Why go heavier?

[Mark in NY]

Hey, back to the old viscosity debate.

Many guys here agree with me but there are guys that feel strongly that even in “sporting driving” conditions, the OEM 5-30 is best for modern “small clearance” engines.
I have gotten great results using 20-50 for 30 years (yikes) and recently switched to 0-40 Mobil-1. This seems to drive some guys nuts, and now with the advent of 5-20 OEM recommendations some consider SAE 40 or 50 molasses.
OK, I finally got around to verifying what I knew all along.
Directly out of the respective Mitchell repair manuals:
Rod bearing “oil clearance” specs:
All figures are in ten-thousanths inch and (mm)
1983 Mustang 5.0 r 0.8 - 1.5 (.020 - .038)
(eng has same specs as the unit that I used to compete with in 1972 SCCA events w/ SAE 50 oil)
2000 Contour SVT 1.1 – 2.5 (.028 - .064)
Decades apart yet very similar nominal rod bearing oil clearance specs. In fact according to Mitchell the newer car has more rod bearing clearance!
So much for that urban legand.
Last week I put a question to the Penzoil tech line: I have a late 90’s car out of warranty, I want the longest engine life, do I use 5-30 or 15-40?
The answer from Penzoil was “ the 15-40 will give the longest engine life.
Now all of the above is a moot point for most drivers that change cars every few years, but for what it is worth – they you are.

 

[sprintman]

Mark in NY
Carefull you'll upset a lot of people here with 'thicker gives longer life'. I'm with you as are most here in Oz. Here 20W50 outsells all other viscosities combined. Valvoline just released Maxlife here, 20W50 only. I called Valvoline re 10W40 availability but they said "who'd buy it"? I wouldn't use 20W50 either now (used 25W70 for a few yrs) but then I sure don't want 5W/CAFE in my motor either. 0/5W40 syn seems like the go but it's horses for courses I guess?

 

[MolaKule]

"doubt engines are designed for oil weight. I'd think they are designed for power, fuel economy, torque curve, max hp...."

The oil grade or viscosity is selected for the engine. No engine is built around the oil.

But the load, clearances, rpm, bearing temp, etc. determine the oil viscosity or viscosity range for the oil.

 

[SteveD2]

MolaKule

In your post above you said that a higher rpm shaft would require a higher viscosity oil, other variables staying the same.

Why? I'd think that the higher speed would have a greater pumping force creating that hydrodynamic wedge of oil that the journal rides on. If so, lower viscosity would be suitable.

I've been trying to think of examples from my practical experience, but none come to mind where the variables all the same...shafts are bigger with lower rpms due to the greater torque, oil leaving the bearing is cooler due to the lower rpm or lower heat from the machinery transfered into the oil, etc.

Ken

 

[MolaKule]

Ken,

"Bearing friction is determined by an empirical equation (derived by test data) called the “McKee” equation as: Eqn. 1, f = 1x10^-10[473(ZN/p)D/C] + k, where k is a constant determined from another equation or chart for various L/D ratios; L is length of bearing (in.), D is diameter of bearing (in.), N is speed of journal in rpm, N’ is speed of journal in rps, Z is absolute viscosity in cP, C is diametral clearance between bearing and journal in inches. For most bearings in automotive use, k = 0.002. p is pressure = W/LD, W is bearing load in lbs. An average value for D/C is 1000. In automobile and piston aircraft engines, the loads W, are: Main – 700-1700 lbs., Crankpin – 1,400-3,400 lbs., and Wrist pin - 2,000-5,000 lbs. The ratio of e to the radial clearance is called the “attitude” and is defined as: Eqn. 2, A = 2e/C = 1- 2Ho/C. Ho is the “Minimum Film Thickness.” So how does one find the viscosity required for a bearing? One uses the “McKee” equation, along with the “Lasche” equation, for bearing heat generation and dissipation, Hd and Hg, and equates these equations to find the viscosity required!"

Notice the friction equation above. The friction increases with revs, N in the equation.

 

[MolaKule]

Ken,

Whoops, got interrupted.

Anyway, the friction f goes into the Hd and Hg equation for heat generated and heat dissipated and is solved for viscosity. So N definatley affects the viscosity requirements.

 

[dickwells]

Mark: Are you sure the clearances you listed are in ten thousandths? .00008 is pretty frickin tight for any bearings I have seen. Thats two microns. RW

 

[Dominic]

Just to add to the oil pressure side of things, my 3800-II V6 runs 70psi idle and 80-90psi easily cruising

LT1 and LS1 motors run such LOW oil pressure compared to me its funny

 

[Patman]

That oil pressure is definitely higher than my LT1! I run about 50psi at idle on a cold start, which comes down to about 18-20psi at idle when warm. When warm, I run about 40psi at 2000rpm on the highway.

 

[Mark in NY]

quote:

---

Originally posted by dickwells:
Mark: Are you sure the clearances you listed are in ten thousandths? .00008 is pretty frickin tight for any bearings I have seen. Thats two microns. RW

---

****,
Those specs are right out of Mitchell.
I checked a few other cars spanning several years. Seems 1 to 2 tens is typical.

Mark

PS I see that on the newly revised Mobil-1 site they are now broadcasting that 0-40 is nationally available in Autozone stores. Just earlier this year they would only say words to the effect - sorry 0-40 is only available at BMW MB etc dealers. I suspect there is much demand out there for heavier-than-CAFE weight oil !

 

[satterfi]

quote:

---

Originally posted by dickwells:
Mark: Are you sure the clearances you listed are in ten thousandths? .00008 is pretty frickin tight for any bearings I have seen. Thats two microns. RW

---

****

You have one too many zeros in there. It's .0008 to .0015.

 

[dragboat]

{Decades apart yet very similar nominal rod bearing oil clearance specs. In fact according to Mitchell the newer car has more rod bearing clearance!
So much for that urban legand.)

Take a look at a early 70's GM Mark IV "454" Factory clearances

.0025 Rod bearing clearance
# 1-2-3-4 main clearance was .0032 and #5 was .0040

Here is spec for a low HP 350 Chevy from 73-77
.0028 rod bearing clearance
Mains- #1 .0020 #2-3-4-.0023 and # 5 was .0030

All are in max limit. See how priorty oiling works there?These engines get rebuilt and often are left with X clearance throught the mains,GM did not design them with this in mind. Now take one of these engines,machine and modify it heavily especially in regards open valve spring pressure,the builder should consider the oil "wt" and other things about" Barrier protection" it imo or someone will be pulling the pin on the grenade that has just been built

I use a Moroso oil pan blanket and run straight wt oil because of this and realize these newer cars are worlds apart but I just wanted to show that all engines imo need a different take on a choice of oil that will be used and how the car is operated because if one engine does well on a oil,the next one might just thrash that oil or allow the motor to be thrashed by oil selection

[ October 06, 2002, 06:29 PM: Message edited by: dragboat ]