Stiff Gear Change When Cold

[Octoposse]

I think it's crept up on me slowly, but the gear change on my 2.5 Lux seems stiff and reluctant from cold in the mornings, then sorts itself out to the usual positive joy after 10 miles or so. No problem in itself, except the worry that it may be causing more wear or damage. Gearbox oil changed 3000 miles ago.

Is this a known problem? Likely to be linkages rather than box?

 

[Copperman05]

I think this is fairly normal in my (limited) experience, 1st and 2nd are particularly stiff on my S2 in the mornings. A short shift linkage will probably exacerbate the issue if one is fitted.

 

[zcacogp]

I don't know what your problem is, but I do know that the linkage is long, not well-designed, and positioned such that it will collect grot in large quantities. I suspect that taking it off, cleaning it and re-greasing it will help a lot.

(That's easier said than done, mind!)


Oli.

 

[barks944]

Since its temperature related isn't it more likely to be related to the gearbox oil?

 

[zcacogp]

Could be, but he says the gear oil was replaced recently.

The grot on the linkage is probably grease-related (grease is sticky and attracts grot). This will soften with heat from the engine, blown back, as the car warms up. (Although my theory could be wrong and I'm just shooting in the dark ... [&:] )


Oli.

 

[pauljmcnulty]

The gearbox, obviously, doesn't get heat from the engine so it takes a good while to fully warm through.

 

[barks944]

I'm not sure the change in the viscosity of the grease in the linkages would have a huge impact, although I too am guessing. My guess is that its the viscosity of the gearbox oil that is making the big difference. Maybe change the grade of oil to one thats thinner at low temperatures. Do they have the same ratings as engine oils e.g. 0w40

 

[zcacogp]

Tom,

Gearbox oils have an SAE rating (50 or 80, I think), but I don't fully understand what this refers to. I think that viscosity is part of it. I do know that they aren't multigrade oils, so don't change behaviour with heat in the same way that engine oils do. A tribologist will be along soon to explain all, I'm sure ...

I still suspect that grot in the system may well explain quite a lot of the problem. To resolve the symptoms as described, I'd be cleaning the linkage and changing the oil, and re-assessing things then.


Oli.

 

[barks944]

Sounds sensible! I've just bought a rebuild LSD gearbox for my turbo that I'm going to fit. I might experiment with a few grades of oil in it and see what happens.

 

[djaychela]

Gearbox oils have an SAE rating (50 or 80, I think), but I don't fully understand what this refers to. I think that viscosity is part of it. I do know that they aren't multigrade oils, so don't change behaviour with heat in the same way that engine oils do.

I think you're thinking about it the wrong way round, though - single viscosity rated oils mean that they are only tested at one temperature. They do change viscosity with temperature, and often more than a multigrade oil. My gearchange is stiff on my 944 when it's cold (more so when the ambient temperature was low a few weeks ago), and it is fine once everything has warmed up; I'm confident this is just down to the viscosity of the oil; my other car (a felicia with a S/C C/R box in it) is much the same - when it's cold you have to take gearchanges slowly, but once warmed up all is well.

 

[barks944]

ORIGINAL: djaychela

Gearbox oils have an SAE rating (50 or 80, I think), but I don't fully understand what this refers to. I think that viscosity is part of it. I do know that they aren't multigrade oils, so don't change behaviour with heat in the same way that engine oils do.

I think you're thinking about it the wrong way round, though - single viscosity rated oils mean that they are only tested at one temperature. They do change viscosity with temperature, and often more than a multigrade oil. My gearchange is stiff on my 944 when it's cold (more so when the ambient temperature was low a few weeks ago), and it is fine once everything has warmed up; I'm confident this is just down to the viscosity of the oil; my other car (a felicia with a S/C C/R box in it) is much the same - when it's cold you have to take gearchanges slowly, but once warmed up all is well.

I don't think Oli has it wrong. He isn't saying that the gearbox oil doesn't change viscosity with temperature but pointing out that they dont display the same charachterisitics as a multi-grade oil. I don't think a single grade oil is on thats only tested at one temperature. A single grade oil displays a somewhat linear change in viscosity with temperature. Multi grade oils have a very different relation ship between viscosity and temperature as temperature changes. This means they don't thin out as you would expect as they get hotter, the idea being that you have a more optimim oil viscosity over a larger temperature range. Effectively the oil behaves like a thin oil when cold and a thicker oil when hot. This is because clever molecules in the oil change shape and all sorts when hot which has an effect on viscosity.

