Hi guys. This one is really rattling my brain, and I need a sanity check - too much doesn't add up, and it seems to me that CGTI forum would be good place to consolidate some info, as well as get it qualified, discussed, etc. Background information: I am taking the 1.8 PL of my rally car apart, to overhaul it, since the oilpan was punctured => need for new bearings, etc. I had another no-oil-pressure-related incident with it a couple of years ago, due to other reasons that not really relevant. This called for yet another rebuild. Engine specs Engine is a PL. Stock deck, stock pistons, stock rods w. ARP bolts. G60 metal gasket (1.6 mm. compressed thickness). 0.9 mm. skim of the head, to up the CR to get close to 11:1 or so (though I haven't cc'd it). Catcams 279/268 cams (http://www.catcams.com/products/camshafts/datasheet.aspx?ENGINE_id=85&CAMSETUP_id=1097). MegaSquirt ECU. I had the limiter at 7500 rpm, and it put out ~173 bhp at the crank @7300 rpm, with around 26 degrees max ignition advance. Problem By the time of the previous rebuild, I noticed erosion on top of two of the pistons, in the small area facing in the driving direction of the car. Some would call this the squish area - but I will be getting to that. This is often a sign of detonation - and I had - from videos - noticed pinging, and upon examination I decided that I had run too much ignition advance (30 degrees max or so IIRC). Thus I didn't do much else to counter the problem, than retard the ignition. After that rebuild, I decided to go conservative on the ignition advance - for a long time I ran with 22 degrees max advance. That should actually be below the factory advance, that according to my research should be around 24 degrees. I run high octane pump gas, and did not notice pinging, though that is by no means a guarantee. At the rollers added another 4 degrees of advance to the ignition, for around 3 bhp extra, landing me at the 26 or so degrees max advance. After this I only got to do one short race, and a freak accident punctured the oilpan and required me to take the engine apart again. This time I saw erosion again - and this time on all 4 pistons - so worse than last time, even though it had a lot less mileage. You will notice that the flat section is very clean compared to the main crown - and I have valve-to-piston contact as well. I have obviously set out to figure out the problem and solve it, once more, and this time I am going about it a bit more thoroughly. Initial thoughts My initial thoughts revolved around piston-to-head contact in the eroded area - though it was counter intuitive. In short: I see "everywhere" that squish gap should be in the 0,5-1,5 mm. range. Some claim that anything over 1,0 mm. won't have the desired effect, and the engine would be prone to pinging/detonation. One quite knowledgeable guy suggest from the picture, that the gap had been really small, basically causing a vapor lock in the area, which in turn could have caused the erosion. I.e. the erosion was possibly not due to detonation. A second theory was that the gap was too small, causing some of the mixture to be caught and not burned as part of the normal burn. This in turn could lead to a detonating mixture in that area. Some research suggested that the squish of the 16V is around 40 thou (1.0 mm.) static when stock. Once you factor in dynamic parameters like piston slap (around 0.3 mm. in my current case) and a rod stretch of maybe 0.09 mm. for each 1000 rpm., I could very well have, if not contact, then maybe a very close gap. Reality check So I had a million numbers from research, and no real evidence of at least some of the realities in my engine. So I measured what the gap in that area was with MY configuration. It came out to 1.35 mm. static, which is a bit on the open side of things. Some actually claim that above 1 mm. should be opened up to above 3 mm., since detonation can occur in the squish band if it is in a supposed danger zone of 1 to 3 mm. As mentioned, I use a G60 head gasket, which has a compressed thickness of 1.6 mm. My research shows that stock 16V gaskets are 1.75 - 1.8 mm. compressed - depending on the manufacturer. Thus stock gap should be more like 1.5-1.55 mm. with room for a bit of piston slap. Theories First of all I noticed from a set of 6A pistons I have lying around, and various other pictures of 2.0 pistons, that the design of the 1.8 and the 2.0 pistons is different in the area in question. Basically the 2.0 pistons are entirely flat, with cut out only just for the valves. The 1.8 pistons have the area around 0.55-0.60 mm. lower than the main crown (I measured with a straight edge and feeler gauges) 1.8: 2.0: According to http://www.not2fast.com/vw/stuff/vw_engines.shtml the 2.0 has a lower compression height, which matches the value of the additional half stroke of the 2.0 over the 1.8. Thus the protusion of those two pistons over the deck (deck height), should be the same. If the compression height is the same on those two pistons, then the 2.0 will get slightly higher CR with the same head, which resonates with the apparent higher CR according to factory numbers. Also, the 2.0 would get better squish from the area in question, by around 0.6 mm. That would bring stock squish gap on a 2.0 down to 0.9-1 mm. That seems more usable . Reverse calculation A reverse calculation can be done with these numbers: The protrusion of the pistons supposedly is around 0.6 mm. - that is calculated from the above link. On the cylinder head, the area in question is level with the sealing surface = 0.0 mm. The stock 1.8 16V head gasket is 1.75-1.8 mm. thick. The calculated gap of a 1.8 then comes to 1.75 mm. (gasket thickness) - 0.6 mm (protusion above deck) + 0.6 mm (difference in height from actual piston crown and supposed squish area) = 1.75 mm. I don't know if the height of the 9A/6A gasket is the same as the 1.8, but if it is, the squish should be around 1.75-0.6 = 1.15 mm. This all hinges on the 0.6 mm. protrusion to be true - I don't know if that is the case - please correct me if someone has an actual measurement, and I am wrong. What area provides the squish/quench in the 16V? That was a long way round to get to a supposed squish gap of around 0.9 to 1.15 mm. stock on a 2.0, and 1.35 to 1.75 mm. stock on a 1.8. But to me that seems to be reality - please correct me if I took a wrong turn somewhere. Going with that, I am seriously doubting that the area in question is actually what provides the squish-effect in the 1.8 version of the 16V engine - or at least that it has a sub-optimal gap. Going back to what I have heard a few places, that 1-3 mm. is a range to stay out of, it would seem that a tuned 1.8, could benefit from at bit of machining of the pistons in that area, opening the gap a couple of mm. A quote from a one guy on VWVortex: Given the stock CR and ignition advance, I would venture the guess that something IS providing proper squish in the engine - it has to with those numbers. I haven't examined this yet, but does the squish actually come from a "band" around the circumference of the piston, as it travels against the head? Given the bigger bore/same combustion chamber on at least 9A/6A, and the apparent same deck height, it would seem that the entire circumference is the squish - not only the flat area near the inlet valves. With the alternate piston crown design of the 1.8, it would even seem that the flat area near the inlet valve, is not really providing much squish, if at all. And that it actually may lead to detonation in that area, when running higher CR, due to the too open gap. The 1.8 also have a chamfer on the crown/side edge of the piston. I wonder what the effect of that is on the squish effect from the band along the actual crown circumference. It seems that the 2.0 does not have the chamfered edge, making the squish area much more effective. To-do I still have a few measurements and things I want to do. Verify gap in my current setup, with new bearings. Clay the piston circumference of my 1.8, to get a feel for what I suppose is the actual squish area, and what the chamfered edge means to the shape of that area. Measure actual piston protrusion. But given all this, I am prone to machining my next set of stock pistons (class regulations say that they have to be stock, but machining is allowed), and lower the height of my "problem area" by at least 1 mm. - possibly 2 - to get out of the supposedly hazardous 1-3 mm. range. In the meantime - please feel free to discuss. I think it is a bit weird to be honest, and all the info I have been able to gather from this and other forums, does not seem to hold up to the realities.