dr bob

Jim--

My suspicion is that the description in the tech literature is a little optimistic. The plating on the washers is hardly thick enough to seriously change the shape of the cones with a 100ºF temp change. The tensioner is at the bottom of the coolant path, where it sees coolant from the pump from the bottom of the radiator, at maybe 160ºF average, maybe more on a really hot day.

As an experiment, someone at home with a stock tensioner apart might measure the height of the stack at ambient temp. Heat the washers in 160º water on the stove, and then quickly measure the stack height before they cool and report the difference. It may be that the lengths of the lever arms multiply any tiny change in stack height to something meaningful in the geometry of the engine, but I'll speculate that the fifference in height is no more than the normal expansion of steel with the same temp change. I'd test my own stack of washers but it's busy right now.

re: Your comment on the Audi (and others) hydraulic dampening only in the compression direction. The 928 has hydraulic dampening only in the compression direction. Of course you do have to keep the oil inside, and that can be a problem if the gaskets are not installed correctly or the top boot leaks. There's actually a little check valve in the bottom piston to pass oil in one direction (up) into the cavity where those spring washers sit. To compress, oil must pass backwards through the check valve or leak past the seal around the top (moving) piston. There's a tiny bypass passage in the housing that allows oil to pass back to the 'reservoir' cavities in the back of the housing against the block, so the piston can be slowly compressed without blowing the top boot apart. Pretty slick, and exactly what's needed so long as it doesn't run out of oil.

One day when I figure out how to post video segments to youtube, I'll share a segment of my belt replacement instruction video where the tensioner is explained as part of a reconditioning effort. In the video, I show each section of the oil path and describe what each is for.

re: Your comment about adding a switch contact. Finding a switch that has the right sensitivity with just the right deadband on such a short stroke is the challenge. A typical microswitch has a maybe 0.100" stroke and 0.080" is deadband required to get it to snap back and reset. It would be possible to multiply the motion with a longer lever arm I guess. But calibration would be a chore and a half, and finding a sealed switch that would be happy in the ultra-not-so-friendly environment inside that cover might also prove difficult. The factory switch is deceptively effective in its simplicity, but depends seriously on the non-linear characteristics of the coned washer spring relative to the linear characteristics of the coil spring that holds the switch contacts. It truly identifies where in the travel the Belleville spring stops applying the correct tension.


It's quite possible that I'm making a mountain out of a molehill. Maybe it's OK to just replace everything when you change the belt, pray that you got a good belt and that no parts will wear beyond limits during the change interval of the belt. Change it every 50k or whatever mileage Gates thinks is a real safe mileage for their product, and just never worry about any of this. That's what current-production cars seem to do.

K's 4Runner with the V-6 has some stated interval for belt change I think. I never worry about it since she just has the dealer do everything. Of course there was that $600 bill for the 60k 'service'. All the normal oil-change stuff plus a coolant change and a lot of extra inspecting. They say they don't do a brake fluid change. Ever. Can't wait to see what they charge for the belt change at 100k. When I asked if they inspected the TBelt and drive stuff, they gave me a really blank look. I feel so good about their blank look that I know that they just never see broken belts or worn drive bits between belt changes. They just never look and therefore never find.

Landseer

Greg, would you recommend upgrading the 85 factory tensioner to an 86 or later unit which has a shift in the force centerline.

Or do you feel that the 85 tensioner in association with new supporting parts is reasonably good enough?

Also, have you formed an opinion yet on the new Gates belt in association with stock tensioner, or is your recommendation Gates original belt?

Thanks in advance. Also, thanks for posting on the DIY threads in general, even though you are in the business to fix the cars.

GregBBRD

I do not update them, unless there are pieces that are worn out.

I have not had the time to investigate the difference, if any, between the "Gates" belt and the "Factory Gates" belt. Apparently, Roger has done this research for us....and I have never known Roger to be anything but completely truthful.

I have a tremendous amount of respect for Roger and his business. We don't always agree, but that's OK....who does always agree? We're big boys and we know that we can freely discuss things and pass ideas back and forth....without calling each other names....that's one reason why I respect him!

Porken's cam tools are brilliant! I suspect/hope his tensioner is equally brilliant. Time will tell. Hopefully, there will come a time when I can use his tensioner without reservation...in the meantime...I just choose to watch.

blown 87

I put one on my car Roger, along with a Porsche belt.
I think it is a good system, but it has two issues that I see.

The replacement for the pivot bolt is a allen screw with nothing to hold it in but RTV is one.
I have told you and Ken, a far better solution is to just reuse the pivot bolt, you can cut the pivot off of it like I did if it makes you feel better.

The second is that I think folks, being people, may not inspect this system as often as they should, and they do not start from square one many times, IE all new gears, and everything else the belt touches.

It still needs to be inspected on a regular basis and IMHO if you run a belt 30,000 miles, you need a new one.
I also think it is a good idea to take a look with a borescope every other oil change to check the gears and for leakage at the tensioner.

As far as the OE vs OEM belts, what can I say, I throw money away by changing my oil every 3,000 miles, and I put on OE belts.

I have seen these tensioners run 200,000 mile on other cars and there is a lower parts count, that means less **** to break, I have also seen them bad right out of the box, or soon there after.

I would love for it to have a warning light, but no other car with a hydraulic tensioner that I am aware of has a warning light.

As far as it collapsing when and if it rolls backwards, the only way I can see that going to happen is if the tensioner is weak, it takes a lot of force and TIME to collapse a good tensioner.

I have recommended to customers that there is a alternative to the stock system and installed this system on customers cars, I bought the last one I did, I bought it from you Roger.

For some reason you think I and Greg Brown are anti progress on these cars, that is not so.

We just see parts for what they are, some are better than others.

