Hydralic -VS- Bi-metal plate tensioners
02-06-2008, 02:23 AM
#16
Rest in Peace
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02-06-2008, 12:56 PM
#17
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To answer the first question. There is no reason to update to the later "shock absorber" tensioner. Porsche added that feature at the same time they were adding a belt tension warning light. The normal flutter of the belt was sending warning signals so they added the baffle / o-ring. What you have is just fine no benefit from any "update".
02-06-2008, 01:29 PM
#18
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There is no 'normal' belt flutter. 
If you want to be able to top it off, the refillable tensioner (w/o-ring) starts in '83, at the same time as the HTD belt was introduced (the tension light, in '85).
If you want to be able to top it off, the refillable tensioner (w/o-ring) starts in '83, at the same time as the HTD belt was introduced (the tension light, in '85).
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."
"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.
"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."
"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.
Original adjuster version needs oil into it also, see WSM 15-20 and 15-21. Later version added channels for oil to flow more easily to different parts of system. It didn't introduce oil at first time.
From '86 model year technical documents:
"Tensioning Roller Console (All Engines)
The center of the tensioning roller is displaced toward the engaging point of the push rod by 8.5 mm. Pressure point and roller center are now on one line, so that bending and inclining of the console are Prevented. The tensioning roller itself has an improved bearing.
Tensioning Roller Housing (All Engines)
The tensioning roller housing is adapted to the tensioning roller console modified by 8.5 mm. Brake bleeder valves are used instead of the former hexagon head bolts to make it easier to add oil and bleed the housing. Filling and bleeding is now very easy by turning both valves counterclockwise by about one turn, and pouring in oil from a plastic bottle and a suitable piece of hose."
So centerline was changed for '86 model year. AFAIK there should be 3 major differences to tensioner housing:
'78-82
- Without outside filling possibility
- Without oil flow channels at the back
'83-85
- With bolt covered outside fillers
- With oil flow channels at the back
- '85 with tension warning system
'86-95
- Centerly (to roller) aligned pushrod
- With brake bleeder valve covered outside fillers
- Not sure if tension warning system changed to later style '87 or already in '86
From '86 model year technical documents:
"Tensioning Roller Console (All Engines)
The center of the tensioning roller is displaced toward the engaging point of the push rod by 8.5 mm. Pressure point and roller center are now on one line, so that bending and inclining of the console are Prevented. The tensioning roller itself has an improved bearing.
Tensioning Roller Housing (All Engines)
The tensioning roller housing is adapted to the tensioning roller console modified by 8.5 mm. Brake bleeder valves are used instead of the former hexagon head bolts to make it easier to add oil and bleed the housing. Filling and bleeding is now very easy by turning both valves counterclockwise by about one turn, and pouring in oil from a plastic bottle and a suitable piece of hose."
So centerline was changed for '86 model year. AFAIK there should be 3 major differences to tensioner housing:
'78-82
- Without outside filling possibility
- Without oil flow channels at the back
'83-85
- With bolt covered outside fillers
- With oil flow channels at the back
- '85 with tension warning system
'86-95
- Centerly (to roller) aligned pushrod
- With brake bleeder valve covered outside fillers
- Not sure if tension warning system changed to later style '87 or already in '86
02-06-2008, 02:17 PM
#19
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Run a 928 engine on a real chassis brake dyno with no covers at a constant 6,000 RPM and it will freak you out how much the belt MOVES ... call it what you wish but the fact is the loose part of the belt from the crank to the passenger cam gear sees a huge variance in tension as the cams compress the valve springs.
02-06-2008, 02:29 PM
#20
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Scary, isn't it! And it's worse when the engine is cold.
But I've kinda forgotten what it looks like. On my cars, it's dead flat to redline, now.
But I've kinda forgotten what it looks like. On my cars, it's dead flat to redline, now.
05-20-2008, 11:21 AM
#22
Drifting
Because it applies basically a constant pressure onto the belt. As the block expands, the roller collapses the piston slowly back into the tensioner. As the block cools, the tensioner pushes the pulley back out to take up the slack. It's a much better design than the stock 928 system.
05-20-2008, 11:35 AM
#23
928 Engine Re-Re-Rebuild Specialist
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Out of curiousity...what's the full price of converting to PorKen's system...out-the-door with all necessary hardware?
05-20-2008, 12:12 PM
#24
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05-20-2008, 12:39 PM
#25
Man of many SIGs
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Because it applies basically a constant pressure onto the belt. As the block expands, the roller collapses the piston slowly back into the tensioner. As the block cools, the tensioner pushes the pulley back out to take up the slack. It's a much better design than the stock 928 system.
05-20-2008, 01:18 PM
#26
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The tensioner/damper hydraulically resets (piston retracts) to a new setpoint after a few seconds of overtension. Short duration overtension is damped to control flapping. With undertension, it reacts immediately, as the belt is turning.
It's just like a suspension shock absorber and spring, but in reverse. Instead of quick in, slow out, the tensioner/damper is quick out, slow in. If you weight the car, and change the amount of piston showing on the shock, it still does the same job.
Worst case - starting a dead cold engine, after shutting it off hot. This is the factory (de)tensioner's weakest moment, and the reason the guide pulley(s) are there. It's also likely the reason why the belt tension light has a 3-minute delay on startup!
It's just like a suspension shock absorber and spring, but in reverse. Instead of quick in, slow out, the tensioner/damper is quick out, slow in. If you weight the car, and change the amount of piston showing on the shock, it still does the same job.
Worst case - starting a dead cold engine, after shutting it off hot. This is the factory (de)tensioner's weakest moment, and the reason the guide pulley(s) are there. It's also likely the reason why the belt tension light has a 3-minute delay on startup!
05-20-2008, 05:29 PM
#27
Drifting
If you try one of Kens Tensioners sold by 928sRus ( a sponsor of this site ) you will not go back to stock. Just hold both in your hand and hear the car run with it in.
05-20-2008, 07:08 PM
#28
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I installed one of Ken's in friends car last week, it is amazing how clean of an install it is.
I also rebuilt a stock one this past Sunday for another local 928, the stock is crude but it does work, just requires a lot more maintenance to keep it working corectly. Oh this second pic is of one badly needing a rebuild.
I also rebuilt a stock one this past Sunday for another local 928, the stock is crude but it does work, just requires a lot more maintenance to keep it working corectly. Oh this second pic is of one badly needing a rebuild.
