oil change

WILDEBILL308 wrote:

427435 wrote:
I guess I have to go into more detail.

A typical journal bearing of a certain diameter, width and operating rpm well have a minimum allowable operating clearance (let's say that minimum CLEARANCE is .001 inch). Then let's say the minimum machining TOLERANCE for the bearing ID and for the shaft OD EACH is +/- .003 inch.

For a nominal 2" rotating shaft and bearing, the shaft will be spec'd something like 1.997" to 2.003". Thus the bearing will need to be spec'd to be from 2.004 to 2.010 to provide a minimum or .001 operating clearance with the smallest bearing going on the biggest shaft.

Now if tolerances can be reduced to +/- .0015 inch, the shaft can then be spec'd to 1.985 to 2.0015 and the bearing can be spec'd to 2.0025 to 2.0055. The minimum clearance is still .001" but the maximum clearance has been reduced from .013" to .007"----------which allows (but does not require) a thinner oil.

Both scenarios, however, require an oil "thin" enough to provide lube when things are at their tightest----------at cold start up.

At cold start up all oils are considerably thicker (by many times) than what they are at operating temps. Some typical viscosities (cSt)of Mobil 1 oils are:

Temp------5w-30------15w-50

-20----------6284--------21,852
0------------2129---------6462
25------------697---------1836
32------------531---------1348
50------------281----------657
100------------71----------138
150------------27-----------46
175------------18-----------30
200------------13-----------20
225------------10-----------15
250-------------7.6---------11.1


Please know that a 5w-20 oil will be very similar to the 5w-30 oil at cooler temps but drop to single digit viscosities at around 212 degrees F -------------which may be fine with a bearing and shaft machined to tight tolerances. This will also improve mpg during CAFE testing.

However, that close tolerance bearing was also be fine with viscosities of several hundred when the engine was starting and warming up-----------and even several thousand if you live in the North.


Bottom line, a 5w-30 oil (or even a 5w-40) will do no damage to your engine where a 5w-20 oil is recommended. It may reduce your MPG by a small percentage, however. It will also provide more lubrication strength under heavy load and at higher temps.

My V10 often runs with the engine temp between 200 and 212 degrees (scan gauge). As I want it to last a long time, I will sacrifice 0.1 or 0.2 mpg to be sure it does.

Mark, great bit of pontification about a fictitious engine. The dimensions that I posted are from Ford. Max main bearing oil clearance 0.0018 that is a long way from your 0.007 max oil clearance. To say that you can have an engine with a 0.013 oil gap is well…..By the way where did you come up with that?
I still recommend Mobil 1 0-20.
Bill

You are worrying about the trees and don't see the forest.

Yes, the numbers are fictitious-----but it is a fact that there is a minimum clearance for a bearing to run at. When manufacturing tolerances are figured, things are spec'd so the clearance never falls below the minimum and the max clearance is whatever the manufacturing tolerances give.

Today, tighter manufacturing tolerances result in smaller MAXIMUM bearing clearances but NOT SMALLER MINIMUM CLEARANCES.

While this allows the use of thinner oils, it does not preclude the use of heavier oils------------as long as the first number is low enough.

Nor is there anything wrong with a 0w-20 oil (especially in cold temps), but a 0w-30 oil is better for engine protection (and a 0w-40 still better).

http://www.mobiloil.com/USA-English/MotorOil/Oils/Mobil_1_0W-40.aspx