GB and I may not see eye to eye on tensioners, but I can sure understand why he does not want to use one, when serviced as designed, there is not much wrong with the stock setup.


BTW
You KNOW I have no problem calling you and saying "I think this part is not of good quality or there is a problem".

Kens tensioner is, IMHO a good product, as is Constintine's clamp and bearings.

I am not convinced on the PK clamp though, at all.

ROG100

Hi Greg,
The feeling is mutual and I always enjoy your views and they also make me think more about the opposite opinion.
I carry a limited amount of liability insurrance but as with all of us I would be at the mercy of the courts when it comes to "sue" happy America. A multi million dollar case would leave me smarting 8>).
In the case of the PKensioner, Ken did his due diligence with the design and ensured it worked for a long time before coming to market. We are talking about a well proven product being adapted to fit our cars. The product has been in service for about 4 years (correct me if I am wrong Ken) with no known issues.
Some are used on race engines but most on daily drivers and garage queens.
Of course there is something that could go wrong - just like one of your highly tuned engines. It would be my job to ensure that all were aware of any faults in an expediant manner particularly when a persons safety could be involved. This applies to any product or service.

As I leaned over your stroker this morning and placed my hand on the inlet manifold to feel the smoothness of the engine, I suddenly remembered you had built it, jumped back just in case it might explode and that you may not have liability insurrance to cover my injuries. Just kidding.

Awesome engine by the way - a true master piece.

ROG100

Blown 87,
I was refering to the fact that you both run shops and have to be very carefull what you fit because once fitted you are responsible for putting it right including the labor. I respect that.
Roger

GregBBRD

Very professional.

Very smart.

SeanR

I've been doing this also. We had one car that had a water leak from that exact spot and it had been well RTV'd. So we cut the pivot bolt flush and used that as the plug on Jim M's GTS.

dprantl

When I was installing mine, I took a look at the allen screw and right away just reused the pivot bolt. Didn't even bother to cut it, there is no need.

Dan
'91 928GT S/C 475hp/460lb.ft

worf928

Interesting thread. There are a couple of things that confuse me though. I've got to go get 6" of snow off my driveway as soon as I quit procrastinating. So I'm going to be lazy and not quote the snippets of messages to which I refer and just launch in with questions:

It seems to me that the stock tensioner cannot increase tension quickly by more than the amount of compression on the spring between the push rod and the washer stack. At rest, before a hypothetical "backwards coughing," the spring is compressed and the washer stack has some compression but not a lot. If the crank turns backwards the belt is going to get loose really, really fast and the stock tensioner isn't going to be able to "lose" more than a 1/2" or so of compression height to keep it in tension.

Am I wrong? Or visualizing this incorrectly? (The motor I have on the stand right now still has pump and the belt on the crank even though the heads are off. We were turning the crank back and forth last week and I found out the hard way how little you can turn the crank backwards before a loose belt binds with itself.)

Next question: "backwards coughing?" How can this happen in practice? I'm not suggesting it cannot. I want to know how and why it can happen. If everything is hooked-up right the starter turns one way, the plugs fire when they are supposed to... so how can the crank move backwards? On the other hand, if plug wires are mis-routed or any number of other things are upgephucked then it could happen, but it seems to me that Bad Things are going to happen in that case no matter what regardless of what the belt does or doesn't do. Any real-world examples so that I can learn?

I haven't put a PKensioner in a 928 yet. I may put one in one of mine soon. If one of the locals asks me to do so, I will. I'm not yet at the point of recommending one. One of my concern's with the PKensioner is the lack of a warning. I have observed and become comfortable with so many failure modes of the stock system that I think I know what many of the warning signs are. And the stock warning system - if it is working (and I always test it) - will give you early indication of a problem. I know what to look for on the stock system once I've got my tools out with or without a tension warning. I don't know what to look for with Ken's system. It's a bit "black boxy" at the moment so I'd be inclined to throw it away every 30k miles and buy a new one. But, I don't have any data or anecdotes to support that.

Last, if one of my cars threw a tension warning, unless it was during the short space between the initial install and the post-install re-tension, I would be very, very unlikely to simply tighten it up a bit and drive it for another year or two. 9.5 times out of 10 the warning system is telling you something really is wrong.

GlenL

It's thrown back by the compression in the cylinder(s). When you are turning by hand has that happened to you? You're pulling hard and when you let up the crank turns back a bit. Once the car is off the engine will spin from momentum until a compression stroke is too strong for the momentum to overcome and then it'll force the crank backwards.

Since no one is going around with video cameras on their timing belts, or not enough people, I'll continue to believe that during the "cough" is when the belt jumps teeth. Makes sense to me.

worf928

Good example. I wasn't think of a "shut-down" case, but a "start-up" case. In any case, yup, I have seen that happen. You'll get one or two crank teeth-worth of "backwards" and the stock system's push rod spring will keep tension on the belt (assuming nothing else is messed up with the belt system.)

It seems that the PKensioner would be no worse than the stock system in this case and perhaps better?

Seems plausible. I continue believe that tooth jumping occurs when you release the throttle abruptly from high rpm with a loose belt. In many cases of warning system trigger, it triggers within a few fractions of a second of a "passing maneuver."

worf928

It was the "coughing" that made me think of only start-up.

Damn snow's not blowing itself off my drive.

Landseer

Its a nice piece, that's for sure.

vbatla

What a great thread this turned out the be. I'm going to start disassembly tomorrow. Couple of other questions; Where does one get the tool used to hold down the crank shaft? From Porsche? Also, since I'm going to be in there, I'm going to to ahead and replace the water pump as well. So: tensioner, water pump, belt will all be new. What do I need to look for regarding the cam gears? I'm going to inspect the crank seal. What other seals / gears should be replaced at this time? Thanks for all the great responses!