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Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
Cam sprockets and oil pump sprockets were new at belt and tensioner installation ~1500 miles/3 years ago. Can't remember if I did new crank gear or not. Have to check receipts. All looks good. I can see how the little pulleys next to the crank gear could help keep the belt from jumping there if slackened. Guess I should have left them in there.
I did remove the passenger side rotor, but reinstalled for reference...they can only go on one way, correct? (All three allen bolts will only thread in if it is in the right location)
I haven't looked at this stuff for three years...can't remember.
Crank gear - like Greg said. Looks askew. You can see a gap - there should never be a gap.
What gap are you referring to? Keep in mind this picture is not directly head on with the crank...it is from slightly above (more head on with tensioner/pulley)
The cams are off the same amount in the same direction so the belt never jumped on those gears. The belt came off the crank sprocket and jumped almost a half engine revolution!
I'm just spitballin here but what if the engine kicked backwards, and because the racing belt is so tough and not stretchable, it forced the PK tensioner backwards enough to cause the belt teeth to slip on the crank gear? It would have to be a very violent pull as I have released and pinned a PK tensioner and the damping does not allow it to move that quickly.
I look at this and it looks like I can see in between the crank and the gear.
It's almost like I can see behind the crank gear. Could it be a shadow.
The Gates Racing Belt fits looser in the teeth, than the stock belts. Roger had the Gates engineers visit me and we discussed this, at length, showing them the "slop". I gave them samples of gears, stock belt, their own racing belt, and other pieces to study the issue. When they left, the guys in my shop all agreed....they didn't really care.
I never heard back, personally. I didn't expect to, actually. Roger told me that they said everything was fine. I trust Roger...without any reservations. I didn't trust the engineers.
I then did some work comparing the expansion rate of the racing belt and the stock belt....to figure out if I needed to compensate the cam timing for the different materials (the Gates engineers claimed that the cam timing should be set the same...which was quickly discovered to not be true....there's several degrees cam timing difference between the two belts when hot (with the same initial setting). (Note that Fiberglass extends when hot. Kevlar shrinks.) It all turned out to be just too much brain damage to swap designs of the belt and to spend the time to figure out the correct initial cam timing because of the two different belt designs.....especially since I've had such a tiny, tiny amount of belt failures.
I use stock belts. (Which includes the stock Gates belts.)
At any rate, that tiny bit of slop between the gear and the belt isn't going to allow the thing to skip cam timing....the tensioner is supposed to keep enough pressure on the belt that skipping should be impossible.
Of course, the Audi tensioner could have been defective and collapsed....anything made can fail (or the "Perfect Storm" could have occurred....read on.) The interesting thing about the stock tensioner is that it is impossible to collapse it below where it is intially set....there's solid metal between the idler arm and the mounting of the tensioner. It can expand with heat, but never get shorter! Not true with the Audi tensioner....something goes wrong internally, allowing the rod to retract, and you've got instant grief!
The "Perfect Storm": Combine the Audi tensioner design with the "shrinking" Gates racing belt when it gets hot....and that poor tensioner is living a life of unstable hell. It has to "retract" when the engine gets hot....not get longer (something I'm not convinced it was ever designed to do.) So, if you shut the engine off with the cams in a position that allows all the slop to be between the 1-4 camshaft and the crank pulley, as the belt gets cold and gets longer....it is possible to have a bunch of slop right at the crank pulley. Have the engine turn backwards, without the "safety" idler at the crankshaft, and you've created the "Perfect Storm" for skipping cam timing.
There's been another "mystery" engine failure with the Porken tensioner. Since no one will talk about it or discuss what happened, it is difficult to know if this failure is similar to that failure.....but I believe that the two failures are similar: Skipped cam timing, valves bent, without anything obvious being broken or wrong.
Pretty obviously, I put a lot of thought into what I do, why I do it, and what pieces I use.....since all I do is 928s these days, I have to do this....just to protect myself from potential disasters. I don't have the money to pay to fix one enigne...much less, multiple engines.
It will be interesting to see if something actually failed, in this situation. However I'm not seeing very much wrong, right now....looks like a currently tight belt, with all the teeth, and all the pieces doing their job correctly. Yet the cam timing is many teeth off, with the associated valve damage.
The interesting thing about the stock tensioner is that it is impossible to collapse it below where it is intially set....there's solid metal between the idler arm and the mounting of the tensioner. It can expand with heat, but never get shorter!
This is not how I understand how the stock tensioner works. The bi-metal Belleville washers supposedly collapse when heated to lessen the tension created by the increase in engine size when the aluminum expands due to heat.
