If we search the depths of the Internet, we can conclude that in the problem list of the B58 engine, one of the first places is held by the problems of the 6th cylinder. Yes, I already wrote that exactly the ignition coil of the 6th cylinder in idle overheats the most (and its failure provokes misfires with impressive expressions – white, thick smoke, even complete crash of the engine; description of the problem read here). But the defect of the ignition coils in no way explains the cases when the 6th cylinder has had damage, thanks to knocking or “incorrect beam of the injector”. Yes, I did not see images which would confirm damages, caused by knocking, exactly in cylinder No.6, but I try to believe what people say.
Yes, you cannot always trust things written on the internet. For example, the service, which misdiagnosed engine damage, has to maintain the version also after the engine is disassembled – otherwise it will have to pay for unjustified work, damaged spare parts, and idleness. And still, if such cases have happened?
Taking a little time, I concluded that the only image/picture source that shows the damage of the piston of the 6th cylinder is WG Motorworks’ entry in FB/Meta, which has been copied by many sources (yes, I will continue to explore videos and other sources). I do have some reservations about this particular case.
Here is the image with the damaged piston. I want to add right away that no damages were detected for the engine block. For the current engine, the piston of the 6th cylinder was replaced, and everything is fine!
Accordingly, the first conclusion – no overheating of the block itself has happened! If the block were to overheat harshly, there would be deformation of its shell, and there would be defects on the walls of the shell (streaks, melting traces). But for this engine, even the block/engine head wasn’t twisted!
Yes, the melted piston looks as if it has seen the nuclear explosion, but I would like to put an accent on one nuance. The melting temperature of the piston is above 600 °C, and the work temperature is at least 2 times lower. For me, it is really hard to imagine the situation when an increase of +300 °C temperature would appear in one local area of the engine block. It is not possible!
The conclusion here is one – the source of the heat was directly by the piston itself! What would that be? We now have one such culprit – oil! If you “push” your car before its engine has warmed up (and the oil is thick), you can “catch” low-speed pre-ignition, which is created on the walls of the piston.
Unfortunately, this example does not stand up to criticism if we try to blame the local overheating of the block, damage created by knocking, or incorrect beam/spray of the injector. As we can understand from the image and the description, the damage to the piston was (and only) on the side, not on the top part (where the injector injects).
Why exactly is the 6th cylinder damaged? And, as claimed by the authors of the entry, it’s not the first case when exactly this cylinder suffers. The explanation is straightforward. And connected to the question, why have those “stupid” engineers moved all engine chains (timing chain) to the back of the engine block?
What happens when the car accelerates rapidly? The oil in the engine crankcase (by inertia) flows to the bottom part of the engine. If the oil pump and oil catcher are placed on the front part of the engine, the oil “hunger” can happen. But precisely in the case of an ample required torque (accelerating rapidly), the best possible oiling is necessary! Solution – moving the oil pump to the back of the engine. As we know, the oil pump is driven by a chain; accordingly, the chains also “move” to the back of the engine. Here is the answer to why the B58 chain is located on the back of the engine. The problem of oil hunger becomes more acute as engines become more powerful. B58 delivers the performance that was once only possible in expensive sports cars. So it is not a surprise that the oiling has to be appropriate for maximum power in all conditions. The old solutions no longer fit. But the previously mentioned inertia can cause problems. In the moment of a rapid acceleration (and even more harshly, if the car drives in the mountains), the oil level in the location of cylinder No.6 increases rapidly. If the oil is thick, there is a significant risk of catching an increased amount of it, which can ignite. Here, a quite simple explanation of why exactly the 6th cylinder is the first, which will suffer due to low-speed pre-ignition.
If we see the image of the piston (above), it becomes clear why exactly this part of the piston was melt.
With a red oval, I marked the damaged place. As we see, in this place the edge of the piston is the thinnest! Less material – less heat capacity/inertia, and it will melt faster, if the burning process happens in this place.
With a red rectangle, I marked the “sides” of the piston – here the “pockets” are created in which the oil from the crankcase can be flushed in. Instead, with a blue rectangle, I marked the “top/bottom” of the piston. Here is the most significant amount of the material (“meat”). First of all, the more the material, the larger its heat capacity – more energy is needed to melt it. Second – a large surface that touches the engine block (via oil film): it is cooled. Third – this part of the piston (“skirt”) pushes back oil leftovers, not allowing them to press between the piston and shell in large amounts.
All symptoms indicate that the engine block has cooled down the piston and was not the reason for its overheating!
Seeing such a picture of the damage, I have suggestions both to users and BMW engineers:
a. It is clearly visible that the part of the piston opposite its finger (by the oil rings) is its weakest part, if the low-speed pre-ignition is happening. If it were slightly strengthened (several mm would be excellent), it could significantly reduce the risk of piston melting.
b. It’s possible that some “ribs”, which would delay “crowding” of the oil in the bottom of the shell of the 6th cylinder at the moments of rapid acceleration, need to be created;
c. suggestion for the users – drive this engine very gently until it reaches work temperature; change the oil regularly (after each 10.000 km, not less often); use high-quality LL oil; use only oil filters, offered by BMW;
d. If the engine is tuned or you use the throttle opening close to 100% when driving into the mountains, be ready for such surprises.
Disclaimer: The image used in this entry is the intellectual property of WG Motorworks.
