.)

Having studied every single transmission failure which Mark Anderson ever had (and there were several), inherently weak internal parts become quite obvious and can/have been dealt with.

The biggest issue is that all of these 928 transmissions are now old and the actual gears have a percentage of hydrogen embrittlement and visual wear, making occasional gear failure more common.
Additionally, some internal pieces are now NLA and are becoming rare, on the used market.
This will become a very limiting issue, in terms of practicality.

Tuomo's issues were that he selected a too early version of these transmissions, which had some pieces with a weaker design. He really never had a chance, regardless of what he did, to the stock gears and internal pieces.

Without much more detail, I have no idea how someone could break multiples of the '87/'88 5 speed transmissions unless they had insane horsepower turning them, or were installing random used/junkyard transmissions that already had issues.
Those are fairly robust units that when fresh, should be very durable.

Worth noting, over the years, I've also had clients who could break a crowbar in a sandbox.
There's not much one can do, for these people, as they will break everything they get their hands on.

Rasant Products

Quote:

Originally Posted by hacker-pschorr

How many 500+hp 928's have you had in the shop?
It's no rumor. For starters, ask @ptuomov how many 5-speeds he's gone through.
Tim Murphy's 5-speed in his 88 didn't last long with 556rwhp and 295's on the back. Then he upgraded to a GTS 5-speed. He hasn't driven it much since.
Mike Simard's 7-liter made short work of a stock 5-speed.
There's more if you do a search on the subject. Driving style and tires definitely play a part here.
Then talk to the track guys, even with moderate HP levels more than a few 5-speeds have met their demise. @Mark Anderson would be the best person to talk to on that subject.

Quote:

Originally Posted by GregBBRD

The weak link on the manual transmission cars has always been the clutches, not the transmissions.

Conflicting information makes it SO hard...

Rasant Products

Sure wish that you lived near me Petza914... We'd set up a corner in the shop for the project, pull the whole damn setup out and figure out how to make it better great and then knock out a few kits for myself and others here...

GregBBRD

I'm not trying to conflict, at all.
The 928 5 speed transmissions, when new, were very robust. Especially by the time the GTS vehicles were built.
The "youngest" GTS transmission is now almost 30 years old. And examples of these are difficult to find and generally need work.

Given the 928 transmission parts situation, at Porsche, some other alternative is probably a good thing to look at, for high horsepower applications.
However, dropping 25-30K into a drivetrain swap is going to discourage many people considering an economical supercharger upgrade. These people are probably going to want to know that they can rebuild their stock automatic or manual transmission (which probably have very high mileage on them and work poorly), and be perfectly fine.
The people who want to make 1,000hp, will probably not blink at that additional 25-30K.

I think that part of your conflict is deciding what market you are trying to initially attract.
If you want to attract the people who are looking for economical horsepower, I think you absolutely need to concentrate on making that function with a stock Porsche transmission, of some sort. If it's a worn out automatic, rebuild it. Put in some Kevlar brake bands, some high performance clutch discs, and do all of the Mercedes upgrades (considerable on a pre-1989 version.) If it ends up needing a different torque converter, deal with that.
If it's a manual with shifting issues, address that. If it is post 1984, install carbon fiber synchros and make it a much better shifting transmission.
If the clutch is not up to the task, you can use a "chatter master" SPEC unit or go with some other proven unit.

When the big dollar guy that wants 1000hp shows up, deal with that, then.

My $.02.

jcorenman

I am thinking that coast-to-coast enclosed transport might be a rounding error for this project...

hacker-pschorr

Not really, unless you are assuming all the cars I listed had stock clutches.

I'm honesty not sure what help it would be to have his car since the whole basis of that kit is designed around a horrible idea = modifying the stock cross member. The horrible welding was just a bonus.

@Dinsdale Piranha went a much better route IMO, he made a new one:
https://rennlist.com/forums/928-foru...swap-pics.html

+1
Tim Murphy and I were often asked to "turn up the boost" on the Murf928 kits. We had the formula, his 88 made 50+hp more than the kits (and that supercharger wasn't maxed out yet) and what was going to be a Stage 4 kit on Todd's red car made 670rwhp.
Aside from what the stock transmission could hold, drivability became the big issue. We decided early on that 450-500rwhp was "plenty" for a bolt on, turn key kit. Multiple cars doing test runs with various devices put down a 0-60 in under 4 seconds and 1/4 slips in the 11's. Todd's 670rwhp supercharged car and his Twin Turbo were practically undriveable until he started modifying the chassis to have a rear weight bias.

We're working on a PnP harness for Motec M130 to go in my 87 so at the very least I'll have traction control with the Murf928 setup I'm installing this winter. I also have a brand new, from Porsche, 1989 LSD trans going in with a slightly higher stall torque converter. This is the same supercharger Tim had on his 88 that has the potential to put down 575+rwhp if I decide to max things out.

