Schocki

You bet! Kompression ratio about 11:1.
It's a 3 liter 16V Variocam.

Whitesands

Thanks Ken.

69gaugeman

I decided to just post on one section of the comments. This is one instance where I agree with Greg that the factory system is more than adequate. All the other white noise in this thread is just that.

I was also aware of failures in the PKT system, but did not bother to do any detailed analysis.

The fear mongering that got the PKT system rolling can also work the other way, as I have seen here in several posts.

I am not aware of a single belt failure when the belt was installed correctly and the tension set and adjusted correctly. MANY failures from water pumps abound as do cam gear failures.

For general information, the oil in the tensioner is used as a heat transfer mechanism. Without the oil the tensioner it will still function perfectly when up to temp. The tensioner will 'theoretically' heat up some small degree slower than the engine itself. The tension 'may' vary during this short period of time. I highly doubt that that short period of time will bring it out of the tolerance window. Happens only at cold start up. The Porsche engineers were being extra cautious.

Same with the tensioner warning light. Considering the primary cause of failures is water pump failures, this warning is extra useful. True VW and Audi do not use a warning light. This is because theirs was designed as a system. Testing proved that mileage based timing belt changes exceeded the life of the tensioner. There is no such testing done with this system.

ptuomov

What fraction of the water pump and cam gear failures is driven by incorrect belt tension?

69gaugeman

Cam gear failures I suspect are very high. Maybe even 100%

Water pump I would say is less. The bearing surface in the water pump is quite substantial. I suspect seal failure and shaft movement is a significant proportion. Which wouldn't be tensioner related. To clarify I have never analyzed a failure. I have not, in fact, ever had one, or even know personally one who has.

On a similar line, how many got the tension error and stopped the engine in time to save the car? Or got the alarm and chose to ignore it <put up my hand> and then had a failure?

That is NOT a failure of the tensioner system. That is a factor of operator error.

You cannot stop someone from hitting themselves in the hand with a hammer. There is no such thing as a safety hammer. Sometimes it is the skill of the operator that makes the situation safe.

Overtightened belts are such a situation. And it is really not that hard to get right, even. The person who over tightens the belt should not even be in there in the first place.

FredR

Your post prompts me to ask a question based on your failure experience. Did you have repeated alarms and ignore them or was it a one off alarm and then a few seconds/minutes later "twang"?

I have often wondered about this. My perception is that if a belt is about to fail it stretches and the system loses tension before going pop altogether. The only scenario I can think of to support instant failure would be if the water pump suddenly seized [possible but unlikely I suspect].

I have experienced a couple of "false alarms" with the stock system during my 16 years of ownership but nothing more. When this happens I pull over, stop the engine, restart it and look for a repeat. If no repeat I remove the inlet tube and take a squint down the vent tube to see if th belt shows any signs of uneven motion at idle.

If the belt is truly failing or has worked loose, surely the alarm will repeat? I have never experienced a repeat alarm so assumed my alarms were false. Nonetheless I did check tension and duly found nothing untoward.

As long as belt tension is checked with the tension tool at TDC on No 1 cylinder it is quite difficult to go wrong.

Regards

Fred

Imo000

I had a pump sieze that made the belt stretch and triggering the warning light. The warning lihht came on way before the temperature hit the red and triggered that light. It saved my engine. The stretched and melted belt was seconds away from snapping. There are pictures of it on here.

69gaugeman

For sure the alarm will repeat. The ONE time I ignored it was because I had just rebuilt the motor and thought maybe it loosened up. I figured a brand new belt wouldn't be a problem. The issue was the oil pump nut came off and the oil pump gear was moving around. When it finally came off the shaft... well.. top end rebuild number 2 after running for 30km.... lesson learned... even I make mistakes. That is one I won't ever again, though.

ptuomov

The factory warning system is unreliable and gives false alarms all the time. This should be obvious from the warm-up time limit that the factory put in the warning system. Whatever happens in the first couple of minutes will stay within those couple of minutes.

As far as the belt tension goes, on planet earth people do things like tension the belt incorrectly. I don't know how it's on your planet. I generally prefer systems that are reliable on the planet Earth populated by humans and that take into account that humans are humans.

James Bailey

When the water pump seizes and I KNOW of MANY including one an 83-84 that my son in law was driving , another S-4 on the Oakland bay bridge etc......The scenario is the car quickly overheats but everything otherwise SEEMS ok so pull over wait for it to cool down check coolant level then in that irresistible human urge to make it back to the cave..... They crank the engine to SEE if it still overheats. During that cool down the nearly red hot smooth water pump pulley has vulcanized and melted itself into the timing belt...the belt then breaks on start up. Jeff was lucky his 928 was none interference the S-4 was not nearly as lucky. The more common timing belt failure is the teeth strip off until there are not enough left to accept the torque (or is it horsepower) of the crank gear trying to turn the camshafts , compress the valve springs, turn the water pump and oil pump.
I also find it interesting that the tension that we measure and talk about is a static measurement on a none running engine. The moment it starts turning all bets are off. No one here knows what those real forces are. Porsche undoubtedly did thousands of hours of testing of the belts to arrive at a satisfactory result and THEN devised a way to test and replicate that tension as measured on a none running engine at a normal temperature. Which has little to do with the tension of a running engine. Each segment of the belt would if you could measure it have a very different tension based on the work it is doing due to drag and the engine RPM. The crank gear to oil pump has the most, oil pump to drivers cam next , cam to water pump, water pump to passenger cam is under less than 1/2 the load of the crank to oil pump segment (but where we measure static tension) and of course the segment where the tensioner picks up the slack which is under the least load of all.
You only need enough tension to keep the teeth of the belt engaged to the teeth of the gear. It is NOT at all like a v-belt where the tightness and friction is all that keeps it from slipping.
In may ways it is like the ten speed bicycle chain where the chain is tight only on the top as it runs from the rear sprocket to the front sprocket as you pedal. The return chain under no load is slack enough to allow you to shift gears from sprocket to sprocket.
Or rubber belts are shrinking under the effect of heat, stretching under load and doing a whole bunch of work. Pretty amazing development really.

69gaugeman

On my planet the tension is NEVER set wrong. It is too easy to get it right. And simple to check once a year.

But I completely understand your point of view.

MainePorsche

This is true on my planet as well.
I check more than once a year.

Jfrahm

I've never had a false with the warning lamp myself. I have found the belt somewhat loose when it's triggered, and I'd see it trigger after a throttle blast or the like. If it went on under other circumstances I'd freak out, shut the car down, and pull timing covers on the side of the road.

Although my other cars (968, Audi, 4runner) have the dampened piston auto-tension setup, and I like that setup, the stock 928 system has been reliable for me and it is pretty easy to check/set the tension.

James Bailey

Right after a high rpm blast where the belt is working it's hardest it has very high tension on the loaded parts and the least on the tension roller run. So the small spring is enough to break the contact and trigger a warning. Wonder how strong that spring is ???