Originally Posted by Iron Man
Saying Al is stronger than Mg is somewhat subjective and doesn't relay all of the necessary info. For one thing, engine blocks are not forged, but cast (the article notes forged Al is stronger...).
Engine blocks are made from a cast Al alloy or a Mg alloy. (Mg is a very common alloy element in Al alloys by the way). Now what neither of you know is the ALLOY that BMW used to make their block, so you cannot compare the two. Also, overall strength is determined by wall thickness, so a lower tensile strength would just require a thicker wall design to compensate. The real overall design question should be, for the given alloy strength and necessary wall thickness, which block design would be lighter (or better meets the overall engine design criteria, don't forget cost as well).
Arguing which one is better is like pissing in the wind. It all depends on the overall design of engine, application and powertrain.
You're correct to a certain extent. The aluminum alloy used in the N54/55 blocks contains other elements, Mg included and the Mg block in the N52 contains other elements as well. The fact is, the Al block is mostly Al and the Mg is mostly Mg. The other elements are in small amounts to serve specific function. Im not saying, nor have I said the N54/55 engines are 'better' just that the N52 block are deemed not suitable for F/I power levels by the man who knows best about the engines. Direct from the horses mouth! Explains it quite simply. Some hate to hear the truth. Not too surprising.
I appreciate the technology that BMW used to make this first and one of a kind lightweight engine. Its a marvel of design and execution to marry 2 alloys together and make them work in such a great way to make power and cut a nice chunk of weight off the front of the car. Being the hp freak I am I needed more power for my DD. If not, I could have been quite happy with a 128. Its a fine package indeed.
I posted some information directly from BMW saying why they wont use the Mg/Al block for turbo cars. There are many here that have been chomping at the bitt for F/I to be made available for their cars when I dont think its coming and thats why I posted. Its not something I made up to piss owners off but thought owners would like to know the truth from the real expert in BMW's engineering/ motor building team. Mr Welter knows the exact alloy BMW alloy formulas, wall thicknesses, design criteria and all necessary data..... and concluded from all that, it wont work. Some take that as insulting when that wasnt the intent. So be it.
article mentions forged early in the article but most of it its talking about cast. And we are talking about the overall design of the engines, application(F/I) and powertrains(N54 turbos) so its all very relevant.