BucketList

It's about like your C4S. About a quart every 1-1.5k miles. Engine purrs like a kitten.

ADias

Change oil brand. Try Liqui Moly Leichtlauf High Tech 5W40 or Motul 8100 Xcess 5W40.

okie981

What causes a piston skirt to collapse? If this has already been posted in this thread, I apologize for asking again.

mikemessi

That is the million dollar question.

HenryPcar

I like to know too, and what exactly is piston skirt collapse ?

Wayne Smith

Check this link ...

http://www.sccoa.com/forums/showthre...sed-skirt-quot


Essentially a collapsed skirt is when the skirt of the piston has actually given way and is closer to the center of the bore than it should be. This is frequently the result of repeated overheats.

Charles Navarro

If you google piston skirt collapse, you'll find a very long list of matches. It's a phenomenon that can happen in any engine.

JE discusses how to evaluate used pistons and they touch on measuring the piston at it's gauge point, which is the point at which the piston diameter can be verified and where you set the total piston to cylinder clearance.

http://blog.jepistons.com/evaluating...are-still-good

While we were using original pistons in the RND program, we would measure every piston. Some wear was to be expected and as we were having the pistons re-coated, we would allow for a small variance in the piston diameter of no more than .0005". We would find within a set sometimes a piston that was significantly smaller at the gauge point (but with no visible wear) than the rest of the pistons out of an engine, which we would reject because of the semi-collapsed skirt.

The how or why is a great question. In my opinion, fatigue is the most likely reason.

mikemessi

Could excessive rocking/tilting within the bore also cause this? The rod bearing wear on this car is suggestive of lspi. Could lspi have possibly played into the equation? Still lots of unknowns and speculation. If this cylinder on this car was prone to running hotter then pre ignition would also be more likely.

Charles Navarro

LSPI (or detonation for that matter) can cause damage to the piston including the piston skirts, so anything is possible.

Rod/stroke ratios, piston skirt size/cam profile, piston offsets, crank angles, etc. all have play in this. Lots of variables with many unknowns and assumptions.

We made one observations early on with Cayenne and 9A1 engines - the pistons have a very hefty pin offset. With the M96/M97, we were able to make the pistons with no offset without having piston noise. We tried this with the Cayenne and 9A1 and the engines sounded like a Porsche diesel tractor. We ended up having to put the offsets into the pistons, leading me to believe (again my opinion) that the forces acting upon the pistons at BDC are greater in these engines due to possibly less favorable engine geometry and lower rod/stroke ratios.

Historically, Porsche has used a fixed deck height (with aircooled and watercooled engines alike) which means as the crank stroke increases, piston pin location must be moved further up towards the top of the piston, resulting in shorter pistons, and typically shorter rods, which results in less favorable rod/stroke ratios. The higher the rod/stroke ratio, the lower the cylinder wear and piston side loading.

okie981

Charles, I recall in earlier posts you mentioned you hadn't seen collapsed skirts on cast pistons (or not many), but had seen numerous forged pistons with collapsed skirts. Is it as simple as cast piston skirts are less likely to be affected by heat when high forces are applied and forged piston skirts are more likely to deform? Not tryign to oversimplify, but wondering if this is a general approximation of the difference in the number of deformed skirts seen between cast and forged. Seems counterintuitive because forged is always better, or so hot rodders have always read in Hot Rod magazine anyway.

As I posted earier, for some reason my #1 cylinder is always pulling more timing than the other 5. This *could* imply the temperatures in that cylinder are warmer, regardless of cause/effect of the warmer temperatures.

Charles Navarro

Correct. We have seen less scoring and I can't recall ever seeing a cast 2.5, 2.7, or 3.2 piston with a collapsed skirt. A forged piston indeed is stronger, but cast pistons by design and manufacturing limitations are heavier and thicker than their forged counterparts.

INTMD8

Maybe this has already been discussed, but I can't help but wonder when things like this occur (and no slight meant at the OP as he bought the car used) if this more often results from people that pay little attention to oil temperature.

Quite a bit of capacity, mine takes a long time to reach minimum "full throttle/high rpm" temp, even though I'm sure the ecu would allow it earlier.

I only think about this because of watching people at car shows, been parked for 4 hours, ice cold and leaving down the street off the limiter.

Maybe no? Just thinking out loud.

Wayne Smith

Exactly what keeps me away from Auto X. It takes ten miles at 70 mph on the freeway to reach temperature and I'm not willing to jump on the throttle before that.

How do people work around this problem at an Auto X?

sandwedge

Mine purred too. It only had a little over 2,000 miles on it when I bought it and burned the same quart/1,200 miles from the day I bought it till the day I sold it 40,000 miles later. No smoke, no problems. Some just use more oil than others and I haven't seen a good explanation as to why. There's a RL member here who bought his -06 997S cab brand new, European delivery and that car has gone through a quart every 800 miles or thereabouts since delivery. He doesn't post much anymore but I think he's got close to 80,000 miles on it now and last I heard, no change in oil consumption since delivered from the factory. It's weird. My -06 C4S used a few drops of oil between changes.

david

Wayne, isn't one data point missing?