H2

928 Colleagues,

One of our esteemed Rennlisters is rebuilding a 928 and it was suggested that he replace the water pump and completed tensioner system with the more modern S4 system. That's quite a job and expense but may be a desirable change for those of us who have older cars and want to keep them forever.

For example, it appears that the S4 water pump and (complete) tensioner system would fit on the 1985S (for example) and perhaps other earlier cars. Since my '85S has a December 1984 tensioner system on it, I was thinking of changing it out anyway to the correct '85-'86 system. But if the S4 system is better, why not go that route instead?

What's the view of members of this group? How many have done this? What's to look out for in such a changeout? I'm thinking that the new system must be a better design or they would not have gone that route for the newer cars. And....there are few things more important than keeping a T-belt in place. What do you think?

Harvey

Bill Ball

Harvey:

The big changeover was MY83 to add hydraulic damping (this is not hydraulic adjustment, just vibration damping), round tooth belt and gears. In MY 85 the warning system was added. Do you have that? After that the changes were minor - bleeders instead of hex head and a stronger warning system spring was added in 87.

Garth S

The complete update to S4 spec tensioner gear can easily be done to any earlier car - all that is required is a complete set of matching parts, and a few $$.
I've updated my '80 to the HTD cogs, and now to the '85 tensioner/carrier arm & bushings/idlers/ etc. To go to the S4 level would require the +8mm offset tensioner, carrier arm, pivot stud, x-brace - and later style water pump: as I said, more $$. When ever the 25 year old water pump decides to cash in, I'll likely complete the upgrade project; however, it will be more motivated by a desire to have commonality of spares in my kit (for a 16V and S4) that any inherent improvement in tensioning.
The one design feature I admire of the "+8mm" tensioner body/carrier arm is the straight line set-up of the tensioner center line to the TB center: this would minimize any tendancy of the previous design to place an off axis load on the pivot bolt. In addition, the cross brace of the S4 from the pump body to the pivot stud presumably stabilizes the stud. I don't know if either of these features has any measurable benifit - they just seem 'right' ....

928ntslow

My question is simply...Why??? If the engine is a non interference engine, who cares if the belt fries? It happened on my 82 and I simply replaced all of the sprockets to the HTD, replaced any questionable rollers, stayed with early rebuilt WP and just got the tensioner boot with adapter to fit the early tensoner. Cost way less (still cost enough though!) and performs great. I checked the tension twice since I had done the job about 8K miles ago and the belt is still nicely tensioned. I even added mods to gain hp and torque....no reason to change. Save yourself some cash and put the money into where it is needed more.

heinrich

Keith, Harv has a 32V, so it is a valve chomper. I think the S4 setup is mo bedda

PorKen

So with the (re)fillable tensioners actual have some dampening, versus the original design?

heinrich

NO KEN.... DOWN BOY!!!!!

You just direct your neurons to a self-adjusting solution a la Chebby / Ford / Chrysler / Audi / Honda ..... FOCUS old man!! <edited ... that sounds a bit better>

worf928

Center t-belt cover?

Bill Ball

Yes. From the MY83 service bulletin

"Major Changes on Light Cylinder Model Series 928 for 1983:
ENGINE
- 928 S model for
- USA and Canada equipped with a 4.7 liter engine developing 174 kW (234 HP).
- Starter mounted on crankcase.
- New cylinder head gaskets.
- Hydraulically cushioned toothed belt tensioner.
- Heavy torque drive (HTD) toothed belt.
- Hydraulic engine mounts."

and further on:


"Toothed Drive Belt Tensioning Mechanism with Hydraulic Damping

The different amounts of drive torque during a revolution of the camshafts produce oscillation in the toothed drive belt, which could cause belt flatter (sic) and in exceptional cases, when belt tightness is not sufficient, even a jumping out of the belt.

The new toothed belt tensioner has been designed to hydraulically dampen this oscillation in the toothed drive belt.
....
The entire tensioning roller housing (2) is filled with engine oil. When toothed belt oscillation causes reaction on tensioning roller (10) and consequently tensioning roller carrier (9), e. g. oscillation in direction of the diaphragm spring set, the push rod (8) and Piston (6) will move in direction of the spring set. The piston now presses against the oil polster and attempts to compress it and the diaphragm springs.

The bimetal diaphragm springs rest on valve carrier (3) and the oil attempts to flow through the bottom valve (4). However, the oil flow also carries the valve along and the chamber is closed completely. Leak oil can still only escape between piston (6) and guide (7). Three relief bores (1) deliver the leak oil outside along the guide behind the valve carrier (3) again. In opposite direction, when the push rod moves away from the tensioning roller housing, the bottom valve opens and allows undamped oil to flow through the valve carrier.
...
If the hydraulic damping fails because of insufficient oil, the toothed belt tensioning mechanism will function as the former system without damping.

The new toothed drive belt tensioning system can also be applied in older models, if the pertinent sealing surfaces of the engine block are not damaged."

Below is an exploded view from the WSM. Numbers don't coincide with above description, but you can see the valve (13 below) in the valve carrier (10)

SharkSkin

Bill, I hate to be a party pooper, but the numbers in the text do not seem to match the callouts in the picture...

928ntslow

OK Heinrich, correct me if I am wrong, but weren't there 16v's in 85? S4's didn't come around until 86.5, but the S2's were 32v which came out in the later part of 85. uhmmm right? Also, aren't all 16v's (barring Euro's) non-interference?

As for the 85S he has, which I THINK is a 16v, what would be the benefits other than filling? Shouldn't the original tensioner be sufficient?

Dave, why do you think Bill keeps having the TB warning light come on in his car?

PorKen

Bill,

Thanks for the info. I guess I 'knew' that, because I upgraded to an '84 tensioner, rebuilt it, and saw the valving, but it didn't 'stick'.
...
I'm pretty sure the Audi early or late 4.2 tensioner/dampener and pulley could be fitted. But they're spendy, and not rebuildable. They might allow a longer belt change interval, however.

I should look at other lesser brands, and see if there's a solution.

heinrich

Keith-Meister .... yes the '85 16V cars were Euro only, called S2

85US through 86.5US were 32V. Euro were 16V till end '86 unless special-ordered. S4 have the long tail lights and are ALL 32V, and came in 1987. Except for one example, Mr Kibort's.

They are Euro 16V '85 - '86.5 (not sure but I think '86.5 are referred to as S3. Those babies have S4 suspension and brakes, whether US 32V or Euro 16V.

All post-'83 engines afaik are interference.

Harv is as 32V as you and I are

928ntslow

Oh boy, I think there may be some ongoing discussions on which cars are interference and which are not. Post 83??? We just replaced Dan B's TB on his 83 and it is a non interference engine. I am guessing that if there is a 45 degree mark on the balancer, then it is interference. The mystery continues....

Thanks for clearing me up on the 16 vs 32v in the mid 80's!

SharkSkin