siscogts

interesting.subscribed.

docmirror

Well, I think the objection in this case is - overruled. The stock tensioner is rather far more complex than it needs to be. My evidence is that the later model is much simpler, and lighter, and lower parts count. The stock tensioner has a piston, beveled washers, seals, lubricant, adjuster, swivel arm, pivot shaft, roller bearing, and belt path surface. A failure in one area of the tensioner can be non-catastrophic such as leaks(common) or falling out of adjustment due to belt stretch, or gear wear, or a combo of several things.

I'm not denigrating any new system, and believe me -- I strongly support the continued engineering work done on the cars to keep them running best. Also, I know, and have already acknowledged that the legacy system is not as robust in the job of belt tension as the modern hydraulic style. I've done enough belt changes with the hydraulic systems to understand the difference. The Ferrari old belt style was a fixed tension roller pre-loaded when the timing components were assembled. However, and it's a big giant 'but', in the case of the 928, unlike many other belt systems the timing belt on this car touches the oil pump and the water pump. While the oil pump seems quite robust and even when it leaks, it doesn't cause many/any catastrophic failures, the water pump is another story.

The 928 engineering team has not yet dug into the water pump issue enough to give me, or some others the warm fuzzy feel that the lack of a tension warning is suitable. I'm sure there are lots of cars out there with the alternate tension system working fine, and I'm not concerned about a failure of the new design, or the belt itself, but I need to know that when something goes wrong in there, I can have a warning before a catastrophic failure occurs. It's not just the belt and gears involved. If the tensioner is robust, even if it leaks and will last the life of the belt(40-50k miles), the reason for changing over is less attractive, given that the warning system goes away.

I suspect in the situation I just found myself that with the new tensioner I would be facing serious engine work, although maybe I would have caught it with the temp rise in time. But - maybe not. Until the water pumps get more reliable, it's going to be a crap shoot.

Mongo

I think in the history of Porsche forums, I have never heard of a failure of the factory tensioner by users on here unless it was poorly maintained, or skipped during multiple TB/WP services.

In defense of Ken's Porkensioner, many cars use self-tensioning and dampening system from Volkswagens to Ferraris with zero failures unless maintenance intervals were not followed. The hydraulic self-dampening design is more modern and also fail proof. I do believe Porsche engineers knew what they were doing (of course) during the construction of their cars, but we are still talking 1970's and 1980's designs here. They can be improved upon, or retrofits of new tech to old.

Some examples of Ken's PKT self-tensioning system on other factory cars:

Ferrari 355:




Audi S6:

bureau13

This brings up a question I've been asking (inside my own head). Why the hell are the water pumps so unreliable? Other cars seem to have mastered reliable water pump technology with no issues, why not this one? And I'll bring it up again, because I'm baffled that more people don't: Engine Guardian! It just seems like a great idea. I traded emails with the guy who makes it and he admitted there were some teething problems in the earlier designs, but that they have been running failure-free lately. Yet I hear next to nothing about them on the list. If you all know something that I'm missing speak up before I go off and put one on my car!

docmirror

Well, no one needs to defend anything here to me or anyone else. In the picture above, is the water pump driven by that large silver wheel, or by the smaller gold wheel below? It looks like the water pump is driven by the smaller gold wheel but the size of the wheel is kinda wrong for the water pump speed.

If that's the water pump drive, what is the big silver wheel for?

PorKen

Another pic of the 30V Audi engine. Silver pulley is a hydraulic pump. (Water pump pulley is black in this pic.)

Audi water pumps aren't that great either, one reason the original belt service interval was reduced from the original 105k mile/170k km. (This long interval goal is apparent in the size of the bearing on the PKT/Audi pulley. Compare to the diminutive -NLA- factory 928 pulley bearing, 92810551212)

ROG100

Current water pumps are very reliable as long as you use a new Porsche or Laso pump with a plastic impeller.
All the "old" issues with WP's were either rebuilds or metal impellers or both.

As with all parts there will be failures - just need to be very low rates/instances - currently the WP's are pretty reliable with few failures.

Mongo

Roger, has anyone at Laso figured out yet if they pinned the impeller with the improved bearing that the lifespan would be significantly longer? If the bearing doesn't fail, usually the impeller does by either just spinning on the shaft, or walking forward, crashing into the block to shatter it. I have always thought running a pin through the impeller and the shaft would be a good fix for this.

GregBBRD

Because the shafts that the impeller and the front pulley are pressed onto are part of the actual bearing, they are extrememly hard.

Drilling a hole through them would require some special equipment.

drnick

How many cars with the stock tensioner system have had belt failure and valve contact? I know its not the OPs question but failure of the tensioner needs to be expanded to look at how many cars are running with too much tension and the damage this causes. Not to mention how many false positives the stock system gives out..

Mongo

I didn't have a pump in front of me when I made this comment. Now that I do, I see what you mean and how much of a PITA it would be as well as a cost driver.

ROG100

Further to what Greg said - it appears to be easier to get a plastic impeller to adhere to the shaft unlike the interference fit of a metal impeller.

I am not aware of any of the plastic impellers coming off the shaft in the last two to three years. Seen about 4 failures due to leaking seal cartridges - however the leaks were pretty obvious in all cases - front weep hole.

polecat702

I'm running the Guardian water pump on my 89, and it's performed flawlessly. Got Ken's tensioner too.


FWIW, before Greg Nettles passed away, we were talking and he had put around 60K on his last TB/WP job in his car. He was using Ken's tensioner, and he told me that everything still looked new, the belt, cam gears, etc. He was sold.

Greg's recommendation is why I bought the PKT from Roger, along with everything needed to complete the job including Ken's timing tool. My cam gears were worn, so I bought a new set from Porsche, and gave my old set to someone on this forum, that couldn't afford a new set. I also replaced my water pump with the Guardian pump.

I've had some other issues but they weren't related to the tensioner, or water pump. Got around 8K miles on the car since the install. I don't have much free time to drive my cars so they sit a lot.

Lizard928

Please keep in mind that the bushings, pivot arm, idler roller, and tensioner roller are all part of the factory tensioner system.

So when any of those pieces fail, it is considered a factory tensioner failure.

It is the same with all pieces. If you do proper maintenance and replace it at the correct time/intervals, you will not have a problem with either system. But IMO the porkensioner is a far superior system with fewer weaker links.

SeanR

That's the part so many seem to forget about and refer to only that cylinder that holds all the disks in place as the tensioner. I've seen numerous failures of the system, seized rollers, bushings no longer there so **** wiggles all over the place, making the belt run in to other parts.

I suppose some think "tensioner" and "tensioning system" are two separate things. I don't.