Quote:
Originally Posted by onewhippedpuppy
My understanding is that bore scoring is more of a metallurgy issue. The cylinder liners were coated with an Alusil or Nikasil insufficiently tough material that breaks down under extreme stress, such as very cold starts where there’s no lubrication in the cylinder bore. It’s why the Mezger based engines of the same era didn’t have issues, as they retain a Nikasil cylinder bore coating just like the air cooled cars. It seems the issue has been traced to cars in cold climates, which would suggest that how you operate the car is probably not a big factor.
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Quote:
Originally Posted by Cajundaddy
Certainly no expert, but bore scoring became an issue at about the same time as widespread long oil change intervals and DFI. Jake or Tony C. probably have data on this.
My personal mitigation:
-Use quality synthetic oil and change it every 5-6k miles.
- Use Top Tier gas whenever possible.
-Never let a car sit and idle to warm up. Get in it and drive gently to 4000 RPM only until temps are up.
- Use sport mode often when car is warm.
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Quote:
Originally Posted by javadog
I've often heard that the 996 and later cars do better if they are routinely revved higher than what is probably typical in an urban US setting.
I also wouldn't be surprised if the modern temp gauges aren't really telling us anything useful. I think they are designed to not give exact temperatures but to report that the temps are normal if they are withing a certain range. So that may explain some of the "sure warms up quickly" observations.
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There's a very, very in-depth 4 part video series on bore scoring that was done by the PCA with folks like Lake Speed, and the LN Eng folks and some others.
This is my synopsis of the video series.
Bore scoring happens because the piston and cylinder are both aluminum. The piston skirts of the affected motors extend below the bottom of the cylinders. The piston skirts have a patch of some ferrous coating so the piston/cylinder contact will be dissimilar metals and so ZDDP can do it's thing of coating/protecting the bore by bonding to the ferrous patch.
Because the piston skirts extend beyond the bottom of the cylinders at the bottom of the stroke, the bottom of the cylinder causes stress on the patch of ferrous coating when the piston changes direction (causing the piston to "rack" in the bore). When the ferrous coating wears off of the pistons, the fact that both the piston and bore are aluminum causes galling and the problem.
When the motors are cold, the oil isn't as good at protecting things so before you go crazy, you need to get the oil up to temp.
THe "S" cars with forged pistons are at even more of a disadvantage because the forged pistons have a different (slower) rate of expansion due to heat than the block, so in the "not fully warmed up" period, the tolerances between the piston and block are increased which means that the racking motion of the piston at the bottom of the bore is enhanced compared to normal up to temp operation.
The issue is worst when the weather is wintery. I think they said "below freezing" but it may have been "below 40" or "in the teens or colder".
The best ways to retard bore scoring are to 1) keep the revs below 3k and the throttle opening low (I think I remember reading/hearing 40%?) when the motor is completely cold and coming up to temp. And to use a good oil with ZDDP (bonds to ferrous metals only) AND Molybdenum (bonds to Aluminum). Change the oil more frequently than 10k, 15k, 20k, usually recommended is around 5k. The additives get used up out of the oil as the oil is run, so even if the base oils are OK with high miles, you will eventually lose the benefit of ZDDP, Moly, and the other additives.
3.4L piston showing the ferrous patch intact
3.4L piston showing the ferrous patch exhibiting patchy wear
3.4L piston showing the ferrous patch essentially gone and the piston galling.