 

[barks944]

ORIGINAL: zcacogp

Tom,

Gearbox oils have an SAE rating (50 or 80, I think), but I don't fully understand what this refers to. I think that viscosity is part of it. I do know that they aren't multigrade oils, so don't change behaviour with heat in the same way that engine oils do. A tribologist will be along soon to explain all, I'm sure ...

I still suspect that grot in the system may well explain quite a lot of the problem. To resolve the symptoms as described, I'd be cleaning the linkage and changing the oil, and re-assessing things then.


Oli.

Just been looking and you can get multi-grade gearbox oil now.

"Many modern gearboxes use a 75W90 gear oil, which is actually of equivalent viscosity to a 10W40 motor oil"

 

[djaychela]

barks944

I don't think Oli has it wrong. He isn't saying that the gearbox oil doesn't change viscosity with temperature but pointing out that they dont display the same charachterisitics as a multi-grade oil. I don't think a single grade oil is on thats only tested at one temperature. A single grade oil displays a somewhat linear change in viscosity with temperature. Multi grade oils have a very different relation ship between viscosity and temperature as temperature changes. This means they don't thin out as you would expect as they get hotter, the idea being that you have a more optimim oil viscosity over a larger temperature range. Effectively the oil behaves like a thin oil when cold and a thicker oil when hot. This is because clever molecules in the oil change shape and all sorts when hot which has an effect on viscosity.

Which is the opposite of what happens with a single-grade oil, hence the gearchange being stiff when the oil is cold, hence my post - the opposite of what you've said is that they (single grade oils) do get thicker with reduced temperature. Oli's post said [oils] "don't change behaviour with heat in the same way that engine oils do", implying that a single-grade oil would remain at the same viscosity regardless of temperature, and pointing the finger elsewhere (the linkage, which I think is unlikely to alter with temperature in the way the gearbox will do). You've been able to get multigrade gearbox oils for a long time now, and this behaviour is why.

 

[Octoposse]

Thanks - I'll have a shot at cleaning the linkages first.

 

[Sandspider]

Mine does this too, especially stiff into second gear. It gets easy after about 3 minutes of motoring. I changed the gearbox oil, and it didn't really make much difference to the stiffness of second gear. I assume it's just the gearbox oil warming up, but not sure why second gear is the stiffest... (First isn't too bad).

 

[944Turbo]

If its been getting gradually stiffer over some months probably related to the colder weather at this time of year, should loosen up when the warmer weather returns. what oil was used?

 

[geoffbateman]

My 2.5 Lux is pretty much the same as others on here then. In summer or winter the change is always a bit stiff into 2nd and sometimes reverse when the car is first started. A short road run soon eliminates this. I have always taken this to be due to the gearbox warming up a bit and 1) arriving at operating tolerances 2) the oil achieving operating viscosity. It is normal to specify the lubricating oil in a gearbox so that it is at the correct/required viscosity within the operating temperature range. The same applies to hydraulic systems, as a general rule. Due to the fact that operating temperatures are invariably above resting temperatures the viscosity of the oil will be thicker at the cold/resting temperature to ensure that the correct viscosity occurs when under load and operating. It is quite common in hydraulically driven vehicles, especially when working on cold climates, to include an oil warm up period as part of the start up procedure to ensure correct operation with maximum lubricity to ensure low start up wear rates. My guess is that the Porsche engineers specced the oil viscosity for operating temperatures and this is why some stiffness can occur with cold gearboxes. The grade of a monograde oil, as correctly stated on here, refers to the viscosity at a single temperature point and the viscosity will alter with temperature at a rate dependant on a property called the Viscosity Index. Synthetic oils tend to have a better VI than mineral oils although some enhanced mineral oils can exhibit very good properties.
I would be very careful about using an oil of a lower viscosity grading (thinner) in order to try and improve cold changing performance, unless the Viscosity index is such that it allows for the correct operating viscosity at the normal operating temperature, otherwise you run the risk of having your oil at a lower viscosity than specified at the operating temperature and may run the risk of accelerated wear.
That's the way I look at it anyway.

 

[Sandspider]

ORIGINAL: 944Turbo

If its been getting gradually stiffer over some months probably related to the colder weather at this time of year, should loosen up when the warmer weather returns. what oil was used?

I can't really remember what it was like in summer! I think it was looser - will check when (if) the weather warms up. The oil I put in was Castrol 85W90 if I remember right - whatever was recommended on here, though not the Swepco stuff).