Originally Posted by onefastman
Now that many more have them only time will tell.
I'm a bit worried about upgraded turbos and stock piston to cylinder clearance. I'm convinced it is like E92fan states, that the piston skirts has scuffed against the cylinder walls. The reason this is the case in his engine but was not shown in e.g. Enrico's engine, is almost for sure E92fan has been running a lot with upgraded turbos in combination with hard driving on the Nurburgring during 8 minutes per lap. The pressure from upgraded turbos is much higher in average than for stock turbos. You just need to touch the pedal on low RPM's to get a lot of boost on upgraded units. This means the average power is a lot higher, which is the purpose for the upgrade, but i also means the piston temperature will be a lot higher. Power does not come for free, the higher the power level is, the higher the piston temperature will be.
The solution is not just to increase the capacity of the cooling system, this will in fact increase the risk for scuffing since the cylinder bore will enlarge less, it is to increase the skirt to cylinder clearance. This can be done also with stock pistons, it is just to hone the cylinder slightly and put in new rings. But since this includes a lot of work and money it is better to put in new pistons at the same time with a bit lower CR and hopefully also stronger than the stock items. The requirement for a correct piston to cylinder clearance for maximum load in the specific application is still there of course. Forged pistons are well known to expand a lot more than cast items. I guess BMW did not designe the N54 to withstand the abuse from upgraded turbos but instead designed it for as little piston and ring wear as possible and to get the engine reasonable quiet.