The bore scuffing was distinctly one sided, the thrust side to be specific. The skirt of the piston showed scuffing and partial loss of the anti-friction coating. I think its pretty safe to say the scuffing was purely a result of piston thrust.
After finally seeing the pictures for myself, I think I'm a bit more qualified now to comment than I was earlier. You're right, Bruce, the scuffing looks distinctly loaded on the trust sides of the pistons. I still stand by my believe that it was PCP-related. Very high gas pressures act on the head and basically want to push the head off the block surface, with only the head bolts/studs holding them down. This tensile load is transfered down the bolts and are taken up by the engine block, and this causes the bore to distort. Of couse, there is a combination of several things that cause bore distortion; I'm mentioning just one.
How do you reduce side thrust, or ultimately prevent scuffing in the future? Offset piston pins can help for one. I believe some of the newer VW engines have this feature. I don't know if blank crowns are available. When I was working at FEV a few years ago, I was aware that they bought Mahle piston blanks that were basically the same ones that go into the TDI, but had a flat crown and no bowl or other pattern machined into them yet. I donno if anyone has the contacts to get something like that.
Second is to just not run such high PCPs. Bore distortion is a very real phenomenon, and that's one of the reasons why PCPs have an upper ceiling. The ALH design limit is 155 bar and 130 bar for the 1Z/AHU TDI. I would hazard that the AAZ would be the same or lower than the 1Z/AHU. I would also estimate that Andy's setup would more than double that figure, albeit in short bursts.
Ring scuffing, in my experience, is always immediately fatal. In searching for the magic "zero ring gap" on a couple of motocross bike engines, I have had the ring ends butt together at high temperature and cause the engine to sieze. Same mechanism as you're suggesting, but brought about by pressure due to lack of ring end gap. The engine will often come free after the ring contracts but the scuffed iron will immediately re-seize as soon as the engine is started, not the case with andy's motor.
Zero-gap rings technically still have "gaps" because they have step-butted-ends. Seizure is most often due to the butted end gaps not being proper to begin with, not an inherent design flaw of zero-gap rings.
Gapless rings would tend to make the ring scuffing problem, if it exists, worse. With little leakage past the top ring all the pressure is carried by it alone. With standard rings the leakage past the top ring is sufficient to load the second ring and the pressure is somewhat shared between them. Cast iron is the best material for rings, particularly when scuffing or marginal lubrication is a concern, I would stick with it. Lubrication might be an issue. 2 stoke detroit diesels are famous for piston scuffing. The detroit "fix" is to run straight weight dino oils in 30 or 40 grades as they typically have the highest HT/HS viscosity. Food for thought?
The second compression ring does very little of the gas sealing work; rather as you correctly say, it reduces the pressure drop across the first ring. In some racing applications, the second ring is either made very low tension and thin or totally dispensed with.
However, the reduced pressure drop due to the contribution of the second-ring, I would say, is relatively small compared to the overall magnitudes of pressures that the top-side of the top ring experiences, and would primarily affect the axial loading of the rings on the piston grooves and not the radial loading on the cylinder. Here I'm looking at both rings as a pair, but concerned only with the top ring. If you have a given 250 bar on the top of the top ring, I would argue that it makes very little difference whether the pressure under the top ring is atmospheric (hypothetical extreme case if the second ring weren't there at all), or a couple of bars.
As for material, yes, the consensus is that good ol' cast iron is ideal for non-plated rings. However, OEM VW rings (at least one or both of the compression rings) are chrome plated, and the tribological surface is the chrome plating and the cylinder, not the base cast iron material. Anyway, I agree that VW has as good OE rings as anyone anywhere, and it would not make sense to try to second-guess them. I think it was Deo who said that Total-Seal simply reworked OEM rings, and if one were so inclined to go overkill on the rings, that might be the route to take.
Intercooling is much improved this time around. Anybody that says high boost is bad hasn't tried it. 
Nitrous is just cheating.
Well, respectly fully I have to disagree here. Any more boost than is required to burn up all the fuel you plan on dumping while aiming for minimal smoke and having decent EGTs only adds to more needless stress to the engine and, in the final analysis, no extra HP made. Note: Please don't misunderstand what I just said! Consider the whole statement rather than just the last 4 words. Do you disagree?
OK, I grant you nitrous is out of the discussion.
The 1.9TD crank is the same as the AHU TDI, save for the sensor wheel. The rods this time around are from the PD. Electron deposition coated anti-fatigue upper bearings are installed in the rods. Standard VW aluminum main bearings will stand anything, no sweat there. The block has been girdled for improved stability.
The PD rods make the one beside it look a twig! And if these are PD100 rods, I hear the PD150 ones are even stronger!
My question stands: have the bearings been looked at?
Cheers,
Dave
Topic: Compound pics (Read 109578 times)