Originally Posted by GregBBRD
Combine the Audi tensioner design with the "shrinking" Gates racing belt when it gets hot....and that poor tensioner is living a life of unstable hell. It has to "retract" when the engine gets hot....not get longer (something I'm not convinced it was ever designed to do.) So, if you shut the engine off with the cams in a position that allows all the slop to be between the right hand camshaft and the crank pulley, as the belt gets cold and gets longer....it is possible to have a bunch of slop right at the crank pulley. Have the engine turn backwards, without the "safety" idler at the crankshaft, and you've created the "Perfect Storm" for skipping cam timing.
Your explanation does not make sense. The Audi tensioner/damper works like a shock absorber. It takes up slack by extending the piston immediately as the belt goes over it and retracts slowly when under constant pressure (up to 30 secs when cold). The tensioner/damper piston range is 11mm. Normally the installed extension is around 5mm.
Originally Posted by GregBBRD
There's been another "mystery" engine failure with the Porken tensioner. Since no one will talk about it or discuss what happened, it is difficult to know if this failure is similar to that failure.....but I believe that the two failures are similar: Skipped cam timing, valves bent, without anything obvious being broken or wrong.
It would be a 'mystery' to me as well.
I know of only two failures, both related to hardware (bolts). Both issues were addressed with the PKT-B or an updated hardware pack.
Are we still thinking those stock idler rollers might prevent this? I know I ALMOST removed them, when doing my timing belt job, but ended up keeping them in place. Kinda glad I did that now...
This is not how I understand how the stock tensioner works. The bi-metal Belleville washers supposedly collapse when heated to lessen the tension created by the increase in engine size when the aluminum expands due to heat.
Your explanation does not make sense. The Audi tensioner/damper works like a shock absorber. It takes up slack by extending the piston immediately as the belt goes over it and retracts slowly when under constant pressure (up to 30 secs when cold). The tensioner/damper piston range is 11mm. Normally the installed extension is around 5mm.
It would be a 'mystery' to me as well.
I know of only two failures, both related to hardware (bolts). Both issues were addressed with the PKT-B or an updated hardware pack.
OK...if nothing I have said makes any sense to you:
What is your specific senario for an engine with skipped cam timing (in the advanced direction), bent valves, with a currently tight belt (with all the teeth), all new gears and pieces, nothing obviously broken or falling off, equipped with your tensioner system that isn't supposed to allow this to happen?
Are we still thinking those stock idler rollers might prevent this? I know I ALMOST removed them, when doing my timing belt job, but ended up keeping them in place. Kinda glad I did that now...
I don't think so....the one engine that I had skip cam timing when it backfired on start had the rollers in place. It did have a "rubber band" instead of a real timing belt, which I believe stretched enough to allow this to happen, but it did have the rollers and jumped two teeth. It also had a pile of compression and displacement (6.5 liter race engine)....which allowed the crank to kick back very quickly.
What is your specific senario for an engine with skipped cam timing (in the advanced direction), bent valves, with a currently tight belt (with all the teeth), all new gears and pieces, nothing obviously broken or falling off, equipped with your tensioner system that isn't supposed to allow this to happen?
Seized water pump with a resulting overheated belt is the only thing that makes sense from the information available so far.
For some reason the belt is now longer than it should be. The tensioner/damper is at its max position. The T/D piston shows normal wear at around 5mm extension which means it was working normally for some time and then the belt became too long to keep controlled.
After Photoshopping for clarity the photo of the 5-8 gear looks like the belt is very shiny. I see reflections of the housing on it. It looks like belt dust in the crank snout photo, but it could as well be dirt. The crank gear looks worn, original.
I'm going to go out on a limb and disagree a bit with Greg vis-a-vis the Belleville stack design.
First, the way I understand the Belleville stack is that it will maintain nearly constant tension under differing temp conditions. As the engine grows, the stock belt remains basically the same length(non-racing). The Belleville stack should compress slightly as the fluid in it heats up. This would be consistent with the known properties of the metal used(Inconel 7xx series?). It would also fit the application, as the tension generally goes up as the engine grows under increased temp. However, I agree that the stock tensioner does have a metal short-limiting length that cannot be shortened unless the adjustment pin is backed out.
Next, I think and am pretty sure that the hydraulic tensioners operate basically the same in that they are designed to compress gradually as the engine grows, and just the opposite, in that they will expand or more accurately extend fairly rapidly to take up any slack under differing throttle loads where slack would tend to build up. This is consistent with where the tensioner is located at the end of the driven loop of the circuit and just before the drive end(crank). So it operates kind of like a shock absorber with a high deflect ratio for off road use.
I removed the small roller cartridge off both of my 928s, but retained the stock tensioner. My decision was based primarily on the extremely low failure rate of a properly maintained and adjusted stock tensioner.
05-19-2016, 09:38 PM
