special tool

Because of my personal experiences in extracting power from these engines, I would NEVER even use a factory turbo RING again.
Nevermind specified factory piston clearances.

I am speaking about high power here (over 500 RWHP you have too much fast heat, or in a high-compression/boosted 944 engine), I realize they are fine for stock/near-stock applications.

Just my opinions, of course, I am sure many know much more than I.

evil 944t

thats true..

evil 944t

Was this a complete n/a motor but turbocharged, ie all factory n/a clearances?

special tool

yes

marcoturbo

ST, what kind of rings should be used in an Alusil block for high power ?

special tool

The ones that Dave/Ski gave me for use with the Mahle Motorsport pistons - these are the only ones which didn't cause problems at high power levels coupled with Dave/Neil clearance specs.

I can't post them here because they are not my intellectual property - and I never have, nor will I ever give someone else's away.
Call them, prices are VERY, VERY reasonable.

My stuff I will tell you all day long.

968turbos2

What kind of oil are you running and also can you post a pic of how the oil was routed to the turbo? Also what kind of trubo Journal or BB? I am wondering if the compression plus the boost cause a high pressure barrier that kept the oil from remaining on cylinder walls and allowing piston/wall contact. On the cylinder walls, is the scoring on opposing sides? If so, look at the top of the scoring on each side do they begin at the same distance from the deck or is one side scored higher than the other?

If you are going to rebuild you should pm me. I can tell you from fresh experience how much you can rebuild/sleeve/forged piston for and have many pics/gotchas etc I can share.

968turbos2

This was probably caused by the fact you used the factory pistons. If you look at 944 turbo pistons, 968 N/A pistons the 944's have an angle straight wal bowl. The pressure from the combustion is then centered in the middle of the piston with a line of force directed at the concave bowls center. Pistons for turbo apps are bowled/dished for that reason as well as lowering the compression ratio.

Maybe the high compression N/A pistons could not direct the pressure towards the center. Are you sure that your ecu has not been compensating for a little knock for 3k miles? Did you verify timing with a light to see if ecu was handling the increased volume. Also running on pump gas with 11 to 1 and 7-10psi boost.....

Ski

Could this have possibly been related to a lack of oil, stemming from the unknown little black particles that didn't end up in the oil pan? Just a thought, or over time, as the cylinders became out of round, that combo with worn rings, the extra heat and pressure - stuff just moved around. We'll probably never know exactly with something like this(unless you find more extreme evidence) but from the looks of the pistons, they were moving side to side in the bores. I'd be curious if there is any debris behind the holes to the oil ring.

Whatever pistons or combo you go forward with, follow the manufacturers recommendation for tolerances unless you or your machinist have excellent inside knowledge of previous projects the same items that you choose. There are several people on here who have great experience with different pistons in different type of bores AND have experience setting the tolerances ( due to expansion rates of different material make up )...for success. Best of luck with new engine.

Laust Pedersen

My guess is that the engine (at least at some point) has been running grossly rich under power, so the oil film has been washed away by unburned gas. The scoring is at an area where liquid would gravitate to.

Fishey

Its a simple piston skirt problem because the design of the piston. The same goes for the fact that the piston is cast and not forged it played a part in this as well. The head went because of heat the damage was done because of heat not because of an oil problem.

-Fishey

special tool

This is an engine assembled at the Porsche factory.

Fishey

That has nothing to do with what I am talking about. The parts inside the motor are the issue not the assembly.

Ski

Ok, I'll bite...what caused the piston skirt to be the problem?

Mikes3.0cabturbo

Laust interesting point on why only the gravity side is galled, on richness its been running data logging whilst tuning, its never got below 10 AFR and quicklly tuned up to low 11's.
Is it possible that the car was carboned up prior to turbocharging, the extra heat/pressure broke down the carbon, maybe mixed with fuel/oil and gone down that side - the pistons were very very clean for 100k mls.

968turbos2, its using stock 951 oiling system for the turbo also running a 60 thou restrictor, oil pressure are 4 bar plus about 3 on idle, oil is changed every 5k's but every 3ks since turboed, mainly castrol 10-60
synthetic. I use an EMS so the fact ECU doesnt control knock anymore - only me and I am pretty sure there has been none, I've been very careful with that.

Ski - thanks I will be taking advice on the new clearances to use dont want to go through this again, just need to assess the bore damage then I will know what piston size I will go for, but I guess I will have to take the block out and to a machinist to tell that - unless there is another trick to use? The bores dont look anywhere near as bad as the pistons.
BTW, the oil control rings are not spotless, but dont seem to show foreign objects in there either.

AFAIK that other turboed 3.0 with stock internals on higher boost have used 968 engines, they have the forged stock pistons and I guess larger clearances? For example 9M supercharged versions they went up to 11psi and tracked them on stock comp.

I measured one of the comp rings at the top of the bore 26 thou gap.

rgds
mike