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Discussion Starter #1
What is the purpose of low(er) friction piston skirts? I believe that's a feature of at least the 2.0 GDI in the Mk3's.

I don't see what it does. The piston skirt doesn't ride on the cylinder bores, the piston rings do.

Is it because pistons moving through the crankcase gases (no, not gasoline) represent a fluid-friction point? Reduce the friction on the piston "body" and it moves through the gasses with less friction?
 

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Could be for lower friction w/ the oil.
 

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Even though the rings technically support the piston there is no way you can cut the skirt completely off, it at some point rides on the walls to stabilize the whole works. Look at the wear points on skirts, you can't have that with no contact. The shorter the skirt gets the more the piston rocks and you lose ring seal with that. The wear greatly accelerates the more it rocks as well.

Friction is usually lowered by both shortening and narrowing the skirt to make less contact overall. Some of todays' normal grocery getter piston skirts look like the trick race pieces of the '70s.
 

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Discussion Starter #4
Even though the rings technically support the piston there is no way you can cut the skirt completely off, it at some point rides on the walls to stabilize the whole works. Look at the wear points on skirts, you can't have that with no contact. The shorter the skirt gets the more the piston rocks and you lose ring seal with that. The wear greatly accelerates the more it rocks as well.

Friction is usually lowered by both shortening and narrowing the skirt to make less contact overall. Some of todays' normal grocery getter piston skirts look like the trick race pieces of the '70s.
Cool. Thanks. So the pistons are designed (if even somewhat) to "ride" the cylinder bores. Are the bores oiled beyond the oil regulator ring for this?

And I see your point. Anything less than perfectly perpendicular to the bore means the rings lose their maximum perpendicular face to the bore
 

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Bores are "oiled" all the way to the top of the compression rings (and a tiny bit escapes past them).

Typical ring description:

Top compression ring, that seals the bore under compression to prevent passage of MOST of the combustion gasses. Generally rides square to the bore in operation, usually chromed (alloy) on the friction surface for wear resistance/smooth sliding & sometimes notched on the top inside edge helping gasses press it out & down to seal.

Oil scraper ring, outer edge at an angle to the bore with the "point" at the bottom. Grey iron typical without coating on edge, it keeps all but a thin oil coating below the compression ring.

Third layer usually (currently) consists of three rings, two oil control rings on either side of a wavy expansion ring with spaces for oil flow. The pair is often coated on the edge & runs square to the bore. The groove for these is punctuated by holes to the inside of the piston for oil flow scraped from the cylinder walls as the piston moves downwards.

Piston is designed to ride as tight to the bore as practical, space needed for oil/expansion as the piston grows when hot.

Piston skirts are the largest part of the piston when cold, piston top expands the most as it heats up with little expansion at the skirt. Clearance is set cold, between the piston skirts & the cyl. bore. Piston ring end gap is also set cold, giving enough expansion space that the ends don't press on each other hot & expand the ring excessively leading to seizure.

Without going into detail, the piston rides on the exh. side of the piston/cylinder during the power stroke, it's not randomly moving side to side during the course of a stroke.

The skirts of the pistons get the most wear from perfect oil layer breakdown, and low friction coatings both reduce this wear & reduce engine friction to aid economy & power.

Reducing friction losses has been a large part of increasing economy/power of engines, this is just one area addressed. I won't get into details on other areas of friction/oil pumping loss reduction, but look at the profile (width around the circle) of the current cam lobes to see a major change in design that reduces energy needed for movement.
 

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The skirts flood with oil scraped down by oil ring with every downstroke. The extra flows back into inside of piston from the holes behind oil ring.
 
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