Further to
42pilot 's point, the limits are very much quantified. Note that while the cc is the same between 50d and 30/35d, the pistons are a different part number. And, we should not gain comfort from excessively stressing a 30d to achieve 50d results.
http://www.etk.cc/bmw/EN/parts/info/...6105/appliance" rel="ugc" target="_blank">http://<a href="http://www.etk.cc/bm.../appliance</a>
However, 30d and 40d share common pistons and many other parts.
http://www.etk.cc/bmw/EN/parts/info/...2585/appliance" rel="ugc" target="_blank">http://<a href="http://www.etk.cc/bm.../appliance</a>
Other engine parts are more common among the 30d/40d/ variants:
- same cylinder head housing
- same crankshaft rod
Again don't aim for 50d output. It has many different components: pistons, timing chain, crankshaft, etc. all major components are a different spec for 50d.
Aiming for 40d output from a 30d should also be done with caution, even though the engines are nearly identical as the " how" is different. And while I am not mechanically inclined, I know from experience that "how" always matters in life.