why different gear oil specified for small block rear drives versus CARC?

[tpeever]

Getting ready to change fluids in our "modern" Guzzis and our manuals for Nevada and 1200 Sport differ in terms of oil specified for rear drive. 1200 Sport manual says 80W90 for the rear drive while Nevada manual says 85W140. Both GL-5. Not trying to substitute for the specified oil or start an endless oil debate, just trying to understand why the difference in specification. Seems like this oil needs to lubricate similar sets of gears operating under similar conditions. Are there differences in engineering of the two drives that would justify a heavier oil in one versus the other?

[Perazzimx14]

Why is the original small block for 1978 different from late model small blocks? They are fundamentally the same especially the cardan.

[bad Chad]

Why would gas engines use different motor oil?  They are all IC motors right?

[tpeever]

Yes, and specs change for the same engine over time. 10W40 used to be specified for the small blocks and now 10W60 is recommended and what I run in both bikes. Still trying to understand if there is an engineering reason underlying the difference in oil spec for the rear drives.

[moto-uno]

https://www.widman.biz/uploads/Transaxle_oil.pdf
  If this link works , you may have something to satisfy some of your curiosities  . Peter

[tpeever]

Thanks! Have seen this article and it is an interesting discussion of GL-4 versus GL-5, sulphur additives and yellow metals etc. Very familiar with this debate with older motorcycle and automotive gearboxes. That is not my question. I am interested in why a thicker GL-5 oil is specified for the rear drives of the small blocks compared to the CARC bikes.

[Mr Revhead]

Probably something to do with the actual load on the gears.
Maybe size, and contact areas dictate the difference

[lucky phil]

Yes, and specs change for the same engine over time. 10W40 used to be specified for the small blocks and now 10W60 is recommended and what I run in both bikes. Still trying to understand if there is an engineering reason underlying the difference in oil spec for the rear drives.

The oil viscosity requirement is dependent on 2 things, The ability of the oil to maintain the oil film on the plain bearings under a given load and temperature and the oil pressure required. I've not seen a modern engine that realistically needs to run a 60 weight oil for the purposes of bearing load. Race engines these days based on road engines now regularly run 20 weigh oils to cut friction and they don't burn bearings or seize. Every manufacturers oil recommendation I've ever seen lists the grade of oil required as a function of the ambient temps the engine will be operated at. Some argue that a 1200 Griso engine for example needs a 60 weight because of the cooling oil galleries around the exhaust ports but I've not seen any evidence or figures that indicate a 60 weight oil has a significantly higher thermal carrying capacity than a 40 weight. I've got lots of experience with gas turbine oils over the years that need to cool the turbine bearing cavities in jets and they dont need or use high viscosity oils, quite the opposite in fact and those temps are way higher than any Griso exhaust will experience. The more viscous the oil the less flow for a given pressure and therefore less heat is extracted is the opposing argument.The other factor is oil pressure and any oil viscosity that will give you around the 10psi/1000 rpm at operating temperature for the given circumstances you are operating the engine in and the oil temperature that generates is whats required. So and air cooled engine in summer in city commuting will be best served by a different grade than the same engine touring in the winter by and large with regards to maintaining the optimal maximum oil pressure. There are two factors that effect Guzzi's in particular and that is they are air cooled which complicates viscosity choices more than a liquid cooled engine because the liquid cooling can maintain the engine in a much narrower temperature range and Older Guzzi's seem to have a sensitivity to idle oil pressure in hot conditions at higher engine temps.
I have been operating a v11 Sport for the last 10 years on Mobil1 0W-40 oil without any issues of any kind whatsoever but there will be people here that will tell me its too thin, which is quite incorrect. When I rebuilt my Daytona engine I logged the oil pressure at varies engine oil temps and decided to run after the break in oil a 10W-60 because  of 2 factors. My hot oil pressure on a 15W-40 was peaking at 58-60 psi which was a little lower than ideal but was of no real concern to me and the idle oil pressure was comfortably above 10psi which is about as low as I'd like to see it not for the concern with regards to a mechanical issue but thats the floor to avoid nuisance oil light indications under extreme conditions in summer in traffic here in Australia. Considering these factors I decided to go with the heavier oil to give me a little headroom at both the upper and lower ends. I could happily run the 0W-40 in the engine as well and not be in the least concerned as 60 psi I know from experience is adequate for this engine even on the track.
So why does Guzzi specify a 10W-60? on balance because they are trying to cover all their bases and possibilities and give themselves "headroom" same as I have done on the Daytona. Is it absolutely necessary, i don't believe so unless you are operating in extreme conditions and as I pointed out in another thread there would be plenty of bikes happily surviving on lower grade oils because dealers just go with the closest grade they have in the workshop if they dont use a 60 weight all the time.   

Ciao           

[Motormike]

When I go to the Harpers site, they state right on their home page that Piaggio now charges over $4000 for a new CARC assembly.  But when I look up various "transmission assemblies" for CARC bikes like the Griso and the Stelvio, prices are all over the place, from $2000 to as little as $900???  If I thought I could buy a complete CARC for as little as $900 I'd hoard a couple.   

[moto-uno]

  Nice response Phil , but I think he was asking about the rear drive box :) . Peter

[lucky phil]

  Nice response Phil , but I think he was asking about the rear drive box :) . Peter

Yea I know but I was answering the specific example of the 10W-40 V 10W-60 engine oil in that post. There's multiple reasons manufacturers make oil grade recommendations including for transmissions and it's often not entirely for technical reasons. I wouldn't be trying to read too much into it quite frankly esp with the Italians.

Ciao