tveltman

I was just reading the OE/OEM/aftermarket thread and I remembered seeing something a while back about individuals trying to beef up the 5 speed. Did this happen and I missed it, or did it fall through, or is it still in the works? I don't really need a new 5-speed, but it would be nice to know they could be available at some point (or already are).

Thanks in advance!

Lizard928

All I will say is that I am currently working on a redesign using the stock case....

There is no other alternatives I am aware of at this time.

IcemanG17

the general consenus is that the 928 5 speed is "good" until around 450ftlbs...maybe less on track with sticky tires......

The corvette transaxle is an option but many have said it will NOT fit without significant mods to the body....

Another option is a modified G50 from a 911.....like those used in mid engined kit cars....but its a pricey option and they are only slightly stronger than the 928 box....

personally I REALLY like the 928 automatic....VERY VERY strong when kept full of fluid at proper temps...remember the 928 auto is a mercedes 722 that was used on many powerful heavy AMG benz cars until the late 90's..... I have over 51 HOURS on track on my 140k+ mileage 4 speed auto (140k when the odo broke..who knows now).... I thought I "lost" 1st gear a while back but I didn't...its still there just sometimes harder to find with the bowden cable disconnected

Hilton

There was another thread recently that mentioned a possible option.. using auto differential and the shorter auto torque tube, and adapter plates. Will see if I can pop it up with a search.

edit: https://rennlist.com/forums/928-foru...mension-2.html

Lizard928

The power that the 928 manual transmission is good until really depends on driving style.

Also I looked at the option of adapting other boxes to an auto diff. Especially looking at a Muncie M22 rock chrusher. But have decided it is better to do something with the stock box....

RyanPerrella

The thing is only a 4 speed though

This is interesting though. One thing that Mercedes has done is that they are using a wet clutch setup now in place of the torque converter and perhaps if someone was very useful they could possibly adapt a modern 7 speed box from the SL and make it fit into the 928. I dont know how they may adapt, ive never seen the new merc box im referring to but with the 928 automatic transaxle it really is a standard auto with an adapter which puts the rear end immediately behind the output of the transmission. This could work on a more modern transmission.

With the elimination of the torque converter this could be a very nice setup.

Not sure what kind of electronics would be required to run it but the box that they use is fairly commonplace now. (I dont recall who makes it ZF or whoever but it is becoming very common throughout the entire line now.)

Also if you really wanted to get tricky, you could also throw in Mercedes mild hybrid system into the mix as that replaces the torque converter or clutch setup with an electric motor so you could perhaps build yourself a hybrid 928. But I dont see the benefit in that, just saying that they make an off the shelf electric motor to fit inside the case.

You can read some more about the transmission here

http://www.benzinsider.com/2008/11/a...agazine-award/

It appears that the bellhousing portion is a secondary part of the case. Thats good because you could fashion a new one to fit the bellhousing off the 928. (Unless this one already does) Considering manufacturers rarely every change bellhousing bolt patterns this may be a non issue.

The output shaft would likely need to be recreated in the proper length to mate to the input of the stock 928 rear diff. An adapter could easily be made out of billet to physically mate the two.

Then any difference in the length wether positive or negative could be altered with a new torque tube or simply shortening the factory torque tube.

I HATE AUTOS! But when i read about what mercedes was doing with the wet clutch in place of the torque converter it really got my interest. This could be a very nice solution.

I also have no idea what these transmissions would cost. New likely insanely expensive, but perhaps in a few years as these cars make their ways to the junk yards they could be had in the $3,000 range. I think they will be common to most 09 and later v8 benz's (for which there are a ton) but the clutch in place of the torque converter I think is limited to AMG models, but that could be purchased separately I would think.

RyanPerrella

as an alternative, I was interested in also trying to use the automatic bolt on rear diff and mate that to the back of a T-56 transasxe. You would likely need a new main shaft but there are enough people making those that it may not be too difficult. But this would likely be a $10,000 project to get the T-56 into the 928 even on the smallest of budgets.

I am liking the 7 speed auto idea a bit more assuming the electronics arent too much of a problem.

