GregBBRD

Well, it must be obvious by now, that Porken and I disagree on what is the best tensioner for a 928 belt.

Clearly there are many, many, miles that have been put on the stock 928 tensioner design.

Unlike the 4 cylinder vehicles (944 to 968), Porsche never felt it necessary to change the basic design of this tensioner (which was completely redesigned for the 4 cylinder models 3 separate times.) This means that Porsche felt that they did a pretty good job of designing this tensioner.

I happen to agree.

There are also many people out there that think Porken's Tensioner is a great design and have used it with success.

I have zero financial stake in this debate about tensioners...it makes no difference, to me, which way you choose to go.

Pick the best one, for you, and install it.

PorKen

'80 revised Bellville washer stack
'80 single crank idler pulley replaces fixed guide piece
'83 tensioner gasket added
'83 oil filling ports, limited damping piston
'85 tension warning system light (with 3 minute delay at startup)
'86 tensioner moved outward for closer offset to pulley center
'87 two crank idler pulleys
'87 stronger plastic tensioner arm bushes
'87 revised tensioner arm pivot with additional support
'92 return to one crank idler pulley



If you like the stock timing belt tensioning system, you'll love the stock flexplate clamp!



I don't sell engines, so it's no biggie to me, but I feel it is irresponsible to not have a belt tensioning system with greater than stock damping capability to go along with a larger cube engine, which will have larger flapping events, especially when (it catches a) cold.

GlenL

You mean that the belt vibrates? I don't see that as a problem that needs solving. The belt will respond to vibrations and when it hits a harmonic it'll vibrate. It's still tight and isn't going to jump a tooth.

From what I've heard, Ken's tensioner is better because you don't need to re-tension the belt. The design also is intended to keep a more consistent tension across temperature. What is the net outcome over the stock tensioner and am I missing something. Does it prevent belts from breaking? What does a 9201 show over temp and mileage?

Porsche made plenty of changes over the lifetime. I've gone to a later oil-filled tensioner myself. (Cheap off eBay.) They didn't issue a recall or service bulletin to retrofit the changes. Or is that not true?

PorKen

Vibration isn't the problem, it's the irregular speed changes at the crank from uneven piston firing or valve spring tension which momentarily elongate the belt on the taught side, creating slack on the loose/tensioner side. Undamped, the belt flutters/flaps with the extra belt length.

With a tensioner/damper like the PKsn'r, the temporary looseness is immediately taken up by the tensioner as the belt passes by. As the cams and the rest of the belt catch up, the belt tension equalizes, and the tension piston retracts slightly to restore average tension.

There is no good flapping.

ROG100

Ken,
I love it when you talk dirty 8>)
Roger

Imo000

So after the tensioner takes up the momentary slack, the belt will be over tightennned untill the piston retracts?

GlenL

The one on the right isn't so bad.

I don't think the tensioner can respond that quickly. It's hydraulicaly damped, isn't it? At 6K rpms pistons are firing at 800 Hz. If it's fully compressed and then released, how quickly does the tensioner become fully extended?

PorKen

The T/D can retract ~2mm maximum immediately after extending, then it takes >30 secs to retract farther if needed to a point where the average tension is restored.

At higher rpms the average tension is higher for a time, as the piston has no time to retract. Temporary higher tension is good to keep the belt on the gears when accelerating. There is no good time for low tension.


Glen... seriously.

Imo000

Ok, I see, so then the tensioner car freely move at least 2mm back and forth at all times?

GlenL

Here's the problem: if the belt is vibrating at 100 Hz (and that's probably low) it needs to move in 5mS to actively damp it. To remove the pulses from cam lobes it needs to be below 1mS. That's awfully quick.

It'd be easier to reduce vibration by adding slack to the belt. Since a vibrating belt will increase tension, I bet it's pulling back a bit, letting the belt go a bit more slack and in that way changing what the harmonic frequency is. It wouldn't be letting the belt tension go lower than normal, btw.

GlenL

You found my beer goggles!

PorKen

Yes. It can move out farther immediately, but takes a while to retract to a new setpoint. Longer when cold. Only when the engine is cold do you see such large variations, though. When warm you can feel the corrections, but hardly see the lever moving.

When retracting the piston on a cold tensioner in a vise, it goes: compress ~2mm, stopped, wait..., compress, stopped, wait..., etc. It takes >5 mins before you can insert the grenade pin.



It can't affect individual events at high speed. It normalizes the average tension as the belt loads up at speed.




You see some weird things with the cover(s) off. EG. at high rpm the belt comes out of the water pump pulley nearly horizontal for a ways (farther and farther with speed), then curves up towards the 5-8 gear!

blown 87

not if it is working right.

GregBBRD

Wow.

You don't understand that the basic design is still the same?

The basic oil filled Bellville washer housing mechanically applying pressure to the belt never changed.

The 4 cylinder engines went from a mechanically locked pulley, to a spring pressure adjusted pulley, to a hydraulic mechanism...over a much shorter period of time.

The stock flex plate retaining system worked great, until it was re-engineered for the 4 speed automatic transmissions. The "new" engineers failed to realize that the clamp could migrate towards the front of the shaft, without the positive stop. Yes, the lack of a positive stop was a problem (sound familiar?) Note that it took years for this to be a problem (sound familiar?)

Of course, Constantine solved this with his great invention.....you want to go there?

As far as what I do is concerned....I'm a pretty big hitter in the world of 928 engines. I've been building these engines for over 30 years. I've literally done hundreds of these engines. I've literally changed over 1000 belts and rebuilt over 1000 tensioners.

I've never had a belt or tensioner fail on a car that I take care of. I've never even had to redo a belt or tensioner in less than 45,000 miles, that I've touched.

Thousands of cars. Millions of miles

I'm willing to bet that I've got more miles on just my big cubic inch engines than you've got total on all the cars equipped with your tensioner.

Call me when you get close, son.

.....and don't ever think about suggesting that I'm irresponsible.....again.

PorKen

The concept is the same, and it never really got better. The design was incrementally changed from the beginning in hopes of improving it, then they gave up, and put a warning system on it. The fact that no other maker has a warning shows a lack of confidence in the system. The fact that the warning system has a 3 minute delay at startup shows that the engineers knew that the belt was uncontrolled at a cold start, and took measures to hide this from the consumer.

The 944 tensioner went from a fixed tensioner, to a fixed tensioner with a spring assist to help tension it (whereby it is locked down) to make it easier to install, to finally, a proper tensioner/damper design. One major improvement, on a much higher volume car.


It does not sound familar. The Audi tensioner/damper has a positive stop. You appear to be deaf in this regard.

Constantine makes an excellent product. Roger sells an alternative which is easily installed which works well, too.


Except for the one which failed on startup.

I would take you more seriously in this discussion, but you have not demonstrated an understanding of how the timing belt acts in motion. That a rubber belt momentarily stretches and returns to shape, and that there are different tension levels at different points along it's length at different times.

Have you never seen the belt flapping at certain rpms with the stock tensioner system, even with a 'properly' tensioned belt, or wondered why all the 'idler' pulleys are there?

"Can you hear me now?"

The Audi tensioner system has been used on hundreds of thousands of cars, with Billions of miles driven. Many mechanics have described it as 'bulletproof' to me. Used on the 928, it is understressed.

If you read carefully, I did not say that you were irresponsible, only that I would feel that way...because I know better.