quattro Digest, Vol 52, Issue 55

[Grant Lenahan]

("unless the oil is soooo incredibly thick that it
doesn't flow at all to the journal bearings, I don't think the type of motor
oils we're talking about are in that catagory")

---snip---

this is exactly the concern.

In cold motors, even today's thin oils cant flow adequately at hgih rpms

All oils are a trade off because they must flow at -10degC and maintain 
some resistance to flow, and maintain high film strength, at +100degC

In meeting these trade offs, it is typically slanted in favor of 
lubricating a warm motor. This inevitably means wear at start up and 
while cold.

And for most folks, that's how motors spend most of their time.

Grant

LL - NY wrote:
> From my engineering education years past (I'm not a practicing engineer
> anymore, just a Physics Teacher), it won't hurt a tight bearing clearance
> engine with thicker oil (unless the oil is soooo incredibly thick that it
> doesn't flow at all to the journal bearings, I don't think the type of motor
> oils we're talking about are in that catagory), it'll just cause
> signifcantly more viscous drag. However, the opposite is not true. If there
> is loose clearance engine designed for heavier weight oils, low viscosity
> oils may not be able to create a thick enough film at the bearing to support
> the loads (unless you can increase the oil flow rate significantly enough
> that oil flow creates a supporting film) and bearing scoring may result.
>
> LL - NY
>
> On 2/25/08, Rick Cone <uglydog56 at yahoo.com> wrote:
>   
>> I wouldn't worry if your engine isn't sludging.  reference 50 year old
>> unrestored engine with crap oil.  Engine clearances are set with a certain
>> oil in mind.  A tight-built engine designed for light oil will suffer damage
>> when run on 20-50.  On the other hand, I had a 327 built by a guy that swore
>> by 20-50.  I tried to run it on lighter oil, and it used like crazy.  Put
>> 20-50 back in it, and the oil usage instantly disappeared.
>>
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