bowerbird

As I continue to marvel t the 928, I wonder why Porsche designed into it what seem like two powerful weaknesses.

Rubber Timing Belt…

Why? It seems like every other V8 has chain driven cams. I know there is a reason for the belt but what is it? Is it because the belt has less rotational mass than an equivalent chain? Its less serious of course on the 16v motors, but on the 32v, like most modem cars, is a ticking time bomb and is quite annoying.

Torque Tube…

I read an extensive thread on torque tube replacement on this forum (can’t locate it now) and wondered again why the design of the driveline would include bearings set only by friction in what seems to be a really heavy and fat steel tube. I don’t have a 928 and have limited mechanical understanding but the setup seems wired at best. From the number of TT replacements discussed here, it seems like the design is possibly a flawed one.


Thoughts?

Landseer

I love both. They sound hard to handle. People sell their cars to avoid them. I buy them and fix them with a beverage in one hand.

dr bob

The timing belt was (and is...) a great solution to driving the cams. It is somewhat self-dampening instead of transmitting vibration as a chain is. It needs no lubrication and is incredibly quiet compared to chains. If you think about statistical life of a car, this little 'weakness' would only need to be replaced once in the average life of the average car. Modern belts last a lot longer, with the original steel reinforcing cords now made of Kevlar. So what part of the timing belt don't you like?

On to the torque tube. The tube itself is there to maintain the rotational relationship between the engine and the differential. It allows the gearbox and diffrential to be mounted in realtively soft supports, so there is much less noise and vibration transmitted to the body. If the diff were allowed to rotate, engine torque reaction would cause misalignment whenever you changed throttle position. Oh, and that annoying suspension/driveline windup with wheel hop and the like? It's missing from your 928 thanks to the connection made by that torque tube. The driveshaft running down the middle is supported at either end by relatively small bearings. Inside the tube, there are at least three relatively large 206-sized ball bearings, situated at the normal flex/vibration nodes of the shaft. The shaft itself is springy, along the lines of the torsion bars used in the 911 rear suspension. I have no doubt that the first test mules had parts from the 911 suspension pile for initial test duty.

If you had been tasked with supplying to the same design requirements, what would you have used?

RevAgent

That may possibly be the best response ever

Landseer

Lol, thanks, and maybe so, but I always envy Dr Bob's responses.

bowerbird

I hear you on the "self dampening" of the belt and chains though typically longer lasting do break. I'm sure spinning a belt takes less force and probably translates into less resistance on the engine too? I'm generally annoyed by belts because they dramatic way in which they often fail, particularly with interference engines. Anymore I won't consider an interference motor with a belt. If I ever do find the right 928 (have been searching for a while) it won't be an 85 on for this reason. Thanks though for the detailed explanation of the TT rationale. I know Id get a concise answer here.

karl ruiter

I think the engineers were tired of hasseling with the chronic cam chain problems of the 911 and tired of trying to make a car with all its weight at one end handle. They moved on to better ideas.

Bill Ball

The torque tube is usually a one-time job if at all. I'm still running the original one untouched in my car. The timing belt is a one day job every 60,000 miles. I can't get real excited about it.

Black Sea RD

The torque tube (TT) design used by Porsche in the 924/944/968 and 928 variants has worked very well through the years and has withstood increases in HP/TQ done by owners, to include Porsche, without complaint.

Their main problem is that the internals are getting old and when the TTs are rebuilt the old parts are re-used and incorrect clearenced bearings are put in which can lead to problems and failures with these rebuilds. The friction fit for the bearing units to capture the bearings were really designed as a one time use affair.

But thankfully there are some good replacements out there nowadays...

Cheers,

SteveG

TB: [Note, I thought the same thing about a 4 cyl Honda when I first heard it had a rubber belt that had to be changed.] Although it is something of a weak point, it gets an inordinate amt. of attention here b/c it is solvable by the average diyer, so it is discussed a lot. If it weren't doable, we wouldn't be here. We find it an "acceptable" cost of ownership.