Also by keeping the 928's diff you can keep the PSD system easily enough although with a 7 speed you would likely want to change the rear end ratio which you may have to do anyway to get the output of the new transmission to mate to the ring and pinion.

I wish i had taken apart the rear automatic transaxle on the 928 i am not familiar with how they bolt up and how many parts there are post torque tube and how many unions and all that. I will consult the PET to get some more ideas though.

Vilhuer

As automatic box is longer than many manual boxes its possible to bolt some other make manual in place of automatic using automatics diff. Only real problem is at 99% of the manual boxes are designed to be bolted right after engine and thus they have large bellhousing for clutch. This will not fit into stock 928 body. Basically its either remove rear seats or make custom casing for what ever box one wants to use. Everything else is just raw work.

entropy_engineering

I would be very interested in having some people here "in the know" post what most of the failures are in the 5 speed box, both early and late style. I remember reading on here (I think mostly from Mark Anderson) about the input shaft cracking at fifth gear.
I build fast Mitsubishis among a few other imports for a living, and on those boxes we preload the bearings tighter for road racing to compensate for thermal expansion of the transmission case. They even feel "stiff" cold. Most of the failures I've seen here on the forum are on road race cars. How many people have managed to break one from power alone? Otherwise a good cooler could possibly help most people, possibly with more bearing preload.
I've also had surprisingly good results with cryogenic processing. It's gotten a bad rap from some as being ineffective, but my results are totally positive. It has to be a good shop that does "deep" cryo not "shallow" to get the desired results. The shop here in Indiana does tons of parts for race teams and all the brake rotors and drums for the local police, fire, etc. It's interesting to note the hardness of properly done parts DOES NOT CHANGE contrary to popular belief (they typically raise 1/2 of one point on the Rockwell scale). I've broken quite a few output shafts launching AWD cars at the track and have found a noticeable improvement in abuse tolerance long and short term this way. I'll be doing the box from my turbo 928 before hitting the track for sure.

RyanPerrella

The gearbox is just generally old

its clunky to shift, only 5 forward speeds, parts are NLA, different ratios are impossible to find

Why reinvest in making the same old wheel when you could perhaps find a more elegant NEW solution.

If you could improve shift quality INTERNALLY (not just through a better shift mechanism) that could be interesting. I guess 5 speeds is ok, you cant really do a 7 speed manual so were only talking adding one overdrive gear here but I think the G28 box is a little long in the tooth.

I would get more excited about a new idea rather than just refreshing the old standby, making new main shafts and what not.

WallyP

The "four-speed" automatic means that it has four forward gear ratios. The torque converter gives from near nothing to near two-to-one reduction in addition to the gear reductions...

The cushioning effect also helps the transmission and differential pieces last much better under shock loading.

It ain't real sexy, but it works! The two-speed Powerglide is a very popular drag racing transmission.

RyanPerrella

I dont buy 928's because they are good drag race cars or because they have smooth shifting lazy automatics.... I guess we see the cars for very different purposes

I suppose though that when you talk about putting in modern transmissions and modifying other parts of the car to comply with the new transmission, your better off just having the newer car and living with the shortcomings of a 30 year old design.

I guess in that case It would be time to move on from the 928 and into something more modern.

IcemanG17

Good points all around.....granted the automatics only have 3 gears you can use since 1st is far too low and can't be selected manually.... but the non stop problem the 5 speeds + clutches is concerning to me....with my interests in endurance racing I have my doubts that the 928 5 speed can hold up...since I see so many failures over and over again....not just in 928's but in ALL the racecars I see....very few survive under race conditions....

Bill Ball

I like 928 S4 automatics.

Jim Devine

I rode in a MBZ clk 63 Black Series. The automatic is incredible!
The owner was at an AMG event at the "RING" & asked an AMG
engineer why they didn't offer manual transmissions. The answer was
" because there are so very few people that could drive them properly" meaning perfect up / down gear changes etc. when driven hard.
With automatics like MBZ offers, that seems to be the future.
The 600 series v12 twin turbo cars when tweeked by Kleeman etc. put out incredible amounts of torque & the trans lasts.