Filling oil just to min line? better mpg?

[undrgnd]

Originally Posted by Christ

Too much oil doesn't prevent the cylinders from moving at all... it just makes it alot harder. While oil doesn't compress, it does move. As one piston is coming down, another is going up, so the fluid is getting forced elsewhere in the crank case. The last thing that would happen from that is a seized piston. More likely what happened was that the flickering oil light was from a failing pump, and intermittent pressure, but without seeing the engine first hand, my guess is as good as the stuff in the cat box.

Christ, you can't be serious! If you really think that when a piston comes down and meets a volume of oil nothing will happen because there is another piston going up, you are in for a shock. Go ahead, add an extra gallon of oil to your car and rev it up. Let us know how much the repairs will be. I know for a FACT that overfilling WILL destroy an engine. At the speeds that the piston is traveling, the oil may as well be concrete.

Underfilling or overfilling by a small amount (~a quart) will not hurt an engine, but I doubt it will help anything either. The nominal dipstick oil level is just that, nominal. It is meant to accommodate all expected engine operating conditions, so it is not an exact, calculated setting. IMHO reducing the oil level in the engine to improve FE is more foolish than removing the seat belts to shed weight for the same goal. I have never met the OP, but I'm going out on a limb and suggest they drop 15lbs and forget about reducing the crankcase oil level. The impact on FE will in all likelihood be the same.

[ShadeTreeMech]

My van's motor lets me overfill it by a quart with no ill effects. It seems to like it, as far as that goes. And as far as whether less oil will allow the engine to heat up faster, there's plenty of heat to go around.

I'd be more comfortable with a lower weight of oil, or as i do, add a quart of Marvel mystery Oil to the crankcase to help keep things clean in there. It likely lowers the viscosity somewhat as well.

I've also loved to second guess the design specs of a vehicle, and sometimes i have to realize much smarter people than I said "you need 5 quarts of this oil to make the engine last longer."

[gone-ot]

...anybody familiar with the technical term "hydraulic lock"?

[Frank Lee]

Yeah. Why?

[gone-ot]

...why? rhetorical emphasis only.

...being basically "incompressible," nasty things *happen* when rotating parts "hit" liquid oil in the crankcase.

...sorta akin to a "metallic bellyflop" for each cylinder as the engine turns.

...as mentioned earlier, that's why crankcase "windage" trays are often (but not always) used in high-rpm engines -- to keep metal and liquid oil totally separated, and metal and oil vapor/foam as far apart as possible.

[daring4]

It would take a crap load of oil to hydro-lock an engine from the bottom of the pistons! When one comes down some are going up, hydro happens when one or more cylinders fills with a liquid on the compression stroke. Over filling the oil pan might reduce mpg's and power from the oil splash, but seriously you will not lock up an engine from a lil overfill.

[daring4]

Engines wouldn't be designed with oil squirters on the rods to spray on the pistons if they couldn't handle it. It's how they cool the pistons. Oil mist is not much different than air, the bottom of a crankcase is not an area of high compression.

[Christ]

Of course the crankcase won't hydro-lock... but if you overfill the crankcase, there is a good chance that oil will leak past the oil control rings and pump into the combustion chambers, which could cause a hydrolock condition on some engines.

[daring4]

I hate to tell Christ thats he's wrong but combustion pressures produce pressure in the crankcase not a vacuum. Oil won't be sucked through the rings into the combustion chamber. Air/fuel is much more easily pulled in through the open intake valve. Pull off your PCV valve at idle and see if it sucks or blows out the hole. 20 bucks says it blows.

If your oil control rings let that much oil in, burning 2 quarts per tank, then you have bigger fish to fry. But they wont let enough in to lock the engine. Thats all I'm trying to get at.

Darin

[Christ]

Well, bud, you can keep believing that, but I've actually seen engines ingest oil when the crankcase was severely overfilled. A quart high, not a chance, but when the oil change specialists at Meineke decide to drain the transmission and refill the engine during an oil change, and now there's nearly 12 qts of oil in the engine, well, that's going to cause a problem.