EspritS4s

With all the supercharger discussions that have gone on lately, I'm curious about where the weak links in the S4 drivetrain are. From reading through the archives, I haven't seen any mention of upgrading clutch/TT/trans. I've seen the 600hp number tossed around a couple of times, but I'm curious if anyone has any insight into what is likely to fail first as the hp/tq is increased.

-Gary

Lizard928

the only thing I have seen was the TT and to fix that Jim from 928 intl added an extra bearing in the tube and that was all, although at the australian 928 page they say the stock 5 speed can handle 750+ BHP

Lagavulin

Can you please provide the link to where you read this?


I had to replace my clutch after supercharging as it starting slipping badly, specifically after adding Louie Ott's X-Pipe (..a very good piece of hardware!). However, the clutch had 53k miles on it and needed replacing anyway.

After removing the clutch it was discovered that the flywheel was grooved like a Ruffles potato chip, and the recommendation by 'the local 928 racer' Jean-Louis was to have it resurfaced, and I did.

The surface of the pressure plate had spots on it where it appeared that the metal had literally smeared, but otherwise it looked ok. I did not change out the throw-out bearing either since I would have to buy the new-style clutch fork to go along with it

In total, I fixed four things: the pilot bearing, resurfaced the flywheel, new disc, and a new clutch slave cylinder.

At the time, I just wanted to get the car back on the road as soon as possible with the thought that during the coming winter, I'd replace the entire assembly with a retro twin-disc setup, thus why I elected not to buy a new pressure plate and throw-out bearing.

Since the installation however, I have had NO problems, and no slippage whatsoever with the single disc setup, and I drive the car hard on a daily basis! So for the time being, it appears that the S4 single disc clutch is good for 600+ crank horsepower on the street.

As for the rest of the drive train and its longevity, there are two things worth noting that I see which does not compromise the drive train from a street perspective.

First is the centrifugal supercharger power delivery which is very linear in nature with an unmatched top-end which does not stress the drive train anywhere near that of a stroked engine and it’s comparative gear crunching low-end torque.

Secondly is traction, or lack of it, as the rear tires go up in smoke as the engine bounces off the rev limiter anywhere past 4000 RPM in second gear (..manual) which is extremely easy on the drive train, but admittedly a little hard on the tires!

With all that said, I do plan on installing a transmission cooler to help dissipate the additional heat produced by nearly doubling the factory horsepower, which should provide an additional margin of safety.

Finally, it has been written that the automatic transmission is more robust than the manual.

MikeN

I've always believed that those claims of "good for 600hp, 700hp", etc. are a bunch of crap. I do however believe that most driveline parts on a 928 may be good for a 10-20% increase in power, but after that I think it's a power vs. reliability scale that doesn't exactly favor long term reliability.

If you look at a GTS automatic you will see that the factory added on an outboard oil cooler for the tranny and retarded ignition timing of the engine in the lower gears........all to save the tranny a bit. Now if the thing was really good for 600-700hp from the start they would not have to do this at all. I think the same goes for all the drivetrain parts.......good and stout with an extra measure of over-engineering.......but even Porsche would be stupid to build in that much over capacity.

Sure, will a 928 tranny take 600hp.......yep, but probably for a short amount of time only.

Lizard928

http://www.landsharkoz.com/tt/ttmanvsaut.htm - bottom of page is where I saw that, and now that I reread it, here is the quote "Is there a manual transmission capable of handling big torque which shifts like a Honda? Devek's car has 375 bhp at the rear wheels; Mark Anderson claims his race car has over 500 bhp. Devek believes that the drivetrain can handle 750 bhp."

EspritS4s

Mike
I don't doubt that what you say is true. It's obvious that higher HP and torque levels will stress the drive train more and result in quicker wear. I'm sure that the people who are pushing the power envelop are expecting things to wear out more quickly. However, there is a difference between parts wearing more quickly and failing altogether. I was hoping that someone with experience breaking drivetrain parts would chime in on this one....

