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timing belt adjustment intervals

 
Old 12-23-2018, 01:54 PM
  #1  
merchauser
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Default timing belt adjustment intervals

TB/WP was done 2 years ago, 7k, and on my 01/2019 schedule was to inspect and adjust my timing belt and tensioner.

yesterday, my timing belt service warning triggered and I decided to service the belt today. sure enough, belt was outside
the kempf tool spec, and a bit loose. . got it set up properly and topped off the oil in the tensioner with STP oil treatment.

not sure I understand what the job of the tensioner is? I guess it is not self adjusting? how often should the tensioner and
belt need to be looked at? I was on a 24 month schedule, but perhaps it should be more often?
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Old 12-23-2018, 04:13 PM
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The factory tensioner has a pretty narrow range of position vs tension on the belt. Hence the recommendation that you check and adjust once after the new belt has seen an initial stretch. After that, if you see the warning you know that something has changed. The PorKen tensioner has a longer and more linear spring rate, working reactively to changes in block size with temp changes and wear. The factory tensioner is a little bit proactive, adjusting position in response to temp change so belt tension stays relatively constant. It still responds to tension changes but it’s very non-linear.

Theres a a ton of discussion history comparing both. If you have the factory tensioner as I do, we suffer through the inspection and possible adjustment after initial break-in stretch, the know the warning light will tell us if belt tension changes too much for the growth.
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Old 12-23-2018, 04:14 PM
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The factory tensioner adjusts for heat expansion of the block. As the block heats up and gets bigger, the bellville washers in the tensioner flatten out and retract a bit.

That keeps the belt from getting too tight when the block expands.
That's all it does.

If you want self adjusting for belt stretch, thermal changes, ect, you need a PKT.

The oil in the factory tensioner is simply to improve heat exchange, so the bellville washers can "see' the heat faster.

There are stories of cars being run hard, then cooled off quickly where an empty tensioner can't keep up and causing problems.
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Old 12-23-2018, 04:33 PM
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^^^^thanks to you both for the education. no big deal to check and adjust. timing belt was replaced, Oct 2016,
and 3 months/500 miles later, the warning light came on. that must have been the break in stretch?

that last adjustment was Jan 2017, so I have gone nearly 2 years since it has needed attention. is that about
normal?

should I be more proactive in terms of time? I have only put on 7k since the belt was done. so I guess its not a mileage
issue. it should also be noted that I have been pretty aggressive in the last 6 months; not sure if that adds to the situation?
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Old 12-23-2018, 04:46 PM
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It shouldnĺt stretch after initial break-in. But I installed a Gates belt, and have seen reports on here of the Conti belt stretching throughout its life and needing re-tensioning outside of the break-in period (from memory 1000 Miles/1500km or close to that distance).
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Old 12-23-2018, 05:16 PM
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^^I have the gates belt as well, but found many posts that suggest you examine shortly after install for an initial stretch.
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Old 12-23-2018, 05:22 PM
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Originally Posted by merchauser View Post
^^I have the gates belt as well, but found many posts that suggest you examine shortly after install for an initial stretch.
just to be clear, the Gates belt stretches in the break-in period (mine needed a very small adjustment). There have been reports of Conti belt needing re-tensioning after the break-in, without any other problems in TB system which could cause reduction in belt tension.
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Old 12-23-2018, 06:04 PM
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the good news is that all is good again and up to proper specs. I really don't like seeing that warning message.
I have read that it does not mean imminent catastrophe, but I immediately turned around and kept revs as low as possible
for the 5 mile drive home. did not skip a beat in getting in there to address ASAP
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Old 12-23-2018, 06:12 PM
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Paul,

When adjusting tension one must ensure that it is set with No1 cylinder at TDC on its firing stroke. Also, if you adjusted the tension after 500 miles there may be a possibility the belt had not reached the fully stretched condition.

Regarding the stock tensioner I suspect that thing is way smarter than many understand. The Belleville washer pack does indeed compensate for thermal growth and the experiments I recorded showed that the thermal growth I calculated and the contraction of the pack as it heats up align nicely. I also believe there is more to the Belleville pack than this feature and the thing I noticed with the pack was that by compressing it with my fingers I could see it contracting some small amount so the pack can and in all likelihood does spring in and out dynamically in service [i.e. it is more than just a fixed rod length applying tension. Why else would Porsche have installed a one way valve at the back of the chamber to allow oil back in but not let it out? I reckon that as the Bellville pack compresses dynamically the oil is forced out of the barrel and then gets back in via the check valve thus dampening the spring motion. Thus to some extent it not only tensions the belt it allows some degree of compression and relaxation with hydraulic damping as the belt tension changes with cam operation. There had to be a reason why Porsche took out US patents on the both the original [non check valve equipped] and later check valve equipped variants given there was nothing particularly new about putting tension on a cam belt.

As I can best understand Porsche advise the belt manufacturer how much force the cam operation is going to generate and the belt manufacturer advises the dimensions of the belt needed to facilitate such operation. Porsche then specify the tensioner to apply the requisite tension.
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Old 12-23-2018, 06:59 PM
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[COLOR=left=#222222]As I can best understand Porsche advise the belt manufacturer how much force the cam operation is going to generate and the belt manufacturer advises the dimensions of the belt needed to facilitate such operation. Porsche then specify the tensioner to apply the requisite tension.[/COLOR]
[COLOR=left=#222222]interesting way of getting from A to B. (FWIW: I did have no. 1 @TDC)[/COLOR]
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Old 12-23-2018, 11:02 PM
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Originally Posted by FredR View Post
...Regarding the stock tensioner I suspect that thing is way smarter than many understand...
Maybe.

I don't pretend to fully understand it.

But, when doing my research on stock vs PKT, one thing that stood out is that nobody used this sort of tensioner any time after the 70s. Not even Porsche.

The 944 has simple adjustable rollers for the timing and balance shaft belts. The belts are short enough that thermal expansion doesn't make enough difference to matter. Yet the 968 went with a hydraulic/dynamic tensioner.
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Old 12-24-2018, 12:29 AM
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Originally Posted by Wisconsin Joe View Post
Maybe.

I don't pretend to fully understand it.

But, when doing my research on stock vs PKT, one thing that stood out is that nobody used this sort of tensioner any time after the 70s. Not even Porsche.

The 944 has simple adjustable rollers for the timing and balance shaft belts. The belts are short enough that thermal expansion doesn't make enough difference to matter. Yet the 968 went with a hydraulic/dynamic tensioner.
Joe,

I can only surmise what might be going on with the stock tensioning unit-for sure it is not an established fact from my point of view- the bit that intrigued me as an engineer was why they fitted the check valve in the body of what I understand to be the tensioners supplied after about 1983 - my initial wild *** guess was that it might have needed modification to support 32 valve operation but the timeline did not seem quite right - whether my analysis of what actually happens dynamically speaking is correct remains to be seen but I am sure Porsche did not fit that feature to inflate the costs [even though it doubtless does!].
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