[E92Fan]

Originally Posted by R1000K3

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.

Not necessarily true - it depends on the type of material used in the piston.

Old-era pistons were notoriously brittle and susceptible to expansion due to the poor alloys used. Hence there used to be a large piston to wall clearance to allow for the expansion. This was regardless of whether the pistons were cast or forged. In those olden days, the cast pistons were better than their forged equivalents, due to the fact that their multiple-piece molds allowed for intricate contours inside and out, resulting in light weight, good expansion control and predictable heat flow through the part. Forgings in comparison were crude, heavy (because of the lack of sophistication in intricate contours) and also made to be loose in the cylinder due to the poor quality of alloys. Forged pistons in those days were dimensionally unstable.

Matters were made worse when silicon was eventually added into the alloys used to manufacture pistons - cast piston manufacturers got hold of the silicon technology first and then the case for cast pistons became even stronger. Forged pistons in comparison did not use silicon in the alloy mix until much later on in the life of the technology. Silicon gives the pistons natural lubricity and limits heat expansion. All modern pistons have silicon in them and cast pistons have historically had the most, with some even having as much as 25% silicon by volume. However there is a disadvantage with silicon - it makes the piston brittle. If you drop a modern cast piston, it would probably crack.

Forged pistons are inherently stronger than cast pistons, given the same material structure, because the forging process compresses the alloy's molecules therefore making the material more dense than a casting. Forged pistons also typically use less silicon content, therefore making the piston less brittle. The result is a piston that can better withstand forceful detonation, and hence why forged pistons are generally used in turbo engines because they can withstand the force of combustion. Advancements in alloy technologies has now resulted in a silicon/nickel mix that when combined with the top alloys available allows for forged pistons of the very highest quality, at least as dimensionally stable as typical cast pistons, and therefore ideally suited to ultra high-performance turbocharged engines.

Originally Posted by Penn999

This is correct, they are more stable to higher temps(max temps, so they dont deform, expand which cause excessive wear) and cast is a molded piece of metal. The stockers need a lower compression tho, IMO. I will be getting forged lowER compression pistons when its time for it.

Hence why I have gone to a 9.5:1 ratio as opposed to the standard 10.2:1

Originally Posted by R1000K3

I've now got uncomfirmed information that E92fan have not been using methanol injection. If this is correct it is less to worry for meth-users with upgraded turbos. Let's wait and see what E92fan will say.

Correct, I don't run methanol at all. I will never have it in my car, because it's too dangerous for me to be driving hard around the 'Ring with the methanol potentially running out with the engine under full load... I'd need a baffled methanol tank, level warnings etc etc..

Does anyone know how much of a difference to cylinder head temperatures (and piston temperatures) methanol makes? Just curious...

Originally Posted by BrianMN

The primary And general use for an EGT sensor is to measure cylinder temps...not sure where you got the notion of monitoring turbo temps, ive ever heard of people concerned about turbo temps. Cylinder temps are much more necessary to monitor When tuning or even just driving hard. If you have the equipment to put more EGT sensors on, I'd probably put another in the front manifold first, then one in the downpipe...but you really aren't going to care about post-turbo temps

Exactly right EGT sensors are being installed during the rebuild, as are sensors in the intake manifold and also pressure transducers either side of the intercooler to measure pressure drop across the Code3/Spearco unit I'm using (same as the Dinan unit)