mark kibort

Having raced the 928 probably more than anyone, I can tell you, I have not seen any signs that the tensioner can fail... its like a bolt....everyone thinks its some dynamic thing. sure, there are some very slow moving parts. (washers, oil, rollers) but if the bearings on the roller are good. you don't even need oil in the tensioner! it works like a fixed shaft, with a little give. mainly the washers are the temp adaptation to make sure the tension is good.
bottomline, it is a great little gysmo. I cant imagine it being a part of a problem. Its got a roller as a moving part, that's it. very bullet!
Ive cobbled some tensioner parts that didn't even fit as they should on scots car and didn't have a problem for years on his car.
as GB suggested, if you have an issue, its due to not checking tension or an aged belt. most often, a bad water pump or defective one.
Im running the "rubberband" Conti belt.. never had a stretch issue or tension issue, and I check periodically. I don't use any oil... im just kind to the engine until its fully warmed. (no oil and the washers don't expand as fast due to temp spreading slower with no oil) there is enough oil in there for lubrication and rust prevention, but nothing in their to damp out any belt vibrations.

Adk46

And furthermore, the fellow that adjusts it is part of the system. That would have been me, with whom I had no confidence. For that reason and others, I chose the PKT.

928porschemangreg

IMHO one difference in the stock tensioner is its ability to allow the 'grow' in the size of the block, whereby the disks in the tensioner flatten with heat, allowing the tension roller to back off easier, with less stress on the belt train as the engine grows in size. With a hydraulic tensioner, there is much more resistance to its compression, so the rapid growth of the engine during heatup, would place a strain on the belt train for a while. This is not so much an issue with smaller V6 and smaller engines.

docmirror

Yes sir, I alluded to that in post 17. It's a whole system which provides the tension via some rather esoteric engineering. As it were, looks like more engineering than is really needed.

BTW, I've got my bits apart and have inspected them and they all look and roll well. I'm going to stay with the stock unit so that I retain the warning.

For others who have switched to the Prokensioner, it's a fantastic advancement to the tension of the 928. If your Porsche bits aren't in top shape as you remove the belt path I strongly recommend just switching to the new style, and put the old complex stuff on the wall of shame. If mine weren't already pristine condition, and only needing a gasket and some lube, I would surely switch to gain the continuous tension that we find on all modern belt systems.


OBTW, the 105k mile change interval was mostly due to the belt being considered part of the CA emission system, and they must be rated to 100k miles initially. You will find that belt interval on almost all cars sold in CA and cars which have NY emission statements. Not that the belt should be left on for 105k on any car, I would advise 50k or so for any toothed belt out there.

Bigfoot928

Has anyone thought about this? The factory tensioner allows the belt to flap under hard acceleration and that means the tension on the waterpump can vary quite a bit. The PKT keeps a more even tension on the belt. This has to be easier on the waterpump bearings. In this line of thought the PKT will allow for fewer catastrophic waterpump failures.

Thoughts?

JEC_31

PorKen,
Have you studied the feasibility of incorporating the warning light into your system?

PorKen

Thought about it, is all - https://rennlist.com/forums/928-foru...ing-light.html

bureau13

There have been a few threads about it. But for a dynamic tensioner system, does a tension warning even make sense? Maybe I misunderstand it, but it seems that the tension is correct, until it isn't, with a dynamic tensioner, and at that point it's too late. However I wonder if perhaps the warning light could be used somehow to warn that the water pump has seized, etc

MFranke

I leave center cover on and listen on the block above the pump. I believe I would pick up a bad bearing from there if there was one. I have yet to have a failing bearing again so time will tell.

However, the stethoscope is really great for how cheap it is. One day hearing abnormal noise near the drivers side cam gear, stethoscope led me to the ps pump, where the three bolts holding the bell pulley were working their way loose. Fixed this and noise went away. Curiously, the exact same problem affected my other S4. These bolts had never been touched to my knowledge, so it is something I'll check periodically now.

928 at last

To the question specifically...
I've been around a while, and never heard of a catastrophic failure of the system. Lots of other reasons, but not the tensioner assembly.

Tom. M

I had an improperly rebuilt tensioner that didn't allow proper tension when hot. This destroyed my cam gear anodizing subsequently cutting the base of the teeth of the belt that led to shearing of the teeth at the crank when the engine was hot and bending two valves. No tensioner warning on that one and the tension checked properly when cold just before it happened.

Looking at the tensioner after it happened, there was a broken belleville washer and the stacks were not setup properly (probably leading to broken washer).

