Originally Posted by f11tw
In November last year, a technical training session on N20 engine was offered to selective customers by a BMW dealer in Beijing, China. During the session the BMW trainer basically confirmed that the top shape of the piston head is different in 20i and 28i models. This is consistent with what one would logically derive from the compression ratio formula - reduced clearance volume resulting in higher compression ratio (11: 1 as in the case of 20i).
Unfortunately the post in the hyperlink below is in Chinese. However I think the pics might still be interesting to have a look at. In addition to an N20 engine torn apart, one gets to see many of the key components in this engine, as well as a few close-up shots of the pistons.
The N20 20i fitted to E84 & F25 have same compression as N20 28i, so do others for certain markets.
The Q&A said in theory if u change the pistons & ECU u'll get a 28i, but they do not support that idea.