Petza914

You're probably right, and it's something I would actually consider to be able to get my car back working again without it having to occupy my lift for the months necessary to take it apart, ship the TT off, get it back and put it back together. If it came back with a smoother and better clutch, that would be a huge bonus. Already being supercharged, there might be value on that side of the equation for Rasant on 16v cars also as I've done some cool things with a cold air intake, dual chamber AOS with heated fluid from the intercooler, etc.

The car is drivable as is for transport.

Adamant1971

What were the percentages when scanned? Assuming you performed an SEM scan since this can not be diagnosed visually. I have a 5 spd 87 so this issue concerns me if I do go forced induction.

GregBBRD

Truthfully, at this point, I'm looking for gear sets that are not damaged or don't have wear at the base of the teeth to build transmissions from.
(Some of the pieces are getting extremely difficult to find and this can be challenging enough.)

I'm certainly not performing a SEM scan [I'm not sure that this would be financially feasible, for a random transmission rebuild. Nor would I have a clue when any particular "scanning percentage" would result in me tossing a perfectly good "looking" (which I may not be able to purchase) gear, into the trash].
I'm probably making a bad assumption that broken 40 year old gears, which fail without any other signs of reasons of failure, suffer from some sort of material deterioration.
Somewhere, somehow, I was led to believe that hydrogen embrittlement was a big factor in high grade steel failures.
In reality, failures could be caused by some sort of additive in some types of gear oil, for all I know.

Adamant1971

Yes, it's hard to know without expensive tests. I am sure it plays a role but how much is the question? There are so many variables.

IcemanG17

Its the mercedes 722 design strong....yes.....is the 4 speed stronger than the 3 speed....yes...and the 5 speed used in the 738ftlbs of twin turbo V12 goodness are just an overdrive version of the 4 speed with electronic control....... Benz used that 5 speed FOREVER on the AMG 65 series cars they put in everything

Do I have any doubts that a Doc improved 3 or 4 speed will hold as much power as he claims it will.....no doubt...for a very long time with minimal maintenance.....the lowly G28-03? he rebuilt for me in 2012 with a tighter LSD still runs in casper on track....

When I put an S4 4 speed with LSD in my old lemons racer I had a well known local mercedes shop go through it....not for more power but just to be fresh......it ran in lemons for 13 YEARS....mostly on Euro S engines....with annual fluid-filters before it let go......So thats 16 to 24 hours endurance racing per weekend, 4 to 5 events per year.....sure sometimes something else broke, but call that 50 hours a year with practice or 650 racing hours.... not too bad....quite a bit better than the record the USA 4.7L lasted 236 hours on track

GregBBRD

Porsche considers a wide variety of their hardware to be a "one use" application.
I've never seen a single one of those pieces of hardware stripped , stretched, or otherwise damaged. (Except for 356 ring gear bolts.)
And yet, I figure that Porsche has a reason for this and thus I replace those pieces of hardware.
I was told, by someone that knows a bunch more about this topic than I do (actually makes some hardware), that this is all about hydrogen embrittlement.

Might actually be about the oil/never seize used when assembled....I have no idea.

Rasant Products

Interesting points...

Okay, so - two cars that I currently own - beyond the 928 world -
1) Mercedes-Benz W124 Estate into which we swapped a 5-speed M-B 722.6 auto from an E55 AMG and that the driver controls (via paddles), with a PCS (Powertrain Control Solutions) standalone system.
2) Alfa Romeo Milano (X-AR transaxle) and now with a forward-mounted, manual 6-speed Getrag.

As for the car in 2) Yes, I know - sacrilege (how dare you disturb the "near-perfect, 50/50 weight-distribution of the transaxle Alfa, in exchange for a Kraut-mixer no less!?) Well, the AR transaxle was fine (once upon a time) for a little 4-cylinder that made 120 horses on a good day, in a stiff wind. The split drive shaft configuration with three GIUBOS and a center-support bearing (which - as I understand it - Porsche even tried out initially for the 928 test-mules themselves), is grossly inadequate for the power that the car now makes from it's 24-valve big-bore motor.

AND, the gear-grinding, slow-shifting syncros drove me nuts, the GIUBOS blowing up and the imbalance issues - all of it sucked and had to go! Now I have a somewhat conventional rear differential setup (albeit with a short input torque tube) and just a short little 2-foot single-piece driveshaft. An E39 M5 rubber donut at the front, a CV at the rear and the system has proven to be bullet-proof! That Getrag shifts like buttah.

As for the car in 1) Please spill the beans. Do any of those bullet-proof internals swap over to the Porsche M-B transaxle perhaps? I went that route because we're still going to go forced-induction on the W124 at some point - and that platform, is STRONG.

Here's the question; is it SO far-fetched to imagine abandoning the transaxle setup in the 928, in favor of a forward-mounted box that can handle the power and a conventional differential and LS hanging out at the rear (from some donor model of vehicle yet to be named...?) Maybe a ZF at the front, or a Getrag as well. A different idea, I know. Go ahead - smash it with rocks.

Would I be disturbing the weight-balance? Sure. The handling of the Alfa is still VERY neutral (corner-balanced and with coil-overs.) The 928 is heavy and would likely be even more prone to under-steer I guess, but still.