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      09-03-2014, 09:31 PM   #9
42pilot
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Quote:
Originally Posted by MattBianco View Post
I need to understand oil consumption. My understanding is gaskets and seals and piston rings keep oil out of the pistons. Any oil missing must have seeped in and burned off (pollution, possible damage to emission system, catalytic converters, etc.), or leaked from a gasket.

After 35 years of driving BMW, Benz, Lexus, Volvo, Merkur (yes I had a Scorpio once, and no teasing allowed), Hyundai (first generation Excel, no jokes allowed) and even new red Scirocco (any jokes here and I will show up to your front door), the only times I had oil consumption was related to leaks. I still have a Lexus LX 470 with circa 200k miles and it has never required a top-up of oil. My current CLK500 and the R500 also lose nothing in their 10k service intervals. I had a 190e about 25 years ago that would lose about a quart every 3000 miles or so, due to a leak. Others were repaired for gasket leaks and oil loss stopped.

I am baffled that oil use can be justified, but as I am not a mechanic, I will defer to a good explanation as to what happens to the mysterious disappearing oil.
Great question and allow me to answer it based on my 12 years of building and designing hot rod turbo Porsche motors (only).

Porsche, BMW, Merc and other high-end car mfg build their motors out of alloys such as magnesium, aluminum, steel and so on. Each of these alloys are used in different areas in the motor. For example, Porsche used forged aluminum pistons inside nikasil aluminum cylinders. Each of these alloys have unique properties that work great in their designed roles, and have different expansion rates. Therefore, the parts need to be designed to work properly at operating temperature and it could mean that, as the engine comes up to temp, it allows oil to seep past (like rings) into the combustion chamber to be burned.

Additionally, Porsche designed their motors to use the smallest amount of oil in their cylinders. The rate was 1200 miles per quart, but what this does is adds additional lubricant where metal to metal is most aggressive. This is why a properly maintained Porsche boxer motor will easily last over 200,000 miles without needing an overhaul and will have very low leak-down values (healthy rings) thereby keeping power relatively high in an older motor.

When building a Porsche motor compared to a domestic motor, it takes about 3x longer due to the attention to tolerances and dry fitting parts such as heads, pistons, valve clearance, deck heights, main and cam bearing clearance, rings, etc. A difference of .0040 in these motors could mean the difference between properly running versus putting a hole in the top of your piston. If you need pics, let me know

BMW and Merc do the same, but with newer technology oils (synthetics), the need for oil use is lower - but it's still there.

My 1979 525hp/500 lb ft torque turbo Porsche used about a qt every 900 miles and during track events, used a quart a day. For your info, a Formula 1 car will use a couple of quarts each race for the same reason - the use of lubrication in the form of light-weight oils takes less horsepower to run the motor and it ensures the engine will last multiple races. They use a dry sump system.

Until recently, most American cars used iron and steel throughout their engines, therefore providing nearly identical expansion rates. Therefore oil usage is low on domestic production engines. That has changed over the past years to aluminum heads, blocks, pistons and so on, so I would imagine oil usage is up on some models.

A properly built, high quality motor will use oil and it will be burned in the combustion chamber.
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