All records from the previous rebuild suggested all was done properly (a 911 shop in SOCAL).

If you have a stock tensioner on your car... pull it off and check it to make sure it's ok and make sure the warning system hasn't been bypassed as probably 50% of them have......

I'm running a PK tensioner on my GT now...no going back to the stock one and will soon be putting one on my track car... I prefer modern technology in this case....

docmirror

I think this is the closest we've come to a catastrophic failure. A field rebuild done improperly led to the tensioner not unloading on a hot engine which resulted in damaged valves. That's one thing about the tension warning system, it only senses a too loose condition, not a too tight condition.

I'm mentioning this because I have more wear on my pax side gear than my drivers side. The tensioner looks fine on the outside, but I haven't viewed the inside with the bellville stack. May have to do that now to insure my washers are set right.

danglerb

Fair chance the "better" part of using an air shock instead of the old style is that its cheaper and needs no adjustment either initially or at 1000 miles.

What I advise people is if you have any doubt about your skill in setting the tension, maybe the PKT is a better option. If you or your shop know how to set the tension, stay with the factory setup.

I think its a mistake to think much about the static tension, when the motor is running the crank gear is the major source of tension pulling the belt over the 5/8 cam gear. Each segment will have less tension as you work around the path back to the crank gear, with the lowest tension between the 1/4 cam gear and crank gear. With the PKT tension is the same on that segment running or stopped, which means the crank to 5/8 tension is increased by the difference between static and dynamic tension of the factory system between the 1/4 and crank. If the stock system flaps under some conditions that means the tension is close to zero, so the crank to 5/8 tension increase is roughly the stock static tension with the PKT.

Dynamic tension between 1/4 and crank might ideally be a much lower value than the static value.

Alan

"Ever heard of a catastrophic failure of the standard tensioner?"

To me the question is a little ambiguous... as you noted Tom has a case where the tensioner was damaged by incorrect assembly - I'm sure a Porkensioner could also be installed badly enough that it would eventually damage itself.

However does it really make sense to include poor installation cases unless one case is far more likely to occur than the other? Not really clear to me either way that is it.

I think the question is more: Has catastrophic engine damage ever been root caused to a stock tensioner issue.

To me the answer is clearly Yes from anecdotal accounts. The root of these issues always seems to be the 'nut behind the wrench' though - not the fundamental design.

The stock tensioner if installed, adjusted & maintained correctly will ensure adequate tension over a range of operating conditions for the life of the belt. It does not dynamically adjust tension in real time, but has a slow heat based adjustment to compensate for heat effects to the belt. The warning system will trigger for tension conditions below the warning threshold (slack belt) but not for overtension cases (always assuming the warning system isn't bypassed).

However if the 'nut behind the wrench' over-tightens the belt - then catastrophic damage to cams and or systemic damage to the drive pulleys/sprockets leading to belt damage & belt failure are possible. One has to assume some of the catastrophic cam damages we have seen are related to overtension, perhaps also cases of water pump bearing damage.

Now this is out of spec use - and human error - but is somewhat likely with this design.

The Porkensioner by its dynamic nature attempts maintain a set tension on the belt which also accommodates heat related changes to the belt. It is unlikely to ever over-tension the belt - but due to belt stretch and/or poor initial adjustment range - it could possibly run out of adjustment range leading to tension loss - without any warning mechanism. However the range of adjustment is quite large. AND - crucially - there is no user tension to set - if it is running within the tensioners range of adjustment the tension will always adjust to a set level (now the equivalency between the auto-tensioner max-min tensions and the operational range of max-min tensions seen with the stock tensioner is a good question - but we may never really know).

So:

A stock tensioner is more likely to result in (user/mechanic caused) but not apparent over tensioning, while under tension cases will be reported by a correctly configured warning light.

A Porkensioner is very unlikely to ever over tension the belt but if the adjustment range bottoms out (say due to unexpected degree of belt stretch) then not apparent under tension cases might be possible. Given the relative ranges of tensioner movement a well adjusted Porkensioner has a larger range of adjustment ability and should not bottom out under any normal belt usage.

I do agree that a warning system for the Porkensioner would be nice - the main case to detect IMO is running out of adjustment on the slack side and there was that other thread Ken referenced discussing possible solutions.

However let's remember that the majority of modern serpentine belt drives use a similar dynamic tensioner without any warning system.

Alan

kmascotto

This is very true...but important to note: How many of these engines are Zero Clearance Engines like the 928. IMHO...few.