komt uit dit draadje van Hartech op rennlist:
https://rennlist.com/forums/997-forum/1 ... gines.htmlMisschien niet direct '996 I love it' maar denk wel hier op zijn plaats als referentie naar de welbekende zwakke punten van het 996 motorblok.
Even de interessante stukken voor de 996 en 997.1 motoren uitgelicht
Uit de openingspost: over Nikasil,Lokasil en Alusil; schaven en scheurenEven de context schtesen: het gaat hier over de cilinderwanden en hun metallurgische microstructuur. bij Lokasil en Alusil worden bij het gieten de cilinderwanden gelegeerd met extra Silicium dat uitprecipiteert als kristallen. Deze Si-kristallen geven de aluminium matrix hun harde en wrijvingsarme eigenschappen.
Bij Nikasil wordt na het gieten van het blok op de aluminium cilinderwand een laagje nikkel gevormd, samen met legeringselementen silicium en koolstof. Si precipiteert uit in deze nikkel als zeer fijne siliciumcarbide deeltjes, dit is een zeer harde en slijtvast materiaal. Na het honen vormt dit tot nu toe gebleken oerdegelijke en slijtvaste cilinderwand.
According to the manufacturers, Lokasil 1 was a matrix with silicon particles 20 to 70 microns in diameter - so the largest was 0.0028" (in Imperial thous) while Lokasil 2 was 30 to 120 microns - so the largest was 0.0048".
Alusil particles (Gen 2 9A1) were much better bonded into the matrix and the size was the same as Lokasil 1.
Nikasil is electroplated and although the result is a matrix in which the silicon particles are a permanent part of the plated surface - even if one did become loose they are only 1/10 of the size of those in Alusil or Lokasil.
I think you can see where this is going because the particle size of Lokasil 2 is larger than the piston clearance - so a loose particle trapped between the piston and the cylinder wall could be bigger than the space it occupies.
But the larger particle size did seem to stiffen the Lokasil 2 cylinders so we see very few cracking, despite the wall thickness being similar and the piston forces of the 3.6 and 3.8 probably higher.
We do not know when Lokasil 2 replaced Lokasil 1 but from the evidence of scoring and lack of cracking it looks like it might have been after the 3.4 996 engines.
When the bores are honed the particles can have the edge machined smoother so that the thickness is less than the particle size - but when the particle becomes free it can turn and then the height or breadth is the original size.
Rather like concrete or tarmac - when a stone gets loose it tends to rub back and forth in the space it occupied and lengthen it into a furrow that then allows more stones (or particles) to get loose and do the same creating a long groove or scratch.
This is why the damage is only on the thrust side of the piston where the gap between the piston and the cylinder is less due to the power pushing the piston against the cylinder wall and squeezing the oil film more.
All silicon particle based cylinder bores will eventually loose some particles over time and when they do - if the oil film is thick enough to provide a big enough space between the piston and the cylinder wall they can slide out eventually. The thinner the oil and the smaller the clearance - the more the problem can be serious.
Of course not all the particles are the maximum size so it can be a lottery which one becomes loose and how big it is.
The original hypereutectic cylinders (Alusil) were used in conjunction with ferrous coated pistons which had a much harder surface than plastic coatings (especially at elevated temperatures).Uit Post #11: factoren die meespelen in cilinderschade(afstanden uitgedrukt in mijlen)
The influence on scoring will depend on a lot more variable factors - listed below.
(1) particle size, particle distribution, bonding strength, honing influence (all out of the publics control or influence).
(2) Bonding quality of the plastic piston coating (also out of the publics control or influence).
The following can be influenced by owners
(3) Thrust loads (highest when the torque is high and the revs moderate - better to operate at peak revs on full load than full throttle at low revs in low gears).
(4) Thickness and viscosity of the oil film that would separate the piston from being too close to the cylinder bore - influenced by oil choice, viscosity, and the running temperature of the coolant (hence a good idea to fit a low temperature thermostat).
Hence an owner from new that never ragged the car too much, fitted a LTT, used a good quality oil of thicker viscosity as the engines wear and who warmed it up and only drove fast at highish revs and was lucky enough to own a car that had good silicon particle distribution, bonding strength and size and had well bonded plastic coatings on the pistons - may get well of 100K before problems.
In contrast any owner (during a multiple owner period) that used thin oils, used high throttle openings from low revs (high torque typical pulling away in 2nd with a tiptronic), who allowed radiators to get clogged and ran with high coolant temperatures and happened to get an engine with poor silicon distribution and/or bonding and poor piston coating, may have a score after 20K.
In between are all the variations and permutations that make predicting failure and advising owners almost impossible except how to minimise the potential problem accepting that despite that an engine with less than perfect internals may still fail prematurely.Dus we gaan er uiteindelijk langyaam allemaal aan
Maar met een licht ovale of licht geschaafde cilinder kan nog wel gewoon doorgereden worden. Revisie dient zich dan op termijn aan. Het goeie nieuws is dat we dan naar 3.8 of 4.0 kunnen gaan
Post #48: Over koeling, radiators en thermostaatFor our various tests we fitted temperature gauges inside various parts of the cylinder blocks and drove the cars in ambient conditions varying between 5 degrees C and 22 degrees C (over a year).
We fitted blanking covers to the radiators in different steps and covering part and full air flow and recorded the results when moving, stationary, following close behind other cars and lorries and on track.
We fitted different thermostats, different auxiliary thermostat housings and altered the coolant flow.
We found that for normal driving in town and up to 50mph one radiator was sufficient. For very fast driving in hot weather two radiators were sufficient. The only time we needed to use the third radiator (we had fitted) was on track following close behind another car and when stationary (at say lights) on a hot day after spirited driving.
From this we established that too much radiator capacity (combined with a thermostat placed at the entry to the engine and not the outlet as was more traditional) resulted in the thermostat being almost closed and the coolant flow being very slow (because the overkill amount of radiator area at the front forced the thermostat to almost close).
The engine splits the internal coolant flow so that by area about 10% goes into the cylinders and mixes back with the other 90% passing through the heads (so the differences cannot be measured unless you have internal gauges - as we did). The 10% splits off from a coolant tube at right angles (so is not best fro flow splitting).
There are therefore circumstances where the engine is at tickover - hardly any coolant is passing through the block and heat soak increases the cylinder surface temperature - thinning the oil and reducing it's ability to maintain a think film between the piston and the bore under the next imminent power run - so if a loose piece of Silicon get stuck in between at that point - this is when damage can occur.
So we manufactured a thermostat housing for the 3rd radiator and advised owners not to rag the engine after a stop when it had been hot (and not to accelerate with too much throttle in 2nd in a tiptronic).