There are no figures available, but I would make the case that the failure rate on cars owned by "knowledgable" diyers is very low, as Bill said, once every 60,000 and you don't really worry about it, just keep an eye on it. No performance car is a "set it and forget it" option.

Dennis Wilson

Agreed. Considering that a 2' steel chain lasts about 100K, on a non-overhead cam engine, before it stretches and starts to skips teeth, a 7' long chain would stretch the same amount in less than 30K miles. AFAIK most overhead cam engines during this time period had belts rather than chains.

Concerning the torque tube, don't forget the safety factor. Porsche (and all auto manufacturers) were faced with a "proposed" 55 mph crash rating by the US DOT.

Dennis

dprantl

Sorry to say, but Porsche, VW and Audi were all quite incompetent at the time with regards to timing chains. Mercedes and SAAB both had OHC engines driven by chains, the former including 4.5 liter V8's in the '70s. With proper engine oil changes, these systems lasted past 200k miles easily, and still do today. It seems Porsche has finally figured this out, as the Cayenne and now Panamera both have timing chains.

Simply put, designing a reliable timing chain system was much more difficult, and Porsche just took the easy way out.

Dan
'91 928GT S/C 475hp/460lb.ft

hacker-pschorr

My mothers V8 Mercedes smashed all the valves when the chain tensioner failed (it was the updated, double chain version).
I have friends with C5 Corvettes that lost their valves due to a timing chain failure.
No system is perfect. I have never owned a car without a timing belt and I have yet to experience a failure.

Belts are cheap and easy to replace (at least on a 928).

dprantl

Of course there are bad apples in every system, and nothing is ever perfect. It also depends on whether the previous owners changed the engine oil regularly. But the fact is that most of the cars have had no problems. I've seen '70's mercedes with over 250k miles and original timing systems still running with no problems. I have owned 3 SAABs in the past, all eventually run over 200k miles, none of which have had their timing chain touched.

Dan
'91 928GT S/C 475hp/460lb.ft

dr bob

I've had several cars with chain-driven overhead cams and more with belts. For some reason the bulk of the chain-driven cars had the engine mounted directly behind the driver's seat, so all that noise was added to the music from the intake, exhaust, piston and combustion noises, and alternator roar. A few had multiple chains, making the noise level even higher. One particular British car had a plywood firewall with jute insulation glued to it to help keep the noise levels within reason.

Here's a reality-- Roller chains stretch a lot more than belts do. Longer chains stretch a lot more than shorter ones, amazingly. Running a roller chain with serious tension isn't practical, since the little bearings wear out much more quickly when the lube is squeezed out of the rollers. Running a chain across longer spans requires guide plates with rub strips to kill oscillations/flapping. Look at the length of the belt on the car, and consider that you would need two chains (one for each cam bank) and still want a belt to drive the water pump and maybe one for the oil pump. The oil pump is likely close enough to the crank to let you use gears; the gears could share the splash bath with the chains, and the spray needed up higher.

Look in the front of those revered M-B chain drives and see that there are (sometimes copper) tubing spray bars inside the cover, supplied with oil from the lifter galley, same source as the chain tensioner. Engineering and assembly marvels when produced. As someone pointed out, Porsche learned a lot from their early 911 efforts. When the drive belt technology became more reliable, they decided that it was suitable for their flagship cars. The backmotor cars had issues with heat and oil control that made belts impractical, while in the relativey dry water-cooled 928 motor with the belts in ventilated housings, the belt makes perfect sense.

FWIW, the chains used to drive the SBC pushrod motor is generally not a roller chain. Maybe in the late hi-perf cars but while the roller chains have lower friction and are a lot lighter, they do stretch a lot more than the pin-and-plate style of drive chain, and aren't as amenable to using spalsh lubrication.