Pistonheads SOTW -B4q Avant with ABC engine

[macspring]

See a 2.6 V6 Audi Avant Quattro is Shed of the Week on Pistonheads.

http://www.pistonheads.com/news/defa...?storyId=28531

While it's too dear for it's condition, the comments by non S2 owners are interesting to read.

[Cottis]

Previous owner went skiing and left the sunroof open = Genius.

Pretty honest write up. Lots of work done to it. I have definitely taxed worse.

[Iolaire]

As a motor engineer I get fed up with so called motoring journalists who clearly have no idea what they are talking about.

This is my take on this article:- http://www.pistonheads.com/gassing/t...vant&mid=62886

Note the bit about the "low tension piston rings (a design feature).

J

[mcandmar]

Where on earth did the low tension piston rings come from. If anything VW/Audi's have always been known for using extremely hard piston rings that take 100k miles to run in.

[dm80]

'Low tension' may well be a relative term though, that has somehow leaked out [sic] after discussions between audi design engineers eons ago. There is not a direct, linear relationship between material hardness and material yield (tensile strength). So the material specced may well be fit for purpose, but made of a lower yield material than a previously specced material, or previously proposed material.

[Iolaire]

Not sure I would agree with you dm about there being no direct relationship. Piston and ring design is an extremely subtle business, and the obvious problem for the designer is expansion.

If you are starting an engine say, in a cold climate where the material temperature is 10 or 20 degrees below, the combustion chamber, of which the piston crown is an integral part when at top dead, is instantly propelled to hundreds of degrees above on ignition.

The heat is then transferred firstly to the top ring and then down to the oil control rings. If you design the piston with too much running clearance it will knock like hell, particularly when cold and use oil like it is going out of style. If you don't have enough clearance it will seize at high temps.

The ring material on the top ring is usually completely different in composition from the oil controls as it has to create compression and withstand enormous heat. The top ring is usually very hard and also has to have exactly the correct combination of tensility and ductility to enable it to "cling" to the bore at very high revs. The oil control rings are usually a composite of 3 parts, 2 scrapers and one expansion. The expansion provides the tension to keep the scrapers tight to the bore, but at the same time permitting oil to cling to the bore for lubrication.

The PH article appears to imply that Audi deliberately fitted oil control rings that have a very low tension to permit the engine to use oil; and this only on the 2.6! Why the hell they would do that is beyond me. This is not a highly stressed, turbocharged engine. There would be no requirement to design it to use oil. In all the years I have worked on these cars I have never come across this, and I think I would have by now if it were the case.

I just hate motoring journalists who have all the enthusiasm but little or no knowledge on their subject. Every time I read a car magazine I see this on just about every article.

Confucius he say, "a little knowledge is a dangerous thing!"

J

[dm80]

Ah, I'm not saying Audi engineers designed 'low tension' rings to deliberately to permit the engine to use oil.

I merely pointed out that while the material used does have to a high hardness property it may be the case that relatively speaking it may also have a lower yield strength property ie: the point at which the material will not return to it's original form - within the elastic limit.

The top ring is usually very hard and also has to have exactly the correct combination of tensility and ductility to enable it to "cling" to the bore at very high revs.

When you say tensility and ductility you mean material yield strength and material hardness. Material hardness ie: toughness is probably the most important property required for the function of the piston rings/seals. The whole issue may have just been a term that has grown arms and legs during discussions in the steamie and has somehow crept onto 12v.org and hence why a motoring journo has plagiarised it as his own without knowing what the **** he's on about.

I agree with your last statement

[dm80]

Hey - f e c k isn't swearing