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Old Sun, Nov-29-2009, 12:21:18 PM   #42
Cleveland
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Originally Posted by elp_jc View Post
To keep the thread going, does anybody have a clue HOW the oil gets to the combustion chamber? It has to be thru the rings or valve guides, but why? I'd understand if we were running 5/20 oil, but with 10/60? Aren't our engines supposedly built to very close tolerances? Something doesn't add up.
The thing most M3 owners don't understand, is that our high-revving engines are very unique powerplants. They cannot be compared to the typical 3-series engines in the non-M cars. The engineering tolerance that are used to build these engines, does create a few necessary compromises that are unavoidable.

The biggest difference between a "M" engine and non-M engine, are the tolerances between the cylinder bore and the the Aluminum pistons /w/ nitrided cro-moly steel piston rings.

Since the S65B40 4.0 liter engine revs to 8400 rpms, the tolerances for piston ring-to-cylinder wall clearances have to more conservative than the tolerances for a normal 3-series engine.

One of the problems associated with in designing a high performance engine with a 8400 redline, is the intense heat that is generated at full load.

You have to engineer in a small relief tolerance, so the piston does not SEIZE inside the cylinder bore.

All internal engine components are made of a wide variety of steel, titanium and magnesium alloys. These reciprocating engine parts are sliding inside smooth uncoated aluminum engine block cylinder bores. These different metal alloys all have different expansion and contraction rates when they heat up.

The BMW engineers have to accurately calculate a safe operating tolerance between all these parts at every temperature point between 0 degrees Fahrenheit and ~260 degrees Fahrenheit.

The biggest challenge for the engineers...is to find a happy medium where the engine will not seize under the most intense testing conditions, while also minimizing the blowby of oil past the piston rings.

That happy medium is why we are told NOT to rev the engine before the engine reaches it's normal operating temperature. (~220 degrees Fahrenheit)

If you rev the engine too high (before it has reached it's normal operation temperature), you will be forcing some oil residual past the outer piston rings into the combustion chamber. (where it is burned as a fuel source)

That is where you 'lose' oil when the engine is cold.

When the engine is operated at extremely high rpms for long periods of time, the break-in procedure will determine whether or not you lose a noticeable amount of oil over time.

Some M engines will lose oil, while others will not.

It all comes down to how well the piston rings were worn-in upon initial break-in.

If the break-in was successful, the edges of the piston rings will seal properly with the cylinder walls to create a very tight seal. But if the edges of the piston rings did not wear-in properly...you will not have a good seal. (resulting in moderate to severe oil consumption)
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