mark kibort

timing belt tension gurus, what do you think the hot vs cold tension is on the timing belt. did a test hot last, and stone cold on a 50 degree morning today. any guessers?? and why?? show your work!

I used the kempf tool and the porsche 9201 tool.

Vilhuer

Is tensioner working and filled with oil?

PorKen

Shouldn't it be the same all the time?


Gates seems to think so - (Gates Technical Bulletin #011)

mark kibort

and you ask , why?? everyone is so caught up with the oil of the tensioner. its only a slight lubricant, and is there for heat transfer. the net net would be that the discs would change shape more slowly until the heat of the engine. (its attached directly to the block by the way) is transfered to the discs. after all its not hydralic, and the damping characteristics is very debatable based on frequency, mass, size of oil exit relief holes and amount of movement.

ive been running a tensioner with very little oil in it, and racing the car for over 12 years. never had any issues. its still oily when its taken apart, but no where near filled up.

Now, to answer your question, yes, this tesioner is filled with oil and working properly.

so, what is your guess. tension cold vs tension very hot??

Ken, the guru of all tensioner, what have you seen with tension hot vs cold? and why?

werent you the one that was estimating the cam timing change due to the expansion of the block, etc? if the the timing changes, that means the distances change for tension, right. Or does the expansion of the discs cold and hot, compensate for this in the tensioner?

anyway, i saw a significant change cold to hot with both the kempf tool and the porsche tool

if gates was right, wouldnt you then be able to check tension cold or hot, not just cold as the factory manual directs?

Vilhuer

Lubrication is exact point. Without enough oil in tensioner washers tend to eat themselve into piston shaft and also "weld" themselves to each other. Quess what that does to washers ability to straighten themselve under heat and releave tightnening belt caused by expanding block and heads.

mark kibort

there are more differnces than i can count of that Gates version and explanation and ours. yes i understand the "bump" control of the actuator of the gates, but our system doesnt have a spring, it has very stiff wavy washers, an the seal of the oil for the two sides of our piston is very differnt than this set up. for all practical purposes, our spring pressure is basically rigid, especially compared to the gates example where that spring tension could acutally be measured, and truely relies on oil for damping

Bill Ball

I've never measured it hot. In theory, with the conical washers, it should be constant as the block and heads heat and cool. That was the INTENT anyway. I wouldn't be surprised if it was found somewhat lower or higher hot.

PorKen

You need a certain amount of belt tension to keep the belt on the gears. More than that, you are wasting HP.

What changes as the engine expands or contracts is free belt length. As the engine warms, the engine expands, moving the cams away from the crankshaft. More belt length is then required to go around all of the gears. With the factory system, the bimetal discs should flatten, freeing up more belt. If not, the belt has to stretch to accomodate, raising tension. As the engine cools, the amount of free belt gets longer, and the discs should expand to take up this length. If not, tension will go down.

The WSM has you set the belt tension when the engine is at 'shop temperature' (perhaps 50F/10C). This tension is likely determined by the working elasticity of the belt, the expected expansion rate of the engine, the contraction range of the bimetal disc pack, plus a 'fudge factor'. It's also possible it allows for how much belt stretch is expected at high rpm, and because the factory system cannot take up any more belt than where it is set at this time, possibly also by how much more contracted the engine will be at very cold temperatures.

As the bimetal disc pack is only an analog of the expected expansion/contraction rate of the engine, is indirectly warmed/cooled, and has a limited range, actual free belt length = belt tension varies. Note that at high rpm, the belt is stretched between the crank and cam gears, freeing up more belt, but this is uncontrolled with the factory system (flapping). Extra, uncontrolled, belt is hoped to be managed by the 'idler' pulleys. (A 3-minute delay at startup ensures that customers are not alarmed by frequent low tension warnings when cold.)

Aside from the bimetal discs, belt tension itself is used as another indirect way of controlling belt length. Most of the time, belt tension is much higher than necessary to keep the belt on the gears, but it is hoped that within the elastic range of the belt, the belt will never be loose at the crank gear.


If the tension is going up when warm, it's likely your bimetal discs have worn a groove into the housing and it is sticking at some point. If tension is going down, something is breaking or broken.


If you had a self tensioning system. IE. a PKT. (We can be certain that Gates is 'right'.)


Exactly.

mark kibort

well, the tension was clearly higher when cold. the tensioner has oil in it and it is clean looking.
it went from 5.0 when hot, to 5.6 cold. makes sense as the ownder did a quarter turn tighten when the light came on, just incase. so it is lightly over tensioned.
seems opposite of what should happen. i would have thought that the tension would go up when hot, due to the expansion of the engine. so the discs get more wavey when hot and flat when cold to take up the tension? the opposite is happening.

PorKen

The bi-metal Belleville washers flatten as they get hot. Concave as they cool.

dr bob

Don't confuse the effect of block expansion with the opposite effect of the heat on the tensioner washers. Obviously they aren't offsetting perfectly, but they are pretty darn close. Washers are compensating faster than the block expands.


Maybe Amsoil in the tensioner wouldn't heat up as much?

mark kibort

makes sense. guess it does work pretty well, considering hot verse cold is only off .5 of a setting.

atb

If this is a conti and not a gates, all bets are off.

PorKen

As MK is pretty much just talking to himself, again, I'll continue talking, to myself.


This was a bit of a revelation, to me, when my fingers typed it yesterday.

Because the 928 timing belt is not actively managed, elasticity/stretch in the belt is used as a reserve, plus or minus, of overall belt length, independent of the detensioning system, with the goal of ensuring that there is never enough loose belt to jump teeth at the crank, or the cam gear(s).

Too much belt is mostly a problem when the engine is cold, but it also occurs at higher rpms, when the belt is pulled taught by the drag of the cams.

The measurement of belt elasticity is tension. This can only be set so high, so as to not cause premature wear, so the belt will be loose if the engine is very cold, or it is run at very high rpm. The guide pulley(s) at the crankshaft are there to try to keep the belt pressed on the gear, even if the belt is loose, but there is no mechanism for the cam gear(s).


With an actively managed belt, where free belt length is controlled as it passes over the tensioner pulley, belt tension can be constant, and on average, lower.

PorKen

As an aside, the fact that the 5-8 gear shows less advance than the 1-4 when the engine is hot might be because the cams are not only moving away from the crank, they are also moving away from the water and oil pumps, and each other.

Being farther away from the water pump and the 1-4 gear does nothing to the 5-8, but being farther from the crank and oil pump does advance it a little.

Conversely, everything advances the 1-4, as all of the belt is on the pulled side.