Why not overfilling a manual gearbox?

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Hi, I understand engine oil overfilling is bad and may render an engine a leaker due to excessive pressure placed on seals. However manual gearboxes are not pressurized and "sealed" like an engine. My manual gearbox has a breather to releave internal pressure if any. So why would it not be recommended to overfill the gearbox?? My gearbox type (french manufacturer) is known to be weak, especially around bearings. For example a higher oil level in the gearbox would mean a better lubrication of the secondary shaft bearings that are placed higher in the box. So wouldn't this be a good thing to increase longevity of the bearings? Would it pose any trouble with synchros/friction? Most boxes work with an "oil dip" but why an oil batch wouldn't be better in some case? Thanks
 
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Some gear oil climbs gear (look into shaeffers gear oil) to lubricate and that should be something worth looking into. Otherwise, I would just simply lay blame on the car/gearbox manufacturer for design faults. No way am I gonna overfill my gearbox in hopes (in vain) that it's gonna make it last longer. REmember: MT gearbox mainly rely on splash lubrication to lubricate.
 
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In reading about my old Dakota and why it takes motor oil, I read that some people claim if you jack it up you can put more than 2 quarts of oil in. They said it shifted better. I haven't tried it, mainly because I usually put in what the manufacturer says to. John
 

kilou

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Heat would expand the oil but if there was that much it would overflow from the breather and not crack a seal. However if the breather doesn't show any sign of oil leakage, then that shouldn't be a problem no? I've also seen many places where people argue overfilling made their tranny shift smoother....I just don't know for how long :) Now the real issue I'm concerned with is about foaming! It's seems overfilling may cause foaming. I can get it for automatic tranny that do work under pressure but not really for manual tranny with a breather.... Does it make sense also for a MT? Yes most manual transmission work with splash lubrication...for a reason probably. But I'd like to understand why a splash lubrication would be better compared to a constant oil bath, especially for bearings. Any idea?
 
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Too much oil in a gearbox will cause the gears to "churn" in the oil more than necessary and create heat which can be excessive and beyond the capabilities of the respective oil cooling system to handle (i.e. oil coolers, air flow cooling over the case, etc) and result in thermal runaway of the oil. In extreme cases the internals can reach temperatures that will cause distortion/melting. It can also cause to much aeration in the oil which could be more than what the stock breather system can handle, which will reduce lubrication capabilities (foamy oil doesn't lubricate well). Theres a fine line between effective cooling/lubrication and thermal runaway when oil system capacities are determined by the engineers.
 
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Fill it to the top of the Mfrs specs - at the top of the range. Be happy. BTW, a car engine is not pressurized, it is the opposite - it operates in a vacuum from the PCV system. This keeps leaks to a minimum.
 

kilou

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Yep but although the crankcase/intake etc is under vaccum, oil is pressurized (oil pump) in an engine...unlike in a manual tranny.
 
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 Originally Posted By: StormRider
Too much oil in a gearbox will cause the gears to "churn" in the oil more than necessary and create heat which can be excessive and beyond the capabilities of the respective oil cooling system to handle (i.e. oil coolers, air flow cooling over the case, etc) and result in thermal runaway of the oil. In extreme cases the internals can reach temperatures that will cause distortion/melting. It can also cause to much aeration in the oil which could be more than what the stock breather system can handle, which will reduce lubrication capabilities (foamy oil doesn't lubricate well). Theres a fine line between effective cooling/lubrication and thermal runaway when oil system capacities are determined by the engineers.
Correct, the extra oil will increase drag resulting in increased HP loss through the gear box. The extra HP will be absorbed into the gearbox as thermal energy resulting in higher lubrication temperatures. It usually takes testing to determine the optimum oil level and to develop the internal deflectors and such to make sure all gears and bearings have adequate lubrication.
 
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