-Gary

Jim bailey - 928 International

Mark Anderson has broken two driveshafts , two transmisions ... Don Hanson has broken transmission gears BUT these are racing on a track ......if you tried to drive that hard on the street .....most likely in less than 1/2 hour you would be dead or in jail . There is just no comparison to the abuse sustained in racing . Even at the low level of the old very brown 1980 My son in law and I would wear out a set of brake pads and tires in 5-6 hours of track time playing on the short track at Willow and we broke a fifth gear (input shaft) . Driving down the highway at 80 mph only takes maybe 80 hp When you are wide open full throttle you are stressing the driveline and on the street that can only be for a few SECONDS at a time because you will soon be going 150 ! If you drag race a 5 speed and rev the engine high , sidestep the clutch even with a stock engine you are going to break things sooner or later . No one KNOWS what the drive line is capable of handling since so few have really tested them . It is worth noting that Porsche kept beefing up the 5 speed increasing the size of the bearings , the gears , adding oil circulation and cooling to the GTS .

Fastest928

Some FACTS

Peak HP is not a factor in trans or torque shafts failures.....TORQUE is.

All transmission are rated in torque capacity for some number of hours, often it is for 24 hours continious....take a look at how hewland labels their gearboxes......by TORQUE, not hp.

Don broke two trans...I built that street engine, I borke two trans, I built that engine also....what did these engines all have in common......a lot of torque!

The 928 trans can easily take 700 or even 1000 hp, depending on rpm

But only about 480 rwtq, around that number or after.....things break...even GTS boxes.

Regards,
Marc

EspritS4s

Marc,
Thanks for the info. I was hoping to get some feedback from the experts. You'll notice in my questions, I always lumped HP/Torque together. Only because the two are not independant, and no one is building a high torque/low horsepower engine. It's obvious that the greatest stress will occur when the most force is being applied. I would also assume that driving style will have much more of an impact at high torque levels.
Regardless, is it your opinion that the gearboxes tend to be the drivetrain component that fails first at high torque levels?

Thanks,
Gary

Ed Scherer

This thread brings back memories of about two and a half years ago...



Ever since that time, I've wondered about how the drive shaft would take even more torque. And then there's the thrust bearing failure issue...

EspritS4s

Ed,
What were the circumstances around that picture?

Thanks,
Gary

Ed Scherer

Nothing unusual, as far as I know. Just taking off from a stop sign with my stock-engined-'90 S4 AT and the shaft snapped.

Now I guess I'm a little abusive on the drive train, but not that bad...

I've kept that one shard that's in the foreground. Pretty interesting to see how the metal broke off. Kind of crystal-like.

Fastest928

The shaft breakage in autos is due to a misfire issues OR the clamp bolt losening. The former will raise it ugly head and snap your cam too....

Marc
DEVEK

Ed Scherer

Yeah, I'm familiar with the former issue; the issue (as I'm sure you know, Marc, but others may not) is covered in a July 9, 1992 TSB entitled "A/T - Central Tube Shaft Broken".

Here's some info from that TSB:

Concern:

Activation of the ignition monitoring system (injection circuit switched off fault codes 1131 or 1231) can lead to breakage of the central tube drive shaft.

General Information:

Oscillations in the central tube system that occur when the ignition monitoring system has switched (flywheel effect) can lead to breakage of the central tube drive shaft. This can occur only in vehicles with automatic transmission where the vehicle is operated at an engine speed of approximately 1000 RPM. Possible causes are:

• Damaged or defective exhaust gas temperature sensors.
• Possible damage to the ignition coil wire left side (in driving direction) between the ignition coil and distributor cap.
• Poor grounding of the mounting plate for the ignition final stages.
• Poor physical connection of the electrical plugs on the ignition final stages.

Whether or not this had anything to do with the failure I experienced, I just don't know.

Fastest928

Thanks Ed...good pointer.

Also, the auto trans can handle close to 600 rwtq, based on asome mercedes work doen a frew years ago by rentech.

Marc