MikeN

I don't think there is any set time for tooth failure, although most of the time I have seen it has been on cars where the whole system had been neglected or not properly maintained. There are exceptions of course.

I know your feeling about the belt. When I got my GT it had 22k miles on it and no one had touched the belt since the factory put it on. Before I drove it home I had it inspected/tensioned at a well respected dealer. According to them it looked great with no cracking or other signs of age. They set the tension and I drove 1k miles home with no problems. As soon as I got home I took a look for myself and found the same.....belt was in great shape.......in fact I could still make out some of the stampings on the back of it. Tensioner
was full of oil and not leaking, all gears and rollers looked great, etc.
I have all the service history since day one, including all the records from the only dealer that serviced it......and no belt service that I could tell had ever been done. Rechecked tension and it was spot on. Buttoned it all back up and
drove it hard 4k more miles......inspecting the belt 2 times within that period.....still the same, and I have never gotten any belt warnings at all. Even tested the system to make sure it was working! Finally changed the belt and rollers this last year at 27K......all the old components still looked great and it really made me cringe to go through it all with everything looking and working perfectly......but didn't want to tempt fate.

So what does this say? I still contend that when the belt system is maintained by the book by knowledgeable mechanics it is pretty robust and won't cause problems. It's only when the car falls through a few owners, and a dozen mechanics along the way......some good.....some not so good......that some troubles can begin......and simply balloon to full failures......which some unfortunate owners have experienced here.

Myself I would not want a chain at all.......even more complexity and more areas for leaks. I'll take the belt due to the fact that I know and understand the system pretty well at this point, know what to look for, and guard against it.
After owning 3 32v 928s I have never had a problem with any of them by just being careful, using some common sense, and using this great forum for help when needed. Just my .02.....

Voytek

Thanks MikeN,
Do You mean You have made 14 years on Your TB? This must be new record! You must be driving fewer miles per year that I do. I think Your assessment of the TB condition adds a lot to understanding of the time degradation factor component in TB failure.

Again, just like in many other fields, a human factor is the key. In commercial aerospace field, over 60% of catastophic events are attributed to the flight crew error. Airplane systems usually work very well. It is often maintetence crew error (Chicago DC-10 accident) or flight crew error (too many to count) is what contributes to a catastrophic event. It appears that the 928 TB system is no different.

Not withstanding MikeN feelings about the chain, does anybody know why exactly Porsche selected TB rather than a chain?

SharkSkin

I don't KNOW but I can guess. Reduced rotating mass, quieter operation, simplicity of design, no stretching problem as with chains, easy maintenance...

ViribusUnits

The biggest problem with the timeing belt is, IMHO the water pump.

Everything other than the water pump is pretty simple, and straight foward. The belt ages is a rather predicatable fashion. The gears are easy enough to inspect. The idlers are like idlers everywhere, and pretty reliable. There is a slight trick to the tensioner because of the way the bolts that holds the tensioner arm on is made. Some of the time, people tighten the bolt in accordance to the thick part, not the skinny part, and promptly break the bolt off.

To be honest, the only other alternative, chains, have their own problems. You've gotta get the tensioner right, make sure it stays lubed, change the chain after so many miles, change the idlers, etc. Basic maintance stuff.

The only thing that the water pump, like all water pumps is completely dependent on a set of seals keeping water out of the bearings. If the seals fail, the bearings will fail very quickly. The seals are in a difficult enviropment, what with trying to seal hot water and glycol on a rapidly spinning shaft. Some fraction of the seals are going to fail prematurely, and thats just a fact of life. The problem is that when the water pump's seals go, it's not a simple process to change it out, and it's highly likely for the water pump to screw up the timeing belt, and cause even worse problems.

I belibe the that if the water pump was driven my a V-belt, or ribed serpintine belt, and accassable w/o haveing to pull the hold front of the engine off, AND with out haveing to deal with 13 or 14 annoyingly thin, lible to get stuck and snap bolts, the 928 would probably have a much better history.

I've often asked myself why did Porsche decide to place the water pump where it is. To my way of thinking, I would have placed the tensioner where the water pump is currently, and had the water pump as part of the coolent mannafold that holds the thermostate. I should be able to change the timeing belt w/o pulling the water pump, as well as change the water pump w/o changeing the timeing belt. I would drive the water pump off of the smog/fan belt. The smog/fan belt would have to be changed to a ribed type similer to whats on the alt.

The only thing I can think of is that nearly every ohc engine I've ever seen has the water pump being driven by the timeing belt. The only one that I've seen where the timeing belt does not drive the water pump was on a 83 Honda Prelude. I don't ofen work on Japanese cars so maybe thats a Japanese thing?.

MikeN

Yep, 14 years on the original belt.....but with only 22k miles......that's probably the only reason it looked in very good condition. Mind you this is not something I'm proud of......just stating the facts. It has now been replaced.

I think there are a large number of 928s out there with TB belts that are much older than 4 years, but with low miles. The belts are pretty tough and age does play a part in their degradation......but to a lot less extent then bad or incorrect maintenance of all the items it rides on. Those are the things that can kill it fast......just my .02

heinrich

14 years ... my 85 had 15 years and 75k miles on her belt. And the belt was perfect, just the cam gears were shot.

heinrich

I'm with Viribus. That pump should never have been made to run on the tbelt. Porken ... to the rescue?

Bill Ball

I think the timing chain fabrication would be prohibitively expensive. Not just expensive, but ridiculously expensive. ALL of the pulleys, rollers and gears would need to manufactured from scratch to run with chain. Considering how long the chain is and its tortuous path, it would have to be one expensive strong but light design. The WP would have to remanufactured with a toothed pulley. Can anyone estimate the cost of such a project including design, tooling of all the gears, chain? Then there is the oiling issue and need for a new, more sealed cover.

Are there any examples of V8's with a chain path like this. I know of cars with individual chains to each bank of cylinders, but not a momma long single chain like this.

heinrich

If it's so easy why don't I just do it then huh?

OK here is how I see it:
Wpump redesign with a cogged roller ... no problem, they are entirely modular and are being rebuilt on a daily basis.
Long chain and oiling: Design a chain track that runs where the current belt runs, just in a perfect triangle. Bolt it on around the chain and oil it up. It can be an almost-sealed unit and botl onto the several mount points for the current covers.
You could even have an additional pulley off the crank, separate for each cam, and cut the triangle out completely. There's a LOT of room when you delete the tbelt; rollers; covers and especially the tensioner. For that matter the chain(s) could run in an oil sump like the crank does.

FlyingDog

What about taking the oil scavenging hose that goes to the oil filler and rerouting it to the chain? Would it provide enough oil? I haven't looked at the heads closely, but you might be able to take some oil from them that would be draining back to the pan. When the oil reaches the bottom, it could drain through the dipstick hole into the pan. You'd need a new place for a dipstick.

Bill Ball

Is it really that easy and cheap to get crank, oil pump and cam gear designed and built to run with Hi-Vo chain?

heinrich

Bill, I think the discussion is at this point fruitless because I doubt anyone is going to design this system. Point of order you're talking about crank GEAR, cam GEARS and oil pump GEARS right? Well those are truly easy to redesign and make.