Originally Posted by f11tw
Let's separate the discussion into two parts:
Regarding the 11:1 compression ratio (high CR) 20i engine on RWD 320i and 520i: although there is no direct evidence against the "binning" theory, at least now there is solid proof that the variance in quality is not "the only" difference between the high CR 20i engine and the 28i engine. BMW needs some limiting factors on the 20i engine so that they can preserve the differentiation between 20i and 28i engines, especially in their bread and butter segments - 3ers and 5ers. IMHO, they cleverly use the different shape of the piston head to achieve the objective - higher compression ratio subjects the engine to higher probability of knocking when adding boost, and it would be more than a small job to replace the pistons. There could be other forces at work which we may not fully understand yet, e.g. limp mode by DME software.
The previous discussion among you, E90fleet, and others have greatly benefited the community in advancing the understanding about N20 platform before 320i is available in US. I myself appreciate that very much. However I think the discussion about the piston was missing previously.
As to why BMW chooses to use the low-compression version of the N20 20i on X1, X3, Z4, and even x-Drive version of 320i and 520i, it would be another interesting discussion and more understanding of the platform may be discovered in that process. However it does not change the fact about the high CR 20i engine. Check realoem for the part number of the pistons in different engines and you'll know.
Yes, at that time, which was quite a long time ago last yr, we did discuss abt the difference in compression ratio & some of the differences in part numbers. There were no detailed discussion abt pistons cos no teardown was available then. If I remember correctly, from the ETK at that time, the early batch of U0 was all of the low compression variety.
As to why they made 11:1 version of the 20i (remember it takes R&D resources to increase the CR of the engine due to need to redesign the pistons), I think it has more to do with efficiency gain. Limiting tuning potential is a useful by-product. As the max boost is reduced, they can increase CR w/o risk of detonation.
As for the low CR version, I think we shall leave it as it is for the time being.