My friend and I were talking about flat engines (Porsche, Subaru) today and why they burn more oil than others, although most high-performance engines burn more oil than conventional engines.

He thought it was because when you shut down a flat motor, the oil on the back of the pistons and rods would just drop to the lowest part of the cylinder and sometimes seep past the rings into the combustion chambers (worse if parked on a hill, and worse in an air-cooled pcar engine since the tolerances were looser IIRC).

I mentioned that Porsche, unlike Subaru (I think), has an "integrated dry sump oil system", and that would help with the problem. He thinks differently, what do you guys think?

-flamesuit on-

Sorry in advance if this is a repost of some sort, most of the threads I had found already were not about why they burned more oil.

 

My 997 is very good with oil consumption, it averages about about 1 litre every 2500mi. when I have to add a little oil. My 2 previous M3's, both I6, one was terrible and the other did great on oil consumption. My 01 M3 oil consumption was 1 litre every 1100mi., while my 04 M3 wouldn't need any oil until after 3k miles. Funny thing is the 01 M3 I did the break in religeously while the M3 at 800mi. I started to enjoy the 8k rpm redline and it never used any oil.

Dave

 

The 6.3 AMG cars burn a BUNCH of oil as does the S54 BMW engine.

 

Hey Dave, I remember you from the old days on the M3 boards!
My M3 was ok on oil, but was 1QT/4,000 miles
My 997, I would estimate that I will have to add a QT by 4500 miles.
I think that since the oil sits in areas that have more seals and junctions on the flat-6, you just get a bit more seepage. Other than that, I think that any motor that pushes 100bhp/liter is going to be high strung enough to burn some oil.

 

I don't think flat engines burn more oil, necessarily. They certainly can leak more oil. In-line and V engines have the oil sitting in the oil pan when the car is not being used. The leak point is really the drain plug for these engines if the plug gasket is not doing its job. In addition, all of the oil from the upper part of the engine ( head, valve train cylinders ) will drain into the oil pan in those designs. With a flat engine you have a split case. So, in addition to the oil drain lug you have the mating surface between the two halves of the engine as a potential leak point. In addition, the oil in the head area and cylnders does not drain as easily into the crankcase. Further, the dry sump system ( and plumbing ) offer additional chances to leak.

As far as actual burning ( cylinder/piston surface) , I observe that the higher perfoming engines have a tendency to do this ( BMW M, etc.). In addition, any car operating at higher rpms will see oil consumption increase throught the valve train area, as well. Higher performance engines also often run on lower viscosity/synthetic oils which, theoretically can leak more easily. Finally, when engines are operating, the oil will find its way past any seal, gasket, O ring ( Porsches have lots of these) thst is not doing its job perfectly. So some of it is leaking and some is getting into the combustion chamber.

 

Thanks everyone, you've all been very helpful.

 

Whats up John...I miss those old Roadfly days...

Dave

 

I haven't been back there in a while! Most of the old crew moved on to P-cars,(a few got the new M3s, like 'greg in oregon') and I think the board started going downhill a bit after everyone left. I am glad that I moved to